June 17, 2014
Institute of Space Technology, Karachi
 Chapter 13: Spatial Models and Modeling
Model
 Description of reality
 Static reproduction that represents basic shape and
form of an object
 A conceptual description of key elements and processes
in a system
 Sophisticated replica of objects, events, or processes
 For this class
 Restricted to computer-based models of spatial features
Computer Based Spatial Models
 Combination of
 GIS
 Computer programming languages
 Spatial and non-spatial analytical tools
Two Broad Classes
 Spatio-temporal Models
 Dynamic in both space and time
 Example: analysis of oil after spill
 Cartographic Models
 Static models
 Involve application of spatial operations
 Example: buffer, interpolation,
reclassification, overly
 Combine data from multiple data layers
Cartographic Models
 Represent spatial features at a fixed point or points of
time
 Most GIS models are cartographic models that are
temporally static
 Provides information through a combination of spatial
data sets, functions and operations
 Reclassification
 Overlay
 Interpolation
 Terrain analysis
 Buffering,
 Map algebra, etc.
Example: Suitability Analysis
 Suitable park sites based on the proximity to Roads and Lakes
and the absence of Wetlands
Flow Chart
 A graphic representation of the spatial data, operations and
their sequence of use in a cartographic model
Application
 Site selection
 Land use planning
 Transportation route and corridor studies
 Design and development of water distribution systems
 Human disease spread
 Pollution response planning
 Endangered species preservation
Designing a Cartographic Model
 Spatial functions and operations are mixed and
matched in cartographic models
 Variation in sequence of same operations will result in
entirely different outputs
 With a small set of data layers and tools, a huge
number of models can be created
 Usually produces a large number of ‘intermediate’ or
temporary data layers that are not needed in final
output or decision making
Designing the BEST Cartographic Model
 Selection of appropriate spatial tools and specification
of their sequence
Conti.. Designing a Cartographic Model
 Based on a set of criteria
 These criteria are usually defined in qualitative terms
 The slope must not be too steep
 Interpretation /translation of criteria into selection
and sequence of spatial operations
 What is meant by “too steep”
 Need to be converted to specific, quantitative
measures
Example: Home Site Selection
 Problem: Ranking sites by suitability for home
construction
 Criteria:
 Slopes should not be too steep
 Southern aspect is preferred to enhance solar warming
 Soils suitable for on-site septic systems
 Sites should be far enough from a main road to offer
some privacy but not so far as to be isolated
 First convert these criteria into more specific
quantitative terms
 Check availability and quality of data
 Do the required data layers exist for the study area?
 Are spatial accuracies, spatial resolution and attributes
appropriate for analysis
 What level of map generalization?
 If required data is not available
 Obtain or develop the required data OR
 Modify the goals
 Explicit ranking of the relative importance of different
classes or types of criteria
General Criteria
Refined Criteria
Slopes Not Too Steep
Slope < 30 degrees
Southern Aspect Preferred
90 < aspect < 270
Soils Suitable for Septic System
Specified list of septic-suitable
soil units
Far enough from Road to Provide
Privacy, But Not Isolated
300 meters < distance to road <
2000 meters
Ranking and Weighting
 Ranking: Assignment of relative values within the
same layer
 How we rank a sandy soil vs. a silty soil in a soil layer
 ‘Weighting’ – assigning the relative values of different
layers
 How we weight the values in an elevation layer vs. the
values in a land use layer
Ranking Within Criteria
 Each Criterion in cartographic model is usually
expressed by a data layer or ‘criterion layer’
 Each criterion layer is a spatial representation of some
constraint or selection condition
 Select site outside floodplain: Floodplain sites = 0,
Upland sites = 1
Discrete vs. Continuous Ranking
 Discrete: when input data are interpreted such that
criterion data layer is a map of discrete value
 Soil = Good, Bad
 Slope = steep or acceptable
 Continuous: ranks vary along a scale
 Soils: Rated from 1 to 100 for construction suitability
Discrete and Continuous ranking
Criterion: desirable sites are greater than 300 but less
than 2000 meters from road
Weighting Among Criteria
 Criteria combined in spatial analysis – in overlay or
addition process
 How to weigh one layer over another?
 How important is slope relative to aspect?
 Will an optimum aspect offset a moderately steep site?
 The relative weightings given to each criterion will
influence the result
 Different weights are likely to result in different suitability
rankings
 Easy to define when importance of the various criteria
expressed on a common scale – money
 Reducing all criteria to a common scale removes
differential weighting among criteria
Assigning Weights based on Importance Ranking
Low to high Elevation: black to
light shade
Low slope: light shades
Aspect: light to dark from
0-360o
Spatio-temporal Models
 Includes time driven processes within the framework
of model
 Feature boundaries, point feature locations, and
attribute variables may change within model run
 Complicated models