gis

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

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

3. Vector Data Structure
lines
polygons

4. 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.
Vector Data Structure

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

6. Raster Data Structure (Grid)

7. 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.
Raster Data Structure

8. 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.
Raster Data Structure

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

10. 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)
Raster Data Structure

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

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

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

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

15. Raster Data Structure

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

17. • 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)
Vector Raster

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

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

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

21. TIN Data Structure

22. TIN Data Structure

23. • 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
TIN Data Structure
Advantages

24. 3 GIS Spatial Data Structure Types

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

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

27. 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.

28. 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.

29. Network Topology

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

31. 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.

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

33. 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.
Should a data layer be topologically structured?

34. 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

35. 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.

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

37. 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

38. 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