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SEMINAR
                   ON
A REVIEW OF CHANGE DETECTION TECHNIQUES


     INDIAN INSTITUTE OF TECHNOLOGY
                 ROORKEE

                                  PRESENTED BY:-
                                 ABHISHEK BHATT
                              RESEARCH SCHOLAR
                        abhishekbhatt.iitr@gmail.com
OUTLINE
This seminar is organized into eight sections as follows:


1. Background and applications of change detection techniques
2. Considerations before implementing change detection
3. A review of seven categories of change detection techniques
4. Comparative analyses among the different techniques
5. A global change analyses
6. Threshold selection
7. Accuracy assessment
8. Summary and recommendations
References
Background
• In general, change detection involves the application of multi-
  temporal datasets to quantitatively analyze the temporal effects

• Change detection can be defined as the process of identifying
  differences in the state of an object or phenomenon by observing it
  at different times. This process is usually applied to Earth surface
  changes at two or more times.

• understanding relationships and interactions to better manage and
  use resources

• Change detection is useful in many applications such as land use
  changes, habitat fragmentation, rate of deforestation, coastal change,
  urban sprawl, and other cumulative changes
Change detection
                                     ► Two main categories of land
                                     cover changes:

                                     ▪ Conversion of land cover from
                                     one category to a different category.

                                     ▪ Modification of the condition of
                                     the land cover type within the same
                                     category (thinning of trees,
                                     selective cutting, pasture to
                                     cultivation, etc.)
source; Norsk Regnesentral website
Applications of change detection
                techniques
• land-use and land-cover (LULC) change
• forest or vegetation change
• forest mortality, defoliation and damage assessment
• deforestation, regeneration and selective logging
• wetland change
• forest fire and fire-affected area detection
• landscape change
• urban change
• environmental change, drought monitoring, flood monitoring, monitoring
  coastal marine environments, desertification, and detection of landslide
  areas
• other applications such as crop monitoring, shifting cultivation monitoring,
  road segments, and change in glacier mass balance and facies.
Considerations before implementing
          change detection
• Before implementing change detection
  analysis, the following conditions must be
  satisfied:
  i. precise registration of multi-temporal images;
  ii. precise radiometric and atmospheric calibration
       or normalization between multi-temporal
       images;
  iii. selection of the same spatial and spectral
       resolution images if possible
Good change detection research should
 provide the following information:

  i.     area change and change rate
  ii.    spatial distribution of changed types
  iii.   Change trajectories of land-cover types
  iv.    accuracy assessment of change detection results.
A review of change detection
               techniques
• Because digital change detection is affected by
  spatial, spectral, radiometric and temporal constraints.

• Many change detection techniques are possible to
  use, the selection of a suitable method or algorithm
  for a given research project is important, but not easy.
The seven change detection technique categories
1.   Algebra Based Approach                     4.   Advanced Models
      •  image differencing                           •  Li-Strahler Reflectance Model
      •  image regression                             •  Spectral Mixture Model
      •  image ratioing                               •  Biophysical Parameter Method
      •  vegetation index differencing          5.   GIS
      •  change vector analysis                       •  Integrated GIS and RS Method
                                                      •  GIS Approach
2.   Transformation                             6.   visual Analysis
      •   PCA                                         •  Visual Interpretation
      •   Tasseled Cap (KT)                     7.   other Change Detection Techniques
      •   Gramm-Schmidt (GS)                          •  Measures of spatial dependence
      •   Chi-Square                                  •  Knowledge-based vision system
                                                      •  Area production method
3.   Classification Based                             •  Combination of three indicators: vegetation
      •   Post-Classification Comparison                 indices, land surface temperature, and
      •   Spectral-Temporal Combined Analysis            spatial structure
      •   EM Transformation                           •  Change curves
      •   Unsupervised Change Detection               •  Generalized linear models
      •   Hybrid Change Detection                     •  Curve-theorem-based approach
      •   Artificial Neural Networks (ANN)            •  Structure-based approach
                                                      •  Spatial statistics-based method
Category I
      Algebra Based Approach

• The algebra category includes
   – image differencing,
   – image regression
   – Image ratioing
   – vegetation index differencing
   – change vector analysis (CVA)
Algebra based Approach……
• These algorithms have a common characteristic, i.e. selecting
  thresholds to determine the changed areas. These methods
  (excluding CVA) are relatively simple, straightforward, easy to
  implement and interpret, but these cannot provide complete
  matrices of change information.

• In this category, two aspects are critical for the change
  detection results:
   – selecting suitable image bands
   – selecting suitable thresholds
Image Differencing

• Concept
   – Date 1 - Date 2
   – No-change = 0
   – Positive and negative values interpretable
   – Pick a threshold for change
Image Differencing
 8    10     8    11

240   11    10    22
                        Image Date 1
205   210   205   54
                                        3       1     1    1
220   98    88    46
                                       143      2     2    0

                                       107     110    0   -168
 5     9     7    10
                                       117      0    -166 -164
 97    9     8    22

 98   100   205   222   Image Date 2
                                             Difference Image =
103    98   254   210                        Image 1 - Image 2
Image Differencing

• Image differencing: Pros
   – Simple (some say it’s the most commonly used method)
   – Easy to interpret
   – Robust

• Cons:
   – Difference value is absolute, so same value may have
     different meaning
   – Requires atmospheric calibration
Image regression
► Relationship between pixel values of two dates is
established by using a regression function.

► The dimension of the residuals is an indicator of where
change occurred.

► Advantage
▪ Reduces impact of atmospheric, sensor and environmental
differences.

► Drawback
• Requires development of accurate regression functions.
• Does not provide change matrix.
Image regression
Image Ratioing
• Concept
   – Date 1 / Date 2
   – No-change = 1
   – Values less than and greater than 1 are interpretable
   – Pick a threshold for change
• Pros
   – Simple
   – May mitigate problems with viewing conditions, esp. sun angle
• Cons
   – Scales change according to a single date, so same change on the ground
     may have different score depending on direction of change; I.e. 50/100
     = .5, 100/50 = 2.0
Change Detection
                                       source: CCRS website, CANADA




Image Difference (TM99 – TM88)     Image Ratio (TM99 / TM88)
Change vector analysis

• In n-dimensional spectral
  space, determine length
  and direction of vector
  between Date 1 and Date2             Date 1



                              Band 4
• No-change = 0 length
                                            Date 2
• Change direction may be
  interpretable
• Pick a threshold for
  change

                                                     Band 3
Change vector analysis
                                     ► Determines in n-dimensional spectral space,
                                     the length and direction of the vector between
                                     Date 1 and Date 2.

                                     ► Produces an intensity image and a direction
                                     image of change. The direction image can be
                                     used to classify change.

