1. How do we represent the world in a GIS database?
GIS Data Structures
How do we represent the world in a GIS database?

2. Objectives To discuss basic data structures for GIS
Details of vector, raster, and TIN data models will be given
To define and discuss topology

3. Basic Data Structures for GIS
1. Vector
2. Raster
3. TIN (triangulated irregular network)
4. Tabular Information
(attribute table)

4. Vector Data Structure
polygons
lines

5. Vector Data Structure
In vector data layers, the feature layer is linked to an attribute table. Every individual feature corresponds to one record (row) in the attribute table.

6. About Image Files Image files contain no data They are the background
You can create data based on images
Not considered a “data” structure

7. Raster Data Structure (Grid)

8. Raster Data Structure
A raster grid can store values that represent categories, for example, vegetation type
The basic grid attribute table has a value and count field
The value field has a code or some real number representing information about the grid cell. In this case it is a code for vegetation.
The count field shows how many grid cells have that same value.

9. Raster Data Structure
A raster grid can store values that represent categories, for example, vegetation type
A grid table can also have additional information, in this case the name of the vegetation type. But is always has the value and count fields.

10. Raster Data Structure
Grids can also store continuous values like elevation

11. Raster Data Structure Elevation grid for area north of Kirkuk, Iraq
From space shuttle radar topography mission (SRTM)
Zoom in and you see the grid cells
These are called:
Digital Elevation Models (DEM)

12. Raster Data Structure So 2 ways of representing elevation:
Vector contour lines
Raster grid

13. Raster Data Structure Sources of raster data
Interpreted satellite imagery, e.g., land cover
Conversion of vector to raster data

14. Raster Data Structure Sources of raster data
Spatial analysis performed on vector data
A density grid derived from the same crime data – interpolation of point data over a continuous surface
A point layer of crime reports

15. Raster Data Structure Sources of raster data
Although an digital aerial photo is in raster format, it has no data.

16. Raster Data Structure

17. Raster and Vector Data Structures
Raster data are described by a cell grid, one value per cell
Vector
Raster
Point
Line
DRM
Zone of cells
Polygon

18. Vector
Raster
Features with discrete shapes and boundaries (e.g., street, land ownership parcel, well)
Database management
Database query and reporting
Network analysis
High quality maps
Continuous surfaces with fuzzy boundaries or with qualities that change gradual over space (e.g., soil, land cover, vegetation, pollution)
Spatial analysis and modeling (e.g., agricultural suitability)

19. TIN Data Structure A 3rd data structure for representing surfaces:
Triangulated Irregular Network (TIN)

20. TIN Data Structure
Elevation points connected by lines to form polygons that contain topographic information

21. TIN Data Structure
Elevation points connected by lines to form polygons that contain topographic information

22. TIN Data Structure

23. TIN Data Structure

24. TIN Data Structure Advantages
Linear geographic features such as streams and ridges are more accurately represented in a TIN
Less points are needed to represent the topography – less hard disk space is needed
Points can be concentrated in important areas where the topography is more variable, or where more detail is required (e.g., small areas of land)
Survey data and known elevations can easily be incorporated into a TIN
Some functions cannot be performed with DEM data, but are easily done with a TIN

25. 3 GIS Spatial Data Structure Types

26. Attribute Data Structure
Attribute table
“Flat File” with columns and rows
Row = geographic feature record
Column = attribute field (item of information about a feature)

27. Attribute field general types
Numeric (integer or decimals)
Text (string)
Date
Blob (binary large object)

28. Attribute data types
Categorical (name):
nominal
no inherent ordering
land use types, county names
ordinal
inherent order
road class; stream class
Note: often coded to numbers (eg. SSN) but can’t do arithmetic
Numerical
Known difference between values
interval
No natural zero
can’t say ‘twice as much’
temperature (Celsius or Fahrenheit)
ratio
natural zero
ratios make sense (e.g. twice as much)
income, age, rainfall
Note: may be expressed as integer [whole number] or floating point [decimal fraction]
Attribute data tables can contain locational information, such as addresses
or a list of X,Y coordinates. ArcView refers to these as event tables. However,
these must be converted to true spatial data (shape file), for example by
geocoding, before they can be displayed as a map.

29. Topology When you edit features in an electric utility
system, you want to be sure that the ends of
primary and secondary lines connect exactly and
that you are able to perform tracing analysis on
that electric network.
Features need to be connected using specific rules.

30. Network Topology

31. Planar topology
Property parcels of land must adjoin each other exactly, without gaps or overlaps. This two-dimensional graph is called a planar topology.

32. Topological relationships
The relationships that do not change if you imagine a map being on a rubber sheet and you pull and stretch the rubber sheet in different directions.
Vector and TIN data can have topological structure.
Raster and images can not have a topological structure.

33. For your project What data layers Vector, raster, TIN, image?
Topological structure (network connectivity or planar topology)?
Attributes?
Minimum required accuracy?

34. Should a data layer be topologically structured?
Some objects are non-topological and can be freely placed in a geographic area.
Examples?
Many objects are primarily stored in a GIS for the purpose of background display on a map, so it is usually not necessary to store them in a topological format.
If roads are a background layer in your GIS, they will probably be simple features. If roads are part of an analysis of a transportation system, they should be topological features.

35. ArcGIS Major Data Formats
Coverages (Arc/Info)
Older
Used with ArcInfo versions 7 and older
Shape files
Developed when ArcView was released
ArcView merged with ArcInfo at version 8
Geodatabases
Developed when ArcGIS was released (version 8)
Shapefiles are still used, but the move is toward geodatabases

36. Arc/Info Coverages
Coverages are an older data structure in which topology could be modeled.
You will still find many data sets in Arc/Info coverage data formats.
But for new data, you should use geodatabase or shapefile formats.

37. Shape files
Shape files can be created with ArcView software.

38. Geodatabases Geodatabases can be created with ArcGIS 8.x , 9.x, and 10
Geodatabases give you more power to specify rules for features and structure topology

39. Summary 3 Spatial Data Structure Types in GIS
Vector
Raster
TIN
Attribute Data Structure – Tables of columns and rows
Topology – needed for spatial data to “know” where other data is