Lecture 3 - Data Models
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  • We know that the world is infinitely complex and that computers have limitations.The job we have is to represent the real world within the limitations of a computer and in a way that is useful for us: These representations are Data Models.We use Raster, Vector and Attribute data in our modelsWe make a model useful by building them with an application in mind.
  • Here’s a vector model in which areas of the earth are generalized as features and represented and stored as polygons.
  • Here there are two “vectors” or “geometries” that are used to abstract the landscape: points and lines.
  • The boundary lines and vertices are used to define the polygons.
  • Lines and nodes are all that is necessary to define the areas – like Polygon IIInstead of defining the boundary between neighboring polygons twice, one for each closed loop polygon, the line is stored only oncetogether with information on which polygons are located to the right and left of the line respectively. Information about the relationships between nodes, lines and polygons are stored in attribute tablesWhy is this representation useful? One example:Say these polygons are census tracts. If we wanted to quickly identify all neighbors of a particular tract, the system would simple go through the list of lines that define that tract then find all of the remaining tracts which are also bounded by these lines. A simple computational procedure.
  • It can be said that because of the way the model constructs polygons through shared points and lines, polygons know who there neighbors are.
  • A GIS topology is a set of rules and behaviors that model how points, lines, and polygons share coincident geometry. For example:  Adjacent features, such as two census tracts, will have a common boundary between them. They share an edge.There are rules that define what is meant by adjacent and generally specify that features can be adjacent but not overlap.
  • A GIS topology is a set of rules and behaviors that model how points, lines, and polygons share coincident geometry. For example:  Adjacent features, such as two census tracts, will have a common boundary between them. They share an edge.There are rules that define what is meant by adjacent and generally specify that features can be adjacent but not overlap.
  • Manage coincident geometry (constrain how features share geometry). For example, adjacent polygons, such as parcels, have shared edges; street centerlines and the boundaries of census blocks have coincident geometry; adjacent soil polygons share edges; etc.Define and enforce data integrity rules (such as no gaps should exist between parcel features, parcels should not overlap, road centerlines should connect at their endpoints).Support topological relationship queries and navigation (for example, to provide the ability to identify adjacent and connected features, find the shared edges, and navigate along a series of connected edges).Support sophisticated editing tools that enforce the topological constraints of the data model (such as the ability to edit a shared edge and update all the features that share the common edge).Construct features from unstructured geometry (e.g., the ability to construct polygons from lines sometimes referred to as "spaghetti").
  • COGO = Coordinate Geometry
  • A GIS topology is a set of rules and behaviors that model how points, lines, and polygons share coincident geometry. For example:  Adjacent features, such as two census tracts, will have a common boundary between them. They share an edge.There are rules that define what is meant by adjacent and generally specify that features can be adjacent but not overlap.
  • CES =Critical Environmental Sites
  • What this discussion of a topological model shows us is that GIS is extensible. At its core, it stores vector and raster information, but how it assembles that data and what it does with it can be modified to address specific purposes.The purposes do not have to be pre-defined.
  • Shape files are special formats for geographic data that were developed by ESRI.Generic database systems are not built that can handle spatial data as easily as non-spatial (“Traditional”) data.This facilitates creating representations as objects with a set of attributes, relationships, rules for how they are constructed, what they can do and what can be done with them.Topology becomes a rule that must be verified because it isn’t directly a function of the model.
  • Objects are a much more robust way of modeling geographic informationCompare the topological model we discussed with an object model of the same thing.
  • Vector – pointsNot a grid, but can be converted to a raster or a different vector format where that approach is necessary.Show Mass Points in ArcScene
  • Take LiDAR and convert it to a raster formatGrid values are an abstraction (amalgam) of the mass point LiDAR data.Useful way to reduce data loadCan be used in conjunction with other raster data for analysis – perhaps land use based on aerial or satellite imagery.Grids overlap in both the Dem and LULC raster.
  • LiDAR data to a vector formatSurfaces with a topologic relationshipStorage differences in computerAmenable to different analysis techniques than DEMSCan have attributes??