                                     ► Typically used when all changes need to be
                                     investigated.
                                     ► Advantage
                                     ▪ Works on multispectral data.
                                     • Allows designation of the type of change
                                        occurring
                                     ► Drawback
source; Norsk Regnesentral website   ▪ Shares some of the drawbacks of algebra based
                                     techniques but less severe
Change vector analysis
Category I. Algebra Based Approach
  Techniques         Characteristics            Advantages           Disadvantages         Examples         Key factors
1. Image       Subtracts the first date     Simple       and      Cannot provide        Forest            Identifies
differencing   image from a second-         Straight forward,     a detailed change     defoliation,      suitable image
               date image, pixel by         easy to interpret     matrix, requires      land-cover        bands and
               pixel                        the results           selection      of     Change and        thresholds
                                                                  thresholds            irrigated
                                                                                        crops
                                                                                        monitoring

2. Image       Establishes relationships    Reduces impacts       Requires to develop   Tropical forest   Develops the
regression     between         bitemporal   of the atmospheric,   accurate regression   change     and    regression
               images, then estimates       sensor and            functions for the     forest            function;
               pixel values of the          environmental         selected      bands   conversion        identifies
               second-date image by use     differences between   before                                  suitable bands
               of a regression function,    two-date images       implementing                            and thresholds
               subtracts the regressed                            change detection
               image from the first-date
               image


3. Image       Calculates the ratio of Reduces impactsof Non-normal           Land-use                    Identifies the
ratioing       registered images of two Sun angle, shadow distribution of the mapping                     image bands
               dates, band by band      and topography    result is often                                 and thresholds
                                                          criticized
Techniques                 Characteristics                   Advantages        Disadvantages       Examples          Key factors
4. Vegetation     Produces vegetation index separately,    Emphasizes             random noise or   Vegetation      Identifies suitable
Index             then subtracts the                       differences in the coherence noise       change          vegetation index
differencing      second-date vegetation index             spectral response                        and forest      and thresholds
                  from the first-date vegetation index     of different features                    canopy
                                                           and reduces impacts                      change
                                                           of topographic effects                   Enhances
                                                           and illumination.




5. Change         Generates two outputs: (1) the           Ability to process    Difficult    to    landscape       Defines
vector analysis   spectral change vector describes the     any number of         identify land      variables       thresholds
(CVA)             direction and magnitude of change        spectral bands        cover change       land-cover      and identifies
                  from the first to the second date; and   desired and to        trajectories       changes         change
                  (2) the total change magnitude per       produce detailed                         disaster        trajectories
                  pixel is computed by determining the     change detection                         assessment
                  Euclidean distance between end           information                              and conifer
                  points through n-dimensional change                                               forest change
                  space
Category II.

Transformation of data sets
Transformations
► Principal Component Analysis


► Alt1: Perform PCA on data from both
dates and analyse the component
images.

► Alt2: Perform PCA separately on each
image and subtract the second-date PC
image from that of the first date.

► Advantage
▪ Reduces data redundancy.

► Drawback
▪ Results are scene dependent and can be difficult to interpret.
▪ Does not provide change matrix.
Kauth Thomas Transformation
• Described the temporal spectral patterns derived from Landsat MSS imagery
  for crops. As crops grow from seed to maturity, there is a net increase in
  NIR and decrease in Red Reflectance. This effect varies based on soil Color
• Brightness Greenness Wetness
• The Brightness, Greenness, Wetness transform was first developed for use
  with the Landsat MSS system and called the “Tasseled Cap” transformation.
• The transform is based on a set of constants applied to the image in the form
  of a linear algebraic formula.
• Brightness – primary axis calculated as the weighted sum of reflectances of
  all spectral bands.
• Greenness – perpendicular to the axis of the Brightness component that
  passes through the point of maturity of all plants
• Yellow Stuff – perpendicular to both Greenness and Brightness axis
  representing senesced vegetation.
Kauth Thomas Transformation


                                                   Typically      the    first    few
                                                   components contain most of the
                                                   information in the data so that
                                                   four channels of LANDSAT MSS
                                                   data or the six channels of the
  http://www.sjsu.edu/faculty/watkins/tassel.htm   Thematic Mapper data may be
                                                   reduced to just three principal
                                                   components. The components
                                                   higher than three are usually
                                                   treated as being information less.


Source; www.sjsu.edu/faculty/watkins/tassel.htm
Category II. Transformation
   Techniques                Characteristics               Advantages            Disadvantages             Examples            Key factors
1. Principal       Assumes that multitemporal data       Reduces data       PCA is scene dependent,      Land-cover         Analyst’s skill in
component          are highly correlated and change      Redundancy         thus the change detection    change urban       identifying which
analysis (PCA)     information can be highlighted in     between bands      results between different    expansion          component best
                   the new components. Two ways to       and emphasizes     dates are often difficult    ,tropical forest   represents      the
                   apply PCA for change detection        different          to interpret and label. It   conversion ,       change         and
                   are: (1)put two or more dates of      information in     cannot      provide      a   forest             selecting
                   images into a single file, then       the      derived   complete     matrix     of   mortality and      thresholds
                   perform PCA and analyse the           components         change class information     forest
                   minor component images for                               and requires determining     defoliation
                   change information; and                                  thresholds to identify the
                   (2) perform PCA separately, then                         changed areas
                   subtract the second-date PC image
                   from the corresponding PC image
                   of the first date

2. Tasselled cap   The principle of this method is       Reduces data       Difficult to interpret and   Monitoring         Analyst’s skill is
(KT)               similar to PCA. The only difference   redundancy         label              change    forest             needed in
                   from PCA is that PCA depends on       between bands      information,        cannot   mortality   ,      identifying
                   the     image    scene,      andKT    and emphasizes     provide a complete           monitoring         which
                   transformation is independent of      different          change matrix; requires      green biomass      component best
                   the scene. The change detection is    information in     determining thresholds       and                represents the
                   implemented based on the three        the      derived   to identify the changed      land-use           change and
                   components: brightness, greenness     components.        areas.           Accurate    change             thresholds
                   and wetness                           KT is scene        atmospheric calibration
                                                         independent.       is required
Techniques          Characteristics           Advantages             Disadvantages           Examples       Key factors

3.           The       GS        method     The association     It is difficult to extract Monitoring    Initial
Gramm–       orthogonalizes      spectral   of transformed      more than one single forest              identification of
Schmidt      vectors taken directly from    components          component related to a mortality         the stable
(GS)         bi-temporal images, as does    with       scene    given type of                            subspace of the
             the original KT method,        characteristics     change.       The      GS                multi-date data is
             produces     three    stable   allows        the   process      relies    on                required
             components corresponding       extraction     of   selection of spectral
             to multitemporal analogues     information that    vectors from multi-date
             of KT brightness, greenness    would not be        image typical of the
             and wetness, and a change      accessible using    type of change being
             component                      other               examined
                                            techniques


4. Chi-      Y=(X-M)T ∑-1*(X-M)             Multiple bands      The assumption that a       Urban        Y is distributed
square       Y:digital value of change      Are                 value      of       Y~0     environmen   as a Chi-square
             image                          simultaneously      represents a pixel of no    tal          random variable
             X:vector of the difference     considered to       change is not true when     change       with p degrees
             of the six digital values      produce        a    a large portion of the                   of freedom ( p is
             between the two dates          single change       image is                                 the number of
             M:vector of the mean           image.              changed. Also the                        bands)
             residual of each band                              change     related     to
             T:transverse of the matrix                         specific         spectral
              ∑-1= inverse covariance                           direction not identified
             matrix
Category-III
Classification based approach
Post-classification
• Post-classification (delta classification)
   – Classify Date 1 and Date 2 separately, compare class values
     on pixel by pixel basis between dates
• Post-classification: Pros
   – Avoids need for strict radiometric calibration
   – Favors classification scheme of user
   – Designates type of change occurring
• Cons
   – Error is multiplicative from two parent maps
   – Changes within classes may be interesting
Composite Analysis
• Composite Analysis
   – Stack Date 1 and Date 2 and run unsupervised
     classification on the whole stack
• Composite Analysis: Pros
   – May extract maximum change variation
   – Includes reference for change, so change is anchored at
     starting value, unlike change vector analysis and image
     differencing
• Cons
   – May be extremely difficult to interpret classes
Unsupervised techniques
                                     ► Objective
                                     ▪ Produce a change detection map in which
                                     changed areas are separated from unchanged
                                     ones.

