Global Digital Elevation Data Merged

A Merged Global Digital Jet Propulsion Laboratory
Topographic Data Set California Institute of Technology

IGARSS 2011

Dr. Michael Kobrick
Dr. Robert Crippen
Dr. Thomas Farr
Jet Propulsion Laboratory

July 27, 2011


Objective:
Generate best quality global digital elevation data set
at highest possible resolution by combining best
existing data

Supported by NASA *MEaSUREs program (M. Maiden)
for enhancement of SRTM data set

* Making Earth Science Data Records
for Use in Research Environments


SRTM: Shuttle Radar Topography Mission

Mission
• Partnership between NASA, NGA, DLR and ASI
• C-band single-pass interferometry
• X-band interferometer by German and Italian space agencies
• Mission duration: Feb., 11 – Feb. 22, 2000 (11 days, 5 hours)

Objective
• Produce DEM of 80% of Earth surface
! 30 meter posting
! Complete coverage ± 60° latitude
! WGS84 EGM96 geoid datum
! 16 m vert., 20 m horiz. accuracy at 90% level


Topographic Coverage

• DTED1 coverage
• 90 meter samples
• ~ 60% coverage
• Poor continuity

• SRTM coverage
• 30 meter samples
• ~ 80% coverage
• Excellent continuity


Accuracy results
Accuracy tests
• NGA veriﬁcation with DTED, other data sources
• USGS comparisons to National Elevations Data sets, GPS arrays
• Comparison to static and dynamic GPS elevations
• Scores of open literature reports comparing to local data sets

All show data exceed 16 m vertical accuracy specs by factor of ~3

Elevations clearly show patterns of tropical forest
clear-cutting with height differences of 4-5 meters


Data distributed by Land Processes DAAC at EDC

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Data Restrictions

Re original NASA/NGA Memorandum of Understanding, DEMs better than 3
arcsec outside U.S territory may not be distributed outside NASA

3 arcsec DEMs and any derived, ancillary or associated products may be
distributed at any resolution, as long as they cannot be reverse-engineered to
better than 3 arcsec DEMs


NASA MEaSUREs Program

Under MEaSUREs NASA has funded effort to create SRTM-derived,
enhanced, merged global data set

! • Void ﬁlled DEMs

! • Enhanced DEM control using ICESat proﬁles

! • Individual and mosaiced radar image data

! • Raw radar and ancillary data set



NASA has funded effort to create SRTM-derived, enhanced, merged
global data set



! • Individual and mosaiced radar voids - cell average 0.52%, median 0.0023%
SRTM image data



Many SRTM DEMs have voids

• Voids (areas with no data) caused
by high slopes or extremely low SNR

• Can be ﬁlled with supplementary DEMs

SRTM voids - cell average 0.52%, median 0.0023%

Original SRTM Voids ﬁlled with ASTER DEM


Potential Void Fill Sources
NED
!

• National Elevation Data (NED), 30m
- Excellent quality
- Available only for U.S., Mexico

• Canadian Digital Elevation Data (CDED), ~ 20m
- Available for virtually all of Canada CDED

• Other sources - various resolutions
- SRTM / X-SAR
- TanDEM-X
- SPOT
- Australian GEODATA Topo
X-SAR
• ASTER GDEM
- Excellent coverage
- Matches SRTM geoid, format, sampling
- Some voids and artifacts


GDEM
With NASA support Silcast Corp. processed several million ASTER
stereo pairs to produce a global DEM

GDEM1 released in mid-2009
- Matches SRTM in posting, datum, formatting
- Coverage exceeds SRTM
- Resolution ~ 120m, versus 50-60m for SRTM
- Numerous artifacts due to cloud mask, stacking method

GDEM2 to be released in mid-2011
- Resolution improved by using smaller convolution kernel
- Many artifacts remain


GDEM coverage

SRTM coverage


GDEM2 - SRTM

Global Land
Cover

EGM96


Comparison
SRTM ASTER GDEM
GDEM1


Comparison
SRTM GDEM1


SRTM w/ void GDEM1


SRTM GDEM1


SRTM ASTER GDEM1


GDEM includes ancillary NUM ﬁle showing number
of pairs contributing to each sample

SRTM GDEM1 GDEM Num

Capetown area, South Africa


GDEM2 Improvement
GDEM1: Original version delivered mid-2009
- Suffered from low resolution (~120 m), cloud and data stacking induced artifacts

GDEM2: Beta version being evaluated now
- Improved resolution with smaller convolution kernel, many artifacts remain
- Included quality ﬁles allow identiﬁcation of most(?) artifacts

