2. Data Models GIS is an abstraction of reality. A perfect copy of reality cannot be recreated in the computer. We create models – sets of constructs for describing & representing select aspects of the real world in a computer. Models are composed of a mix of raster, vector, and attribute data. Model is tailored to a specific function.
3. node B node C Polygon I Polygon III node A Polygon II node E node F node D Polygon V Polygon IV node G Coding Vector Data Vector Mode Model of Reality Reality
4. node B node C Polygon I Polygon III node A Polygon II node E node F node D Polygon V Polygon IV node G
5. node B node C Polygon I Polygon III node A Polygon II node E node F node D A topologic vector model records the points and linesshared between polygons as unique items, thus every oneof the points and lines are recorded in the data only once.
6. node B node C node A Polygon II node E node D Polygon 2 is on the right sideof the line ABCED.
7. Polygon I Polygon III node A Polygon II node E node F node D Polygon V Polygon IV Polygon 2 knows it’s adjacentto Polygons 1, 3, & 4. It shares a line segment with each. Polygon 2 knows it touchesPolygon 5. It shares node E with Polygon 5.
8. What is Topology? Shared Geometries, Adjacency and Overlap Where points, lines, and polygons share individual vertices. Move a point and it moves a vertex in a line/polygon, and vice versa. Two polygons that share vertices are considered adjacent. Overlapping (or non-overlapping) features can be located, and then marked as errors.
9. node B node C Polygon I Polygon III node A Polygon II node E node F node D Polygon V Polygon IV node G Strict Topology Features are composed from a common set of points and lines. Altering the vertices of one polygon affects polygons that share those vertices. Harder to introduce gaps or slivers.
10. Topology Can you think of a reason why topology would be important to model? Where in the real world is this concept important? Let’s take a look at some examples.
12. Parcel Overlap Example The boundaries of two properties should never overlap, and there should never be a gap between them, unless intentional. Clear error in parcel boundaries.
13. Policy-based Topology Rules In the NJ State Plan, CESs and the Environmentally Sensitive Planning Area both represent areas of environmental importance. Thus, CESs should never be placed on top of the ES Planning Area. In our utility network, poles hold up the transmission lines. The transmission line features must always share a vertex with the utility pole point features.
14. GIS is extensible With modern GIS, a polygon is not just a polygon. Software can be adapted to fit your model of reality. The software can be easily extended to create new data types and perform new analyses. GIS can be adapted to store, model, and display data about any observable phenomenon on the Earth.
15. Objects GIS Features as Objects is a recent method of representing aspects of the real-world in GIS Example of the shift from specialty data to DBMS that are spatially-aware Non-planar, temporally shifting, topologically linked, rule-based actions Still important to check for topology to ensure as a quality control step
16. Vector Geometry as Objects Parcels Planar geometries with attribute information Parcels as objects in a Cadastral “carpet” Objects with topology rules (“don’toverlap, unless”) Members of “regional” features (zoning, municipality) Composed of surveyed parts (COGO, benchmarks) Keys that link to attribute tables (owner(s), assessments, plans, etc)
17. Attributes as Objects Not only can multiple sets of geospatial features interact with rules, the attributes can be linked with one another, with their own set of rules and actions Ownership record linked to GIS parcel Search on multiple owners, records Removal of parcel warns about “orphan” owner Functions that can be performed by GIS analyst can be embedded in the actual database
18. Explore Models Let’s take a look at several GIS data models. Take note of the storage method: Raster Vector (and vector type: point, line, polygon, etc…) Also take note of the model family: Topological Model Object Model Both
19. Elevation using LIDAR LIDAR data is 3D elevation data recorded from an airplane. Stored as “mass points” – even a small area is composed of thousands of point features. No real need for attributes, simply XYZ points. Points can be joined together to create a surface model of a landscape.
20. Elevation DEMs Digital Elevation Models, or DEMs, often refer to a raster representation of elevation. Each cell in the raster grid contains a value that is the height of the cell above a fixed point (i.e. sea level).
21. Elevation using TINs Triangulated Irregular Networks, or TINs are vector models that represent elevation. The study area is composed of individual triangles, composed of a network of shared nodes and edges The surfaces of the triangles attempt to represent the surface, so in areas of gradual elevation change, there are fewer triangles.
24. Networks Analysis can be performed across a network, represented by a feature dataset of points and lines. Road network or water, sewer, utility, rail, etc… Optimal route – shortest, lowest cost, avoiding left turns, follow height and weight restrictions, time of day restrictions, include real-time traffic… Multi-modal – walk/bike to bus stop, bus to train, walk from train to final destination.
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27. Models Diagrammed GIS models can be depicted in a schematic form, similar to a flow chart. Shows the interconnected nature of the classes that make up the overall model. Some models can be constructed within ArcGIS using ModelBuilder.
30. Creating GIS Models Abstractions of reality naturally have shortcomings. Models tailored to a specific task can be used to explore phenomenon or predict effects. Developing a data model to solve a problem is how GIS has become a decision-making platform. Consider how you could study an abstract set of data using GIS to solve real-world issues.
Notas del editor
Simple routing, one vehicle, multiple stops
Service area routing – Maintenance calls are routed to the closest facility