                                     ► The changes sought are assumed to result in
                                     larger changes in radiance values than other
                                     factors.

                                     ► Comparison is performed directly on the
                                     spectral data.

                                     ► This results in a difference image which is
                                     analysed to separate insignificant from
                                     significant changes.
source; Norsk Regnesentral website
Supervised techniques
                                      Objective
                                     Generate a change detection map
                                     where      changed    areas     are
                                     identified and the land-cover
                                     transition type can be identified.

                                      The changes are detected and
                                       labelled using supervised
                                       classification approaches.

                                      Main techniques:
                                     • Post-classification comparison
                                     • Multidate direct classification
source; Norsk Regnesentral website
Post classification comparison
                                     ► Standard supervised classifiers are used to
                                     classify the two images independently.

                                     ► Changes are detected by comparing the two
                                     classified images.

                                     ► Advantage
                                     ▪ Common and intuitive.
                                     ▪ Provides change matrix.

                                     ► Drawback
                                     ▪ Critically depends on the accuracy of the
                                     classification maps. Accuracy close to the
                                     product of the two results.
                                     ▪ Does not exploit the dependence between
                                     the information from the two points in time.
source; Norsk Regnesentral website
Post classification comparison
Multidate direct classification
                                     ► Two dates are combined into one multitemporal
                                     image and classified.
                                     ► Performs joint classification of the two images
                                     by
                                     using a stacked feature vector.
                                     ► Change detection is performed by considering
                                     each
                                     transition as a class, and training the classifier to
                                     recognize all classes and all transitions.
                                     ► Advantage
                                     ▪ Exploits the multitemporal information.
                                     ▪ Error rate not cumulative.
                                     ▪ Provides change matrix.
                                     ► Drawback
                                     ▪ Ground truth required also for transitions.
source; Norsk Regnesentral website
Supervised vs. Unsupervised
                         Supervised                  Unsupervised
Level of change     Change detection at          Change detection at data
                    decision level.              level.
detection
Change              Provides explicit labeling   Separates ‘change’ from
                    of change and class          ‘no change’.
information         transitions
Change              Obtained directly from       Obtained through
                    the classified images.       interpretation of the
computation                                      difference image.
Ground truth        Requires ground truth.       Requires no ground truth.

Spectral            Multispectral.               Most methods work on
                                                 one spectral band.
information.
Data requirements   Not sensitive to             Sensitive to atmospheric
                    atmospheric conditions       conditions and sensor
                    and sensor differences.      differences.
Category III. Classification based approach
  Techniques               Characteristics                 Advantages           Disadvantages            Examples        Key factors
1. Post            Separately      classifies   multi- Minimizes              Requires      a great     LULC change,   Selects sufficient
classification     temporal images into thematic       impacts           of   amount of time and        wetland        training sample
comparison         maps, then implements comparison    atmospheric,           expertise to create       change         data for
                   of the classified images, pixel by  sensor and             classification            and urban      classification
                   pixel                               environmental          products. The final       expansion
                                                       differences            accuracy depends on
                                                       between                the quality of the
                                                       multitemporal          classified image of
                                                       images; provides a     each date
                                                       complete matrix of
                                                       change
                                                       information
2. Spectral–       Puts multi-temporal data into a Simple               and Difficult to identify       Changes in     Labels the
temporal           single file, then classifies the    timesaving           and label the change        coastal zone   change classes
combined           combined dataset and identifies and in classification    classes;           cannot   environments
analysis           labels the changes                                       provide a complete          and forest
                                                                            matrix of change            change
                                                                            information
3. EM              The      EM     detection   is   a    This method was Requires estimating            Land-cover     Estimates the
detection          classification-based method using     reported        to the a priori joint          change         a priori joint
                   an expectation maximization (EM)      provide     higher class probability.                         class probability
                   algorithm to estimate the a priori    change detection
                   joint class probabilities at two      accuracy      than
                   times. These probabilities are        other      change
                   estimated directly from the images    detection methods
                   under analysis
Techniques              Characteristics                  Advantages             Disadvantages              Examples          Key factors
4. Unsupervised   Selects spectrally similar groups of   This       method      Difficulty           in Forest hange         Identifies the
change            pixels and clusters date 1 image       makes use of the       identifying         and                      spectrally similar
detection         into primary clusters, then labels     unsupervised           labelling        change                      or relatively
                  spectrally similar groups in date 2    nature        and      trajectories                                 homogeneous
                  image into primary clusters in date    automation of the                                                   units
                  2 image, and finally detects and       change    analysis
                  identifies changes and outputs         process
                  results
5. Hybrid         Uses an overlay enhancement from       This method            Requires selection          LULC change      Selects suitable
change            a selected image to isolate changed    Excludes               of thresholds to            , vegetation     thresholds to
detection         pixels, then uses Supervised           unchanged              implement                   change           identify         the
                  classification. A binary change        pixels          from   classification;             and              change and non-
                  mask is constructed from the           classification to      somewhat complicated        monitoring       change areas and
                  classification results. This change    reduce                 to                          eelgrass         develops accurate
                  mask sieves out the                    classification         identify         change                      classifi’n output
                  changed themes from the LULC           errors                 trajectories
                  maps produced for each date

6. Artificial     The input used to train the neural     ANN         is    a    The nature of hidden        Mortality        The architecture
neural            network is the spectral data of the    nonparametric          layers     is     poorly    detection   in   used such as the
networks          period      of       change.       A   Supervised             known; a long training      Lake , land-     number of hidden
(ANN)             backpropagation algorithm is often     method and has         time is required. ANN       cover change,    layers,        and
                  used to train the multi-layer          the ability to         is often sensitive to the   forest change,   training samples
                  perceptron neural network model        estimate        the    amount of training          Urban hange
                                                         properties of data     data     used.     ANN
                                                         based      on   the    functions     are     not
                                                         training samples       common in         image
                                                                                processing software
Category IV. Advanced models
 Techniques             Characteristics                   Advantages             Disadvantages          Examples       Key factors
1. Li–Strahler The Li–Strahler canopy model         This method combines        This        method     Mapping        Develops the
reflectance    is used to estimate each conifer     the techniques of digital   requires a large       and            Stand crown
model          stand crown cover for two dates      image processing of         number of field        monitoring     cover images
               of     imageries      separately.    remotely sensed data        Measurement data.      conifer        and identifies
               Comparison of the stand crown        with traditional sampling   It is complex and      mortality      the      crown
               covers for two dates is              and field observation       not available in                      characteristics
               conducted to produce the             methods. It provides        commercial image                      of vegetation
               change detection results             statistical results and     processing                            types
                                                    maps       showing    the   software. It is only
                                                    geometric distribution of   suitable         for
                                                    changed patterns            vegetation change
2. Spectral     Uses spectral mixture analysis to   The      fractions   have   This method is         Land-cover     Identifies
mixture         derive       fraction    images.    biophysical meanings,       regarded as an         change,        suitable
model           Endmembers are selected from        representing the areal      advanced      image    seasonal       endmembers;
                training areas on the image or      proportion      of   each   processing             vegetation     defines suitable
                from spectra of materials           endmember within the        analysis and is        patterns and   thresholds for
                occurring in the study area or      pixel. The results are      somewhat               Vegetation     each       land-
                from a relevant spectral library.   stable, accurate and        complex                change         cover
                Changes are detected by             repeatable                                         using    TM    class based on
                comparing the ‘before’ and                                                             data           fractions
                ‘after’ fraction images of each
                end member. The quantitative
                changes can be measured by
                classifying images based on the
                endmember fractions
Category V. GIS based approach
Techniques         Characteristics             Advantages          Disadvantages         Examples      Key factors
3.           Incorporates image data and    Allows access of      Different     data     LULC        The accuracy of
Integrated   GIS data, such as the          ancillary data to     quality      from      and         different data
GIS and      overlay of GIS layers          aid interpretation    various    sources     urban       sources and their
remote       directly on image data;        and analysis and      often degrades the     sprawl      registration
Sensing      moves results of image         has the ability to    results of LULC                    accuracies
method       processing into GIS system     directly    update    change detection                   between the
             for                            land-use                                                 thematic images
             further analysis               information in GIS