SRTM GDEM1 GDEM2 GDEM2 N>2


NUM = 3

• Voiding samples with NUM < 3 seems to
eliminate most artifacts

• Deletes ~ 7% of GDEM samples

• GDEM2 and SRTM voids only loosely
correlated

Distribution of NUMs in GDEM2


Voids

SRTM GDEM2 N>2

SRTM/GDEM2 Combo



global data set






ICESat Data
• Comparisons of ICESat proﬁles w/ SRTM data have revealed possible bias at
latitude extremes
• Other published work suggests possible SRTM bias in high elevation
Malaspina
glaciated terrain (this deserves more attention)
• SRTM ocean residuals (should center on zero) show curious latitude
dependence
• Residuals are well within accuracy specs, but systematic nature is obvious
• Important to study of glacier changes with implications for climate change

SRTM ocean residuals from unedited data
Residual latitude dependence


ICESat Data
• M. Simard working with Carabajal, Sauber et al at GSFC to develop database of ICESat
topographic proﬁles for additional ground truth

• Requires careful modeling of laser echoes, differentiation of vegetation, bare Earth
returns

• Will be applied as additional ground control for void-ﬁlled DEMs

• Processing using new control data to begin mid-2011


‘Bonus Data’

• Rasterized version of SRTM Water Body Data (signiﬁcant improvement to
World Vector Shoreline) - complete, ~15000 ESDRs to be shipped Aug., 2011


Void-filling Summary

• Plan as proposed: Void fill SRTM on best effort basis (~20% of tiles)

• GDEM1 available: Void fill much of SRTM with manual artifact detection

• GDEM2 available: Void fill most of SRTM with semi-automated artifact
detection

• Will allow data generation greatly exceeding original MEaSUREs plan

and

• Expanded plan...


Expanded Plan

• Original NASA-NGA MOU has expired, creating possibility of distributable full-
resolution data set

• At NASA request generated set of options for SRTM data “reprocessing”

• A task plan for an expanded effort is now under review at NASA Hq

• Plan will involve additional resources and an additional year beyond end of
MEaSUREs program in 2012

• Resulting data set to be called NASADEM


Elements of NASADEM
• First step is interferometric reprocessing of SRTM raw data with improved algorithms, faster
computers
- Better handling of Beam Adaptive Tracker and other ancillary data
- Improved phase unwapping algorithms
- Leverage NASA ʻCloudʼ computing resources

• ICESat database will provide vastly improved ground control

• GDEM2 quality incorporating NUM files is sufficient to merge (not just void-fill) with SRTM,
most voids will disappear

• Remaining voids will be filled with new ASTER acquisitions or other data sets
- EDC has agreed to provide additional ASTER DEMs as required from archive or new
acquisitions
- R. Crippen has developed technique for detecting and eliminating artifacts in individual
ASTER DEMs

• SRTM water mask will allow incorporation of NGA coastline and other water body information


Raw radar echoes NASADEM ASTER GDEM2

Artifact removal

SAR processing (updated)
Current SRTM Modified GDEM2
Global DEM

Updated strip DEMs
Void-fill
SRTM
Metric Data

Continental Adjustment (mostly) void- Remaining
IceSAT filled SRTM (ver3) void map
Data Set

Acquire new
ʻNewʼ SRTM Global DEM NED, CDED, et al LPDAAC ASTER DEMs

New ASTER DEMs
SRTM Water
Body Data

Modified DEMs Artifact removal

Merge corrected DEMs

Key:
Pink: New data set or process NASADEM + associated products:
Yellow: Existing data set or process
Slope, Aspect, Curvature, Images,
Process
Errors, Correlation


Elements of NASADEM

• Data will be distributed and freely available at full 30
meter posting!

• New data types included
- Correlation maps
- Error metrics
- Terrain corrected images, slopes, et al

• Processing will be continent-by-continent, with release
throughout latter half of 2013



50.02a
global data set 151.07d






Radar Image Task Status

Basic Image Set
• Includes image, incidence angle map for each subswath crossing each cell
• Subswath inclusion allows differentiating polarizations
• Numerous ﬁles/cell, total data set ~ 700 gigabytes (compressed!)
• Distributable at 1 arcsec
• Complete: > 365,000 ESDRs en route to Land Processes DAAC

Data take number (may be from 2 to ~20)
34.02a 50.02a 135.07d 151.07d

Images
Individual data take (per subswath)

stacks for each cell
incidence
angles


SRTM Image Data Example Application

Image Incidence angle

User-deﬁned
region of interest

Scatter plot generates backscatter curve


Central Canada

Single-swath mosaic

Average of all swaths


GILF KEBIR
TM SRTM


SRTM Radar Image of Lake Baikal

Global Digital Elevation Data Merged

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