GIS Data Structures
How do we represent the world in
a GIS database?
Basic Data Structures for GIS
1. Vector
2. Raster
3. TIN (triangulated
irregular network)
4. Tabular Information
(attribute table)
Vector Data Structure
polygons
lines
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.
About Image Files
• Image files contain no
data
• They are the background
• You can create data
based on images
• Not considered a “data”
structure
Raster Data Structure (Grid)
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.
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.
Raster Data Structure
Grids can also store continuous values like elevation
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)
Raster Data Structure
So 2 ways of representing elevation:
Vector contour lines
Raster grid
Raster Data Structure
Sources of raster data
Interpreted
satellite imagery,
e.g., land cover
Conversion of vector to raster data
Raster Data Structure
Spatial analysis performed on vector data
Sources of raster 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
Sources of raster data
Although an digital aerial photo is in raster format, it has no data.
Raster Data Structure
Raster and Vector Data Structures
Raster data are described by a cell grid, one value per cell
Vector
Raster
Point
Line
Zone of cells
Polygon
Vector
• Features with discrete
shapes and
boundaries (e.g.,
street, land ownership
parcel, well)
• Database
management
• Database query and
reporting
• Network analysis
• High quality maps
Raster
• 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)
TIN Data Structure
A 3rd data structure for representing surfaces:
Triangulated Irregular Network (TIN)
TIN Data Structure
Elevation points
connected by
lines to form
polygons that
contain
topographic
information
TIN Data Structure
Elevation points
connected by
lines to form
polygons that
contain
topographic
information
TIN Data Structure
TIN Data Structure
3 GIS Spatial Data Structure Types
Attribute Data Structure
Attribute table
“Flat File” with columns and rows
Row = geographic feature record
Column = attribute field (item of information about a feature)
Attribute field general types
• Numeric (integer or decimals)
• Text (string)
• Date
• Blob (binary large object)
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.
Network Topology
Planar topology
Property parcels of land must adjoin each other
exactly, without gaps or overlaps. This twodimensional graph is called a planar topology.
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.
For your project
• What data layers
• Vector, raster, TIN, image?
• Topological structure (network connectivity
or planar topology)?
• Attributes?
• Minimum required accuracy?
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.
ArcGIS Major Data Formats
• Shape files
• Coverages (Arc/Info)
• Geodatabases
Shape files
Shape files can be created with ArcView software.
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 formats.
Geodatabases
Geodatabases can be created with ArcGIS 8.x and 9.x
Geodatabases give you more power to specify rules for features
and structure topology
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