4. GIS       Integrates past and current    This        method    Different GIS data     Urban       The accuracy of
approach     maps of land use with          allows                with       different   change      different data
             topographic and geological     incorporation of      geometric              And         sources and their
             data. The image overlaying     aerial                accuracy        and    landscape   registration
             and      binary     masking    photographic data     classification         change      accuracies
             techniques are useful in       of current and past   system degrades                    between the
             revealing quantitatively the   land-use data with    the quality of                     thematic images.
             change dynamics in each        other map data        results
             category
Category VI. Visual analysis
   Techniques             Characteristics                Advantages            Disadvantages       Examples    Key factors


1. Visual        One band (or VI) from date1      Human        experience   Cannot provide       Land-use      Analyst’s
interpretation   image as red, the same band      and knowledge are         detailed change      change,       skill and
                 (or VI) from date2 image as      useful during visual      information.         forest        familiarit
                 green, and the same band (or     interpretation. Two or    The        results   change ,      y      with
                 VI) from date3 image as blue     three dates of images     depend on the        monitoring    the study
                 if      available.    Visually   can be analysed at        analyst’s skill in   selectively   area
                 interprets      the     colour   one time. The analyst     image                logged
                 composite to identify the        can         incorporate   interpretation.      areas and
                 changed areas. An alternative    texture, shape, size      Time-                land cover
                 is to implement on-screen        and            patterns   consuming and        change
                 digitizing of changed areas      intovisual                difficulty in
                 using visual interpretation      interpretation       to   updating      the
                 based on overlaid images of      make a decision on        results
                 diff. dates                      the LULC change
Category VII. Other change detection
                 techniques
1. Measures of spatial dependence (Henebry 1993)
2. Knowledge-based vision system (Wang 1993)
3. Area production method (Hussin et al. 1994)
4. Combination of three indicators: vegetation indices, land surface
temperature, and spatial structure (Lambin and Strahler 1994b)
5. Change curves (Lawrence and Ripple 1999)
6. Generalized linear models (Morisette et al. 1999)
7. Curve-theorem-based approach (Yue et al. 2002)
8. Structure-based approach (Zhang et al. 2002)
9. Spatial statistics-based method (Read and Lam 2002)
Factors to consider when choosing a method
► Objective of the change detection?
▪ Monitor/identify specific changes
▪ More efficient mapping at T2
▪ Improved quality of mapping at T2
► What type of change information to extract?
▪ Spectral changes
▪ Land cover transitions
▪ Shape changes
▪ Changes in long temporal series
► What type of changes to be considered?
▪ Land use and land cover change
▪ Forest and vegetation change
▪ Wetland change
▪ Urban change
▪ Environmental change
Factors to consider…
► Expected amount of changes
► Available data at date 1 and date 2
      • Remote sensing data
      • Temporal, spatial and spectral characteristics.
      • Differences in characteristics btw. date 1 and date 2.
      • Classified maps
      • Ground truth
► Environmental considerations
      • Atmospheric conditions
      • Soil moisture conditions
      • Phenological states
► Accuracy requirements
Comparing the Different Techniques
   Two types of change detection either detect binary change/non-change, or
    the detailed “from-to” change between different classes.

   Different change detection techniques are often tested and compared
    based on an accuracy assessment or qualitative assessment.

   no single method is suitable for all cases.

   A combination of two change detection techniques can improve the
    change detection results (image differencing/PCA, NDVI/PCA,
    PCA/CVA).

   The most common change detection methods: image differencing, PCA,
    CVA, and post-classification comparison.
Global change analyses and image
                resolution
   For change detection at high or moderate spatial resolution: use Landsat
    TM, SPOT, or radar.

   For change detection at the continental or global scale, use coarse
    resolution data such as MODIS and AVHRR.

   AVHRR has daily availability at low cost; it is the best source of data for
    large area change detection.

   NDVI and land surface temperatures derived from MODIS or AVHRR
    thermal bands are especially useful in large area change detection.
Threshold Selection
   Many change detection algorithms require threshold selection to determine whether a
    pixel has changed.

   Thresholds can be adjusted manually until the resulting image is satisfactory, or they
    can be selected statistically using a suitable standard deviation from a class mean.
    Both are highly subjective methods.

   Other methods exist for improving the change detection results, such as using fuzzy
    set and fuzzy membership functions to replace the thresholds.

   However, threshold selection is simple and intuitive, so it is still the most
    extensively applied method for detecting binary change/no-change information.
Accuracy Assessment
   Accuracy assessments are important for understanding the change detection
    results and using these results in decision-making.

   However, they are difficult to do because reliable temporal field-based datasets are
    often problematic to collect.

   The error matrix is the most common method for accuracy assessment. To
    properly generate one, the following factors must be considered:
     1. ground truth data collection,
     2. classification scheme,
     3. sampling scheme,
     4. spatial autocorrelation, and
     5. sample size and sample unit.
Summary and Recommendations
   The binary change/no-change threshold techniques all have difficulties in
    distinguishing true changed areas from the detected change areas. Single-
    band image differencing and PCA are the recommended methods.

   Classification-based change detection methods can avoid such problems,
    but requires more effort to implement. Post-classification comparison is a
    suitable method when sufficient training data is available.

   When multi-source data is available, GIS techniques can be helpful.

