GEORELATIONAL VECTOR DATA MODELS
Data Models
•A model is a gross simplification of the real world
•Spatial data is stored in either a Raster Data Model or a Vector Data Model
•The Vector Data Model uses points, lines and polygons to represent the real world.
Viewing geographic phenomena as discrete entities or objects
VECTOR DATA MODEL
Prepares data in two basic steps:
•First, it uses points and their x- y-coordinates to represent spatial features as points, lines and areas
(polygons).
•Second – it organizes geometric objects and their spatial relationships into digital data files for computer
access
Vector Data Models
•Vector Data Models represent geographic features as points, lines, and polygons
•Map features are positioned as sequences of X,Y coordinates
•The location and shape of features and boundaries are precisely defined
Vector Data Models
•Advantages
Entities/objects represented as discrete
Topology can be easily described
Coordinate transformation relatively easy
•Disadvantages
Complex data structure
Overlaying computationally ‘expensive’
Time-consuming to display
Difficult for spatial modeling
Vector Data Model
•Point: features recorded as single x,y coordinate pairs or geographic location
•Line: features are an ordered series of connected line segments
•Polygon: features are an ordered series of at least 3 connected lines with same start and end point
VECTOR DATA MODEL
•ESRI has introduced new vector data models with each new software package:
ArcINFO – coverage
ArcView – shapefile
ArcGIS – geodatabase
•Coverages and shapefiles are examples of the Georelational Data Model
•Geodatabases are examples of the Object-Based Data Model.
Georelational Vector Data Model –
•The Georelational Data Model stores spatial and attribute data separately in a split system
•Spatial data in ‘graphic files’
•Attribute data in ‘relational database files’.
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•The link between the two ‘systems’ is usually a “Feature Label” or ID
•The Georelational Data Model has been used for years in GIS, however the new Object-Based Data Model
is encroaching into the realm.
GEORELATIONAL VECTOR DATA MODEL – Representation of Simple Features
•Point – 0 dimension, with the property of location
Also called: Node, Vertex, 0-Cell
•Line – 1 dimension with property of length
Two end points and points between define shape of line
Also called: Edge, link, chain or 1-cell
•Area – 2 dimensional with property of area and perimeter
Composed of connected lines, standing alone or sharing boundaries with other areas
May contain holes (a national forest with private lands, for example)
Also called: polygon, face, zone, or 2-cell
GEORELATIONAL VECTOR DATA MODEL - TOPOLOGY
•Topology explicitly defines the spatial relationships between features.
•One explanation is through mathematics – the Graph Theory
Using diagrams or graphs, the arrangements of geometric objects and the relationships between the
objects can be examined.
•Adjacency and Incidence
GEORELATIONAL VECTOR DATA MODEL
•The topological data structure determines how and where points and lines connect in a GIS through nodes
(topological junctions)
Order of connectivity defines shape of the arc or polygon
 The GIS accesses topological information to determine relationships between data
Use of a digraph (directed graph) explains concept of adjacency and incidence
•Adjacency: arcs joining corresponding nodes in a digraph
•Incidence: indicating direction of arc – either to or from a node
GEORELATIONAL VECTOR DATA MODEL – GRAPH THEORY
•Vertex 1 is incident with side ‘a’
•Vertex 2 is incident with side ‘a’, ‘b’, and ‘d’
•Vertex 3 is incident with side ‘b’ and ‘c’
•Vertex
4 is incident with side ‘c’ and ‘d’
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GEORELATIONAL VECTOR DATA MODEL – INCIDENCE GRID/MATRIX
Vertex 1
Vertex 2
Vertex 3
Vertex 4
Side 'a'
1
1
0
0
Side 'b'
0
1
1
0
Side 'c'
0
0
1
1
Side 'd'
0
1
0
1
GEORELATIONAL VECTOR DATA MODELS - ADJACENCY
•Using the same graph as before, a grid showing how vertexes can be joined by edges (lines) would be
represented as:
Vertex 1 is adjacent to vertex 2
Vertex 2 is adjacent to vertex 2, 3 and 4
Vertex 3 is adjacent to vertex 2 and 4
Vertex 4 is adjacent to vertex 2 and 3
GEORELATIONAL VECTOR DATA MODEL – ADJACENCY MATRIX/GRID
Vertex 1
Vertex 2
Vertex 3
Vertex 4
Vertex 1
0
1
0
0
Vertex 2
1
0
1
1
Vertex 3
0
1
0
1
Vertex 4
0
1
1
0
GEORELATIONAL VECTOR DATA MODEL
•Arc-Node topology developed to store information
 Used to encode information used in U.S. Census Bureau TIGER files (Topologically Integrated
Geographic Encoding and Reference)
•Points – 0-cells
•Lines – 1-cells
•Area – 2-cells
GEORELATIONAL VECTOR DATA MODEL
•TIGER (Topologically Integrated Geographic Encoding and Referencing system) databases
Each 1-cell (line) in a TIGER file is a directed line
•Line has a specific starting point, a direction, an end point, with an explicit left or right side
Each 2-cell (area) and 0-cell (point) is aware of the 1-cell (line) attached to it.