  • Showing Slope by color
  • Showing elevation by color
  • Simple routing, one vehicle, multiple stops
  • Service area routing – Maintenance calls are routed to the closest facility

Lecture 3 - Data Models Presentation Transcript

  • 1. Data Models Introduction to GIS – Spring 2013 1 M. Corbalis
  • 2. Data Models • Geographic Information Systems use an abstraction of reality. • We create models – sets of constructs for describing and representing selected aspects of the real world in a computer. • Models are composed of a mix of raster, vector, and attribute data. (rules too.) • Model is tailored to a specific function. Introduction to GIS – Spring 2013 2 M. Corbalis
  • 3. Coding Vector Data node node B C Polygon I Polygon node III Polygon A II node node F D Polygon V Polygon IV node G Reality Vector Model Introduction to GIS – Spring 2013 3 M. Corbalis
  • 4. Coding Vector Data node node B C Polygon I Polygon III node A Polygon II node node E F node D Polygon V Polygon IV node G Introduction to GIS – Spring 2013 4 M. Corbalis
  • 5. Topologic Vector Model node node B C Polygon I Polygon III node A Polygon II node node E F node D Polygon VA topologic vector model records the points and lines Polygon IVshared between polygons as unique items – so, every one node Gof the points and lines are recorded in the data only once Introduction to GIS – Spring 2013 5 M. Corbalis
  • 6. Topologic Vector Model node node B C node A Polygon II node E node DPolygon II is on the right sideof the line ABCED. Introduction to GIS – Spring 2013 6 M. Corbalis
  • 7. Topologic Vector Model Polygon 2 “knows” it’s adjacent to Polygons 1, 3, & 4. It shares a line segment with each. Polygon I Polygon node III A Polygon II node node E F node D Polygon V Polygon IV Polygon 2 “knows” it touches Polygon 5. It shares node E with Polygon 5. Introduction to GIS – Spring 2013 7 M. Corbalis
  • 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. Introduction to GIS – Spring 2013 8 M. Corbalis
  • 9. Strict Topology node node• Features are composed B C from a common set of Polygon points and lines. I Polygon node III• Altering the vertices of A Polygon II node node one polygon affects E node F polygons that share D Polygon those vertices. Polygon V IV• Harder to introduce node G gaps or slivers. Introduction to GIS – Spring 2013 9 M. Corbalis
  • 10. Topology?• Reasons topology would be important to model?• Where in the real world is this concept important? Introduction to GIS – Spring 2013 10 M. Corbalis
  • 11. Cadastre Example benchmark survey (COGO) parcels zones Introduction to GIS – Spring 2013 11 M. Corbalis
  • 12. Topology Applied : Parcel Overlap• The boundaries of two properties should never overlap, and there should never be a gap between them, unless intentional.• Clear error in parcel boundaries. Introduction to GIS – Spring 2013 12 M. Corbalis
  • 13. Policy-based Topology Rules• In the NJ State Plan, CESs and the Environmentally Sensitive Planning Area both represent areas of environmental importance. – 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. Introduction to GIS – Spring 2013 13 M. Corbalis
  • 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. Introduction to GIS – Spring 2013 14 M. Corbalis
  • 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 Introduction to GIS – Spring 2013 15 M. Corbalis
  • 16. Vector Geometry as Objects• Parcels – Planar geometries with attribute information• Parcels as objects in a Cadastral “carpet” – Objects with topology rules (“don’t overlap, unless”) – Members of “regional” features (zoning, municipality) – Composed of surveyed parts (COGO, benchmarks) – Keys that link to attribute tables (owner(s), assessments, plans, etc) Introduction to GIS – Spring 2013 16 M. Corbalis
  • 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 Introduction to GIS – Spring 2013 17 M. Corbalis
  • 18. Exploring 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 Introduction to GIS – Spring 2013 18 M. Corbalis
  • 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. Introduction to GIS – Spring 2013 19 M. Corbalis
  • 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). Introduction to GIS – Spring 2013 20 M. Corbalis
  • 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. Introduction to GIS – Spring 2013 21 M. Corbalis
  • 22. TIN Model of Campus Introduction to GIS – Spring 2013 22 M. Corbalis
  • 23. TIN Model of Campus Introduction to GIS – Spring 2013 23 M. Corbalis
  • 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. Introduction to GIS – Spring 2013 24 M. Corbalis
  • 25. Networks Introduction to GIS – Spring 2013 25 M. Corbalis
  • 26. Networks Introduction to GIS – Spring 2013 26 M. Corbalis
  • 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. Introduction to GIS – Spring 2013 27 M. Corbalis
  • 28. Models Diagrammed Introduction to GIS – Spring 2013 28 M. Corbalis
  • 29. NJ DEP Wastewater Model Introduction to GIS – Spring 2013 29 M. Corbalis
  • 30. NJ DEP Wastewater Model Environmental Science Data Modeling Other Discipline Specific Data Models Introduction to GIS – Spring 2013 30 M. Corbalis
  • 31. 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. Introduction to GIS – Spring 2013 31 M. Corbalis
  • 32. Raster Introduction to GIS – Spring 2013 32 M. Corbalis