   Advanced techniques such as LSMA, ANN, or a combination of change
    detection methods can produce higher quality change detection results.
References
•   MACLEOD, R. D., and CONGALTON, R. G.,A quantitative comparison of change detection
    algorithms for monitoring eelgrass from remotely sensed data. Photogrammetric Engineering
    and Remote Sensing, vol. 64, pp 207–216. 1998,
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•   LAMBIN, E. F., and STRAHLER, A. H., 1994a, Change-vector analysis in multitemporal
    space: a tool to detect and categorize land-cover change processes using high temporal
    resolution satellite data. Remote Sensing of Environment, 48, 231–244.
•   GREEN, K., KEMPKA, D., and LACKEY, L., 1994, Using remote sensing to detect
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    urban fringes. IEEE Transactions on Geoscience and Remote Sensing, 31, 136–145
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    analysis using remote sensing: a test for the spatially resolved area production model.
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•   LAWRENCE, R. L., and RIPPLE, W. J., 1999, Calculating change curves for
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    Environment, 67, 309–319
•   MORISETTE, J. T., KHORRAM, S., and MACE, T., 1999, Land-cover change
    detectionenhanced with generalized linear models. International Journal of Remote Sensing,
    20,2703–2721
•   YUE, T. X., CHEN, S. P., XU, B., LIU, Q. S., LI, H. G., LIU, G. H., and YE, Q. H., 2002, A
    curve-theorem based approach for change detection and its application to Yellow River
    Delta. International Journal of Remote Sensing, 23, 2283–2292.
•   ZHANG, Q., WANG, J., PENG, X., GONG, P., and SHI, P., 2002, Urban build-up land
    change detection with road density and spectral information from multitemporal Landsat
    TM data. International Journal of Remote Sensing, 23, 3057–3078
•   READ, J. M., and LAM, N. S.-N., 2002, Spatial methods for characterizing land cover
    and detecting land over changes for the tropics. International Journal of Remote Sensing, 23,
    2457–2474.
•   JENSEN, J. R., and TOLL, D. L., 1982, Detecting residential land use development at
    the urban fringe. Photogrammetric Engineering and Remote Sensing, 48, 629–643.
•   CHAVEZ, P. S. JR, and MACKINNON, D. J., 1994, Automatic detection of vegetation
    changes in the southwestern United States using remotely sensed images.
    Photogrammetric Engineering and Remote Sensing, 60, 571–583.
•   PILON, P. G., HOWARTH, P. J., BULLOCK, R. A., and ADENIYI, P. O., 1988, An
    enhanced classification approach to change detection in semi-arid environments.
    Photogram- metric Engineering and Remote Sensing, 54, 1709–1716.
References……….
•   FUNG, T., and LEDREW, E., 1987, The application of principal component analysis to
    change detection. Photogrammetric Engineering and Remote Sensing, 53, 1649–1658.
•   FUNG, T., 1990, An assessment of TM imagery for land-cover change detection.
    IEEE Transactions on Geoscience and Remote Sensing, 28, 681–684.
•   GONG, P., 1993, Change detection using principal component analysis and fuzzy set
    theory. Canadian Journal of Remote Sensing, 19, 22–29.
•   FOODY, G. M., 2001, Monitoring the magnitude of land-cover change around the southern
    limits of the Sahara. Photogrammetric Engineering and Remote Sensing, 67, 841–847
•   ABUELGASIM, A. A., ROSS, W. D., GOPAL, S., and WOODCOCK, C. E., 1999,
    Change detection using adaptive fuzzy neural networks: environmental damage
    assessment after the Gulf War. Remote Sensing of Environment, 70, 208–223.
•   DAI, X. L., and KHORRAM, S., 1999, Remotely sensed change detection based on
    artificial neural networks. Photogrammetric Engineering and Remote Sensing, 65, 1187–
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•   GOPAL, S., and WOODCOCK, C. E., 1996, Remote sensing of forest change using
    artificial neural networks. IEEE Transactions on Geoscience and Remote Sensing, 34, 398–
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•   GOPAL, S., and WOODCOCK, C. E., 1999, Artificial neural networks for detecting
    forest change. In Information Processing for Remote Sensing, edited by C. H. Chen
    (Singapore: World Scientific Publishing Co.), pp. 225–236.
References……….
•   WOODCOCK, C. E., MACOMBER, S. A., PAX-LENNEY, M., and COHEN, W. B.,
    2001 Monitoring large areas for forest change using Landsat: generalization across space, time
    and Landsat sensors. Remote Sensing of Environment, 78, 194–203.
•   LIU, X., and LATHROP, R. G. JR, 2002, Urban change detection based on an artificial neural
    network. International Journal of Remote Sensing, 23, 2513–2518.
•   ROGAN, J., FRANKLIN, J., and ROBERTS, D. A., 2002, A comparison of methods
    for monitoring multitemporal vegetation change using Thematic Mapper imagery.
    Remote Sensing of Environment, 80, 143–156.
•   KRESSLER, F., and STEINNOCHER, K., 1996, Change detection in urban areas using
    satellite data and spectral mixture analysis. International Archives of Photogrammetry and
    Remote Sensing, 31, 379–383
•   ADAMS, J. B., SABOL, D., KAPOS, V., FILHO, R. A., ROBERTS, D. A., SMITH, M.
    O., and GILLESPIE, A. R., 1995, Classification of multispectral images based on fractions
    of endmembers: application to land-cover change in the Brazilian Amazoˆ n. Remote
    Sensing of Environment, 52, 137–154.
•   PETIT, C. C., and LAMBIN, E. F., 2001, Integration of multi-source remote sensing data for
    land cover change detection. International Journal of Geographical Information
    Science, 15, 785–803.
•   LO, C. P., and SHIPMAN, R. L., 1990, A GIS approach to land-use change dynamics
    detection. Photogrammetric Engineering and Remote Sensing, 56, 1483–1491

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A review of change detection techniques

