GIS Data Structures

How do we represent the world in a GIS database?

Objectives

• To discuss basic data structures for GIS

• Details of vector, raster, and TIN data models will be given

• To define and discuss topology

Basic Data Structures for GIS

1. Vector

2. Raster

3. TIN (triangulated irregular network)

4. Tabular Information(attribute table)

Vector Data Structure

lines

polygons

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

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)

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, IraqFrom 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

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

Sources of raster data

Although an digital aerial photo is in raster format, it has no data.

Raster Data Structure

Raster Data Structure

Raster and Vector Data Structures

PointPoint

LineLine

PolygonPolygon

VectorVector RasterRaster

Raster data are described by a cell grid, one value per cell

Zone of cells

• 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

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

TIN Data Structure

• 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 StructureAdvantages

3 GIS Spatial Data Structure Types

Attribute table

“Flat File” with columns and rows

Row = geographic feature recordColumn = attribute field (item of information about a feature)

Attribute Data Structure

Attribute field general types

• Numeric (integer or decimals)

• Text (string)

• Date

• Blob (binary large object)

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.

TopologyWhen you edit features in an electric utilitysystem, you want to be sure that the ends ofprimary and secondary lines connect exactly andthat you are able to perform tracing analysis onthat 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 two-dimensional 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?

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?

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

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.

Shape files

Shape files can be created with ArcView software.

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

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