GIS Data Structures

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GIS Data Structures

• How do we represent the world in a GIS database?

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Basic data structures for GIS
Vector
Raster
TIN (triangulated irregular network)
Tabular information (attribute table)
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About image files
Image files contain no data They are the
background You can create data based on
images Not considered a data structure
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Vector data structure
polygons
lines
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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.
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Raster Data Structure (Grid)
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Raster and Vector Data
Raster data are described by a cell grid, one
value per cell
Vector
Raster
Point
Line
Zone of cells
Polygon
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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.
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A raster grid can store values that represent
categories, for example, vegetation type
A grid table can also have additonal information,
in this case the name of the vegetation type. But
is always has the value and count fields.
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Grids can also store continuous values like
elevation
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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)
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So 2 ways of representing elevation
Vector contour lines
Raster grid
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Sources of raster data
Interpreted satellite imagery, e.g., land cover
Conversion of vector to raster data
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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
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Sources of raster data
Although an digital aerial photo is in raster
format, it has no data.
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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 graduall over space (e.g.,
  soil, land cover, vegetation, pollution)
• Spatial analysis and modeling (e.g., agricultural
  suitability)

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A 3rd data structure for representing
surfaces Triangulated Irregular Network (TIN)
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TIN Data Structure
Elevation points connected by lines to form
polygons that contain topographic information
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TIN Data Structure
Elevation points connected by lines to form
polygons that contain topographic information
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Attribute table Flat File with columns and
rows Row geographic feature record Column
attribute field (item of information about a
feature)
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Attribute field general types

• Numeric (integer or decimals)
• Text (string)
• Date
• Blob (binary large object)

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

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

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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.
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ArcGIS major data formats

• Shape files
• Coverages (Arc/Info)
• Geodatabases

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Shape files
Shape files can be created with ArcView 3
software.
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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.
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Geodatabases
Geodatabases can be created with ArcGIS 8.3 and
9.0 Geodatabases give you more power to specify
rules for features and structure topology