  • 1. SEMINAR ON A REVIEW OF CHANGE DETECTION TECHNIQUES INDIAN INSTITUTE OF TECHNOLOGY ROORKEE PRESENTED BY:- ABHISHEK BHATT RESEARCH SCHOLAR abhishekbhatt.iitr@gmail.com
  • 2. OUTLINE This seminar is organized into eight sections as follows: 1. Background and applications of change detection techniques 2. Considerations before implementing change detection 3. A review of seven categories of change detection techniques 4. Comparative analyses among the different techniques 5. A global change analyses 6. Threshold selection 7. Accuracy assessment 8. Summary and recommendations References
  • 3. Background • In general, change detection involves the application of multi- temporal datasets to quantitatively analyze the temporal effects • Change detection can be defined as the process of identifying differences in the state of an object or phenomenon by observing it at different times. This process is usually applied to Earth surface changes at two or more times. • understanding relationships and interactions to better manage and use resources • Change detection is useful in many applications such as land use changes, habitat fragmentation, rate of deforestation, coastal change, urban sprawl, and other cumulative changes
  • 4.
  • 5.
  • 6. Change detection ► Two main categories of land cover changes: ▪ Conversion of land cover from one category to a different category. ▪ Modification of the condition of the land cover type within the same category (thinning of trees, selective cutting, pasture to cultivation, etc.) source; Norsk Regnesentral website
  • 7. Applications of change detection techniques • land-use and land-cover (LULC) change • forest or vegetation change • forest mortality, defoliation and damage assessment • deforestation, regeneration and selective logging • wetland change • forest fire and fire-affected area detection • landscape change • urban change • environmental change, drought monitoring, flood monitoring, monitoring coastal marine environments, desertification, and detection of landslide areas • other applications such as crop monitoring, shifting cultivation monitoring, road segments, and change in glacier mass balance and facies.
  • 8. Considerations before implementing change detection • Before implementing change detection analysis, the following conditions must be satisfied: i. precise registration of multi-temporal images; ii. precise radiometric and atmospheric calibration or normalization between multi-temporal images; iii. selection of the same spatial and spectral resolution images if possible
  • 9. Good change detection research should provide the following information: i. area change and change rate ii. spatial distribution of changed types iii. Change trajectories of land-cover types iv. accuracy assessment of change detection results.
  • 10. A review of change detection techniques • Because digital change detection is affected by spatial, spectral, radiometric and temporal constraints. • Many change detection techniques are possible to use, the selection of a suitable method or algorithm for a given research project is important, but not easy.
  • 11. The seven change detection technique categories 1. Algebra Based Approach 4. Advanced Models • image differencing • Li-Strahler Reflectance Model • image regression • Spectral Mixture Model • image ratioing • Biophysical Parameter Method • vegetation index differencing 5. GIS • change vector analysis • Integrated GIS and RS Method • GIS Approach 2. Transformation 6. visual Analysis • PCA • Visual Interpretation • Tasseled Cap (KT) 7. other Change Detection Techniques • Gramm-Schmidt (GS) • Measures of spatial dependence • Chi-Square • Knowledge-based vision system • Area production method 3. Classification Based • Combination of three indicators: vegetation • Post-Classification Comparison indices, land surface temperature, and • Spectral-Temporal Combined Analysis spatial structure • EM Transformation • Change curves • Unsupervised Change Detection • Generalized linear models • Hybrid Change Detection • Curve-theorem-based approach • Artificial Neural Networks (ANN) • Structure-based approach • Spatial statistics-based method
  • 12. Category I Algebra Based Approach • The algebra category includes – image differencing, – image regression – Image ratioing – vegetation index differencing – change vector analysis (CVA)
  • 13. Algebra based Approach…… • These algorithms have a common characteristic, i.e. selecting thresholds to determine the changed areas. These methods (excluding CVA) are relatively simple, straightforward, easy to implement and interpret, but these cannot provide complete matrices of change information. • In this category, two aspects are critical for the change detection results: – selecting suitable image bands – selecting suitable thresholds
  • 14. Image Differencing • Concept – Date 1 - Date 2 – No-change = 0 – Positive and negative values interpretable – Pick a threshold for change
  • 15. Image Differencing 8 10 8 11 240 11 10 22 Image Date 1 205 210 205 54 3 1 1 1 220 98 88 46 143 2 2 0 107 110 0 -168 5 9 7 10 117 0 -166 -164 97 9 8 22 98 100 205 222 Image Date 2 Difference Image = 103 98 254 210 Image 1 - Image 2
  • 16. Image Differencing • Image differencing: Pros – Simple (some say it’s the most commonly used method) – Easy to interpret – Robust • Cons: – Difference value is absolute, so same value may have different meaning – Requires atmospheric calibration
  • 17. Image regression ► Relationship between pixel values of two dates is established by using a regression function. ► The dimension of the residuals is an indicator of where change occurred. ► Advantage ▪ Reduces impact of atmospheric, sensor and environmental differences. ► Drawback • Requires development of accurate regression functions. • Does not provide change matrix.
  • 19. Image Ratioing • Concept – Date 1 / Date 2 – No-change = 1 – Values less than and greater than 1 are interpretable – Pick a threshold for change • Pros – Simple – May mitigate problems with viewing conditions, esp. sun angle • Cons – Scales change according to a single date, so same change on the ground may have different score depending on direction of change; I.e. 50/100 = .5, 100/50 = 2.0
  • 20. Change Detection source: CCRS website, CANADA Image Difference (TM99 – TM88) Image Ratio (TM99 / TM88)
  • 21. Change vector analysis • In n-dimensional spectral space, determine length and direction of vector between Date 1 and Date2 Date 1 Band 4 • No-change = 0 length Date 2 • Change direction may be interpretable • Pick a threshold for change Band 3
  • 22. Change vector analysis ► Determines in n-dimensional spectral space, the length and direction of the vector between Date 1 and Date 2. ► Produces an intensity image and a direction image of change. The direction image can be used to classify change. ► Typically used when all changes need to be investigated. ► Advantage ▪ Works on multispectral data. • Allows designation of the type of change occurring ► Drawback source; Norsk Regnesentral website ▪ Shares some of the drawbacks of algebra based techniques but less severe
  • 24. Category I. Algebra Based Approach Techniques Characteristics Advantages Disadvantages Examples Key factors 1. Image Subtracts the first date Simple and Cannot provide Forest Identifies differencing image from a second- Straight forward, a detailed change defoliation, suitable image date image, pixel by easy to interpret matrix, requires land-cover bands and pixel the results selection of Change and thresholds thresholds irrigated crops monitoring 2. Image Establishes relationships Reduces impacts Requires to develop Tropical forest Develops the regression between bitemporal of the atmospheric, accurate regression change and regression images, then estimates sensor and functions for the forest function; pixel values of the environmental selected bands conversion identifies second-date image by use differences between before suitable bands of a regression function, two-date images implementing and thresholds subtracts the regressed change detection image from the first-date image 3. Image Calculates the ratio of Reduces impactsof Non-normal Land-use Identifies the ratioing registered images of two Sun angle, shadow distribution of the mapping image bands dates, band by band and topography result is often and thresholds criticized
  • 25. Techniques Characteristics Advantages Disadvantages Examples Key factors 4. Vegetation Produces vegetation index separately, Emphasizes random noise or Vegetation Identifies suitable Index then subtracts the differences in the coherence noise change vegetation index differencing second-date vegetation index spectral response and forest and thresholds from the first-date vegetation index of different features canopy and reduces impacts change of topographic effects Enhances and illumination. 5. Change Generates two outputs: (1) the Ability to process Difficult to landscape Defines vector analysis spectral change vector describes the any number of identify land variables thresholds (CVA) direction and magnitude of change spectral bands cover change land-cover and identifies from the first to the second date; and desired and to trajectories changes change (2) the total change magnitude per produce detailed disaster trajectories pixel is computed by determining the change detection assessment Euclidean distance between end information and conifer points through n-dimensional change forest change space
  • 27. Transformations ► Principal Component Analysis ► Alt1: Perform PCA on data from both dates and analyse the component images. ► Alt2: Perform PCA separately on each image and subtract the second-date PC image from that of the first date. ► Advantage ▪ Reduces data redundancy. ► Drawback ▪ Results are scene dependent and can be difficult to interpret. ▪ Does not provide change matrix.
  • 28. Kauth Thomas Transformation • Described the temporal spectral patterns derived from Landsat MSS imagery for crops. As crops grow from seed to maturity, there is a net increase in NIR and decrease in Red Reflectance. This effect varies based on soil Color • Brightness Greenness Wetness • The Brightness, Greenness, Wetness transform was first developed for use with the Landsat MSS system and called the “Tasseled Cap” transformation. • The transform is based on a set of constants applied to the image in the form of a linear algebraic formula. • Brightness – primary axis calculated as the weighted sum of reflectances of all spectral bands. • Greenness – perpendicular to the axis of the Brightness component that passes through the point of maturity of all plants • Yellow Stuff – perpendicular to both Greenness and Brightness axis representing senesced vegetation.
  • 29. Kauth Thomas Transformation Typically the first few components contain most of the information in the data so that four channels of LANDSAT MSS data or the six channels of the http://www.sjsu.edu/faculty/watkins/tassel.htm Thematic Mapper data may be reduced to just three principal components. The components higher than three are usually treated as being information less. Source; www.sjsu.edu/faculty/watkins/tassel.htm
  • 30. Category II. Transformation Techniques Characteristics Advantages Disadvantages Examples Key factors 1. Principal Assumes that multitemporal data Reduces data PCA is scene dependent, Land-cover Analyst’s skill in component are highly correlated and change Redundancy thus the change detection change urban identifying which analysis (PCA) information can be highlighted in between bands results between different expansion component best the new components. Two ways to and emphasizes dates are often difficult ,tropical forest represents the apply PCA for change detection different to interpret and label. It conversion , change and are: (1)put two or more dates of information in cannot provide a forest selecting images into a single file, then the derived complete matrix of mortality and thresholds perform PCA and analyse the components change class information forest minor component images for and requires determining defoliation change information; and thresholds to identify the (2) perform PCA separately, then changed areas subtract the second-date PC image from the corresponding PC image of the first date 2. Tasselled cap The principle of this method is Reduces data Difficult to interpret and Monitoring Analyst’s skill is (KT) similar to PCA. The only difference redundancy label change forest needed in from PCA is that PCA depends on between bands information, cannot mortality , identifying the image scene, andKT and emphasizes provide a complete monitoring which transformation is independent of different change matrix; requires green biomass component best the scene. The change detection is information in determining thresholds and represents the implemented based on the three the derived to identify the changed land-use change and components: brightness, greenness components. areas. Accurate change thresholds and wetness KT is scene atmospheric calibration independent. is required
  • 31. Techniques Characteristics Advantages Disadvantages Examples Key factors 3. The GS method The association It is difficult to extract Monitoring Initial Gramm– orthogonalizes spectral of transformed more than one single forest identification of Schmidt vectors taken directly from components component related to a mortality the stable (GS) bi-temporal images, as does with scene given type of subspace of the the original KT method, characteristics change. The GS multi-date data is produces three stable allows the process relies on required components corresponding extraction of selection of spectral to multitemporal analogues information that vectors from multi-date of KT brightness, greenness would not be image typical of the and wetness, and a change accessible using type of change being component other examined techniques 4. Chi- Y=(X-M)T ∑-1*(X-M) Multiple bands The assumption that a Urban Y is distributed square Y:digital value of change Are value of Y~0 environmen as a Chi-square image simultaneously represents a pixel of no tal random variable X:vector of the difference considered to change is not true when change with p degrees of the six digital values produce a a large portion of the of freedom ( p is between the two dates single change image is the number of M:vector of the mean image. changed. Also the bands) residual of each band change related to T:transverse of the matrix specific spectral ∑-1= inverse covariance direction not identified matrix
  • 33. Post-classification • Post-classification (delta classification) – Classify Date 1 and Date 2 separately, compare class values on pixel by pixel basis between dates • Post-classification: Pros – Avoids need for strict radiometric calibration – Favors classification scheme of user – Designates type of change occurring • Cons – Error is multiplicative from two parent maps – Changes within classes may be interesting
  • 34. Composite Analysis • Composite Analysis – Stack Date 1 and Date 2 and run unsupervised classification on the whole stack • Composite Analysis: Pros – May extract maximum change variation – Includes reference for change, so change is anchored at starting value, unlike change vector analysis and image differencing • Cons – May be extremely difficult to interpret classes
  • 35. Unsupervised techniques ► Objective ▪ Produce a change detection map in which changed areas are separated from unchanged ones. ► The changes sought are assumed to result in larger changes in radiance values than other factors. ► Comparison is performed directly on the spectral data. ► This results in a difference image which is analysed to separate insignificant from significant changes. source; Norsk Regnesentral website
  • 36. Supervised techniques  Objective Generate a change detection map where changed areas are identified and the land-cover transition type can be identified.  The changes are detected and labelled using supervised classification approaches.  Main techniques: • Post-classification comparison • Multidate direct classification source; Norsk Regnesentral website
  • 37. Post classification comparison ► Standard supervised classifiers are used to classify the two images independently. ► Changes are detected by comparing the two classified images. ► Advantage ▪ Common and intuitive. ▪ Provides change matrix. ► Drawback ▪ Critically depends on the accuracy of the classification maps. Accuracy close to the product of the two results. ▪ Does not exploit the dependence between the information from the two points in time. source; Norsk Regnesentral website
  • 39. Multidate direct classification ► Two dates are combined into one multitemporal image and classified. ► Performs joint classification of the two images by using a stacked feature vector. ► Change detection is performed by considering each transition as a class, and training the classifier to recognize all classes and all transitions. ► Advantage ▪ Exploits the multitemporal information. ▪ Error rate not cumulative. ▪ Provides change matrix. ► Drawback ▪ Ground truth required also for transitions. source; Norsk Regnesentral website
  • 40. Supervised vs. Unsupervised Supervised Unsupervised Level of change Change detection at Change detection at data decision level. level. detection Change Provides explicit labeling Separates ‘change’ from of change and class ‘no change’. information transitions Change Obtained directly from Obtained through the classified images. interpretation of the computation difference image. Ground truth Requires ground truth. Requires no ground truth. Spectral Multispectral. Most methods work on one spectral band. information. Data requirements Not sensitive to Sensitive to atmospheric atmospheric conditions conditions and sensor and sensor differences. differences.
  • 41. Category III. Classification based approach Techniques Characteristics Advantages Disadvantages Examples Key factors 1. Post Separately classifies multi- Minimizes Requires a great LULC change, Selects sufficient classification temporal images into thematic impacts of amount of time and wetland training sample comparison maps, then implements comparison atmospheric, expertise to create change data for of the classified images, pixel by sensor and classification and urban classification pixel environmental products. The final expansion differences accuracy depends on between the quality of the multitemporal classified image of images; provides a each date complete matrix of change information 2. Spectral– Puts multi-temporal data into a Simple and Difficult to identify Changes in Labels the temporal single file, then classifies the timesaving and label the change coastal zone change classes combined combined dataset and identifies and in classification classes; cannot environments analysis labels the changes provide a complete and forest matrix of change change information 3. EM The EM detection is a This method was Requires estimating Land-cover Estimates the detection classification-based method using reported to the a priori joint change a priori joint an expectation maximization (EM) provide higher class probability. class probability algorithm to estimate the a priori change detection joint class probabilities at two accuracy than times. These probabilities are other change estimated directly from the images detection methods under analysis