The TIGER database includes all spatial relationships between points, lines and areas.
•Block groups can be associated with roads,
•Addresses can be located on either the right or left side of a street
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GEORELATIONAL VECTOR DATA MODEL
•Topology in GIS software –
•Topology introduced to separate GIS from CAD
CAD cannot support topology, even though data is maintained in separate layers and each layer can be
created, drawn, edited and symbolized separately
•A coverage is the standard, topological vector data format used in ARC/INFO.
Coverages are grouped by point, line, and polygon
Three topological relationships supported by ARC/INFO overages
•Connectivity – nodes connect arcs
•Area definition – a series of connected arcs defines an area
•Contiguity – direction of arcs established, with left or right polygons identified
GEORELATIONAL VECTOR DATA MODEL
•Data Structure for typical Point, Line or Polygon coverage in ArcINFO
•The Arc List shows the relationship between polygons and their arcs
GEORELATIONAL VECTOR DATA MODEL
•Non-Topological Vector Data Model –
•Non-topological vector data is now a standard for many commercial GIS packages including: MapInfo,
ArcView, GeoMedia and others.
•Non-topological data format for ArcView is shapefile.
No files are stored to describe spatial relationships among the geometric objects even though…
•Point data is stored as a pair of x,y coordinates
•Line data is stored as a series of points
•Polygon/area data is stored as a series of lines
GEORELATIONAL VECTOR DATA MODEL
•Shapefiles have duplicate arcs for shared boundaries
•Geometry of shapefile stored in two files:
The .shp file storing the feature geometry
The .shx file maintaining the index of the feature geometry (each feature may have a different number of
coordinates)
•Other files within a shapefile:
The .dbf file stores attribute information for files in a dBASE file
The .sbn and .sbx files store the spatial index of a feature. ArcView partitions the geographic features into
spatial ‘bins’.
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•The record
number of each feature is stored and ArcView uses the file to speed up spatial queries and
redraws the theme
GEORELATIONAL VECTOR DATA MODEL
•TIN – triangulated irregular network
•A vector data structure for terrain mapping
•The TIN ‘mimics’ surface with a series of non-overlapping triangles.
•The two basic data elements of a TIN are (a) elevation points with x,y, and z values and (b) edges (lines)
connecting the points to form the triangle
The triangle is assumed to have a constant gradient
The x, y points are location, and the z value represents elevation
Slope and aspect for each triangle are computed from the x,y and z values at the three points of the
triangle.
TIN in GIS
•TINS are sets of adjacent, non-overlapping triangles derived from irregularly spaced x- y-coordinates and
z-values.
•TIN models provide control over terrain slope, aspect, surface areas, volumetric and cut-fill analysis and
generating contours.
•The TIN's vector data structure is based on irregularly-spaced point, line and polygon data interpreted as
mass points and breaklines while storing the topological relationship between triangles and their adjacent
neighbors.
TIN to CONTOUR LINES
GEORELATIONAL VECTOR DATA MODEL - REGIONS
•A geographic area with similar characteristics (physical or cultural)
•Data models for geographic regions will need to work with two spatial characteristics:
Spatially joint or disjoint area within any one region
Two regions that overlap or cover the same area
•A simple polygon coverage won’t work for this reason.
Regions must be subclassed and each subclass must be related to underlying polygons and arcs
GEORELATIONAL VECTOR DATA MODELS - ROUTES
•Routes are linear features – highways, rivers, bike paths.
•Routes have measurement systems that allow linear measures to be used on a projected coordinate system.
Using known starting points (a milepost, for example), transportation agencies can locate accidents,
bridges, pavement conditions, even traffic conditions along a stretch of highway using Routes
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Linear attributes are called Events and must be associated with routes for displaying and analysis with
other spatial features
ROUTES IN GIS
GEORELATIONAL VECTOR DATA MODEL
•In summary, the Georelational Vector Data Model has been the standard for GIS data historically.
Spatial Data and Attribute Data are stored separately in graphic files or relational databases, respectively.
•However, the Object-based Data Model is challenging this standard as advances in computer technology
now allow storage of both spatial and attribute data in a single system.
This eliminates any problem with data synchronization in a split system
•The Georelational Data Model (e.g., a shapefile) is still an important vector data model.
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