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Spatial Models and ModelingJune 7, 2013Institute of Space Technology, KarachiChapter 13: Spatial Models and ModelingModelDescription of realityStatic reproduction that represents basic shape and form of an objectA conceptual description of key elements and processes in a systemSophisticated replica of objects, events, or processesFor this classRestricted to computer-based models of spatial featuresIt may be..Spatial models3Computer Based Spatial ModelsCombination ofGISComputer programming languagesSpatial and non-spatial analytical toolsTwo Broad ClassesSpatio-temporal ModelsDynamic in both space and timeExample: analysis of oil after spill

Cartographic ModelsStatic modelsInvolve application of spatial operationsExample: buffer, interpolation, reclassification, overlyCombine data from multiple data layers

Analysis of oil after spill: current, winds, wave action, and physics of oil separation and evaporation on exposure to air might be combined in a model to predict the changing location of an oil slick. The actions of objects as they move across an environment may also be represented in a spatio-temporal model. 5Cartographic ModelsRepresent spatial features at a fixed point or points of timeMost GIS models are cartographic models that are temporally staticProvides information through a combination of spatial data sets, functions and operationsReclassificationOverlayInterpolationTerrain analysisBuffering, Map algebra, etc.

Temporally static: Although we may use them to analyze a change over a period of time e.g. vegetation change over a 10-year period. Each data layer represents the vegetation boundaries at a fixed point in time. Polygon boundaries for a given layer do not change. Data in base layers are mapped for a given period. These data are the basic for spatial operations that may create new data layers. 6Example: Suitability AnalysisSuitable park sites based on the proximity to Roads and Lakes and the absence of Wetlands

Most common type of ~models7Flow ChartA