  • 42. Techniques Characteristics Advantages Disadvantages Examples Key factors 4. Unsupervised Selects spectrally similar groups of This method Difficulty in Forest hange Identifies the change pixels and clusters date 1 image makes use of the identifying and spectrally similar detection into primary clusters, then labels unsupervised labelling change or relatively spectrally similar groups in date 2 nature and trajectories homogeneous image into primary clusters in date automation of the units 2 image, and finally detects and change analysis identifies changes and outputs process results 5. Hybrid Uses an overlay enhancement from This method Requires selection LULC change Selects suitable change a selected image to isolate changed Excludes of thresholds to , vegetation thresholds to detection pixels, then uses Supervised unchanged implement change identify the classification. A binary change pixels from classification; and change and non- mask is constructed from the classification to somewhat complicated monitoring change areas and classification results. This change reduce to eelgrass develops accurate mask sieves out the classification identify change classifi’n output changed themes from the LULC errors trajectories maps produced for each date 6. Artificial The input used to train the neural ANN is a The nature of hidden Mortality The architecture neural network is the spectral data of the nonparametric layers is poorly detection in used such as the networks period of change. A Supervised known; a long training Lake , land- number of hidden (ANN) backpropagation algorithm is often method and has time is required. ANN cover change, layers, and used to train the multi-layer the ability to is often sensitive to the forest change, training samples perceptron neural network model estimate the amount of training Urban hange properties of data data used. ANN based on the functions are not training samples common in image processing software
  • 43. Category IV. Advanced models Techniques Characteristics Advantages Disadvantages Examples Key factors 1. Li–Strahler The Li–Strahler canopy model This method combines This method Mapping Develops the reflectance is used to estimate each conifer the techniques of digital requires a large and Stand crown model stand crown cover for two dates image processing of number of field monitoring cover images of imageries separately. remotely sensed data Measurement data. conifer and identifies Comparison of the stand crown with traditional sampling It is complex and mortality the crown covers for two dates is and field observation not available in characteristics conducted to produce the methods. It provides commercial image of vegetation change detection results statistical results and processing types maps showing the software. It is only geometric distribution of suitable for changed patterns vegetation change 2. Spectral Uses spectral mixture analysis to The fractions have This method is Land-cover Identifies mixture derive fraction images. biophysical meanings, regarded as an change, suitable model Endmembers are selected from representing the areal advanced image seasonal endmembers; training areas on the image or proportion of each processing vegetation defines suitable from spectra of materials endmember within the analysis and is patterns and thresholds for occurring in the study area or pixel. The results are somewhat Vegetation each land- from a relevant spectral library. stable, accurate and complex change cover Changes are detected by repeatable using TM class based on comparing the ‘before’ and data fractions ‘after’ fraction images of each end member. The quantitative changes can be measured by classifying images based on the endmember fractions
  • 44. Category V. GIS based approach Techniques Characteristics Advantages Disadvantages Examples Key factors 3. Incorporates image data and Allows access of Different data LULC The accuracy of Integrated GIS data, such as the ancillary data to quality from and different data GIS and overlay of GIS layers aid interpretation various sources urban sources and their remote directly on image data; and analysis and often degrades the sprawl registration Sensing moves results of image has the ability to results of LULC accuracies method processing into GIS system directly update change detection between the for land-use thematic images further analysis information in GIS 4. GIS Integrates past and current This method Different GIS data Urban The accuracy of approach maps of land use with allows with different change different data topographic and geological incorporation of geometric And sources and their data. The image overlaying aerial accuracy and landscape registration and binary masking photographic data classification change accuracies techniques are useful in of current and past system degrades between the revealing quantitatively the land-use data with the quality of thematic images. change dynamics in each other map data results category
  • 45. Category VI. Visual analysis Techniques Characteristics Advantages Disadvantages Examples Key factors 1. Visual One band (or VI) from date1 Human experience Cannot provide Land-use Analyst’s interpretation image as red, the same band and knowledge are detailed change change, skill and (or VI) from date2 image as useful during visual information. forest familiarit green, and the same band (or interpretation. Two or The results change , y with VI) from date3 image as blue three dates of images depend on the monitoring the study if available. Visually can be analysed at analyst’s skill in selectively area interprets the colour one time. The analyst image logged composite to identify the can incorporate interpretation. areas and changed areas. An alternative texture, shape, size Time- land cover is to implement on-screen and patterns consuming and change digitizing of changed areas intovisual difficulty in using visual interpretation interpretation to updating the based on overlaid images of make a decision on results diff. dates the LULC change
  • 46. Category VII. Other change detection techniques 1. Measures of spatial dependence (Henebry 1993) 2. Knowledge-based vision system (Wang 1993) 3. Area production method (Hussin et al. 1994) 4. Combination of three indicators: vegetation indices, land surface temperature, and spatial structure (Lambin and Strahler 1994b) 5. Change curves (Lawrence and Ripple 1999) 6. Generalized linear models (Morisette et al. 1999) 7. Curve-theorem-based approach (Yue et al. 2002) 8. Structure-based approach (Zhang et al. 2002) 9. Spatial statistics-based method (Read and Lam 2002)
  • 47. Factors to consider when choosing a method ► Objective of the change detection? ▪ Monitor/identify specific changes ▪ More efficient mapping at T2 ▪ Improved quality of mapping at T2 ► What type of change information to extract? ▪ Spectral changes ▪ Land cover transitions ▪ Shape changes ▪ Changes in long temporal series ► What type of changes to be considered? ▪ Land use and land cover change ▪ Forest and vegetation change ▪ Wetland change ▪ Urban change ▪ Environmental change
  • 48. Factors to consider… ► Expected amount of changes ► Available data at date 1 and date 2 • Remote sensing data • Temporal, spatial and spectral characteristics. • Differences in characteristics btw. date 1 and date 2. • Classified maps • Ground truth ► Environmental considerations • Atmospheric conditions • Soil moisture conditions • Phenological states ► Accuracy requirements
  • 49. Comparing the Different Techniques  Two types of change detection either detect binary change/non-change, or the detailed “from-to” change between different classes.  Different change detection techniques are often tested and compared based on an accuracy assessment or qualitative assessment.  no single method is suitable for all cases.  A combination of two change detection techniques can improve the change detection results (image differencing/PCA, NDVI/PCA, PCA/CVA).  The most common change detection methods: image differencing, PCA, CVA, and post-classification comparison.
  • 50. Global change analyses and image resolution  For change detection at high or moderate spatial resolution: use Landsat TM, SPOT, or radar.  For change detection at the continental or global scale, use coarse resolution data such as MODIS and AVHRR.  AVHRR has daily availability at low cost; it is the best source of data for large area change detection.  NDVI and land surface temperatures derived from MODIS or AVHRR thermal bands are especially useful in large area change detection.
  • 51. Threshold Selection  Many change detection algorithms require threshold selection to determine whether a pixel has changed.  Thresholds can be adjusted manually until the resulting image is satisfactory, or they can be selected statistically using a suitable standard deviation from a class mean. Both are highly subjective methods.  Other methods exist for improving the change detection results, such as using fuzzy set and fuzzy membership functions to replace the thresholds.  However, threshold selection is simple and intuitive, so it is still the most extensively applied method for detecting binary change/no-change information.
  • 52. Accuracy Assessment  Accuracy assessments are important for understanding the change detection results and using these results in decision-making.  However, they are difficult to do because reliable temporal field-based datasets are often problematic to collect.  The error matrix is the most common method for accuracy assessment. To properly generate one, the following factors must be considered: 1. ground truth data collection, 2. classification scheme, 3. sampling scheme, 4. spatial autocorrelation, and 5. sample size and sample unit.
  • 53. Summary and Recommendations  The binary change/no-change threshold techniques all have difficulties in distinguishing true changed areas from the detected change areas. Single- band image differencing and PCA are the recommended methods.  Classification-based change detection methods can avoid such problems, but requires more effort to implement. Post-classification comparison is a suitable method when sufficient training data is available.  When multi-source data is available, GIS techniques can be helpful.  Advanced techniques such as LSMA, ANN, or a combination of change detection methods can produce higher quality change detection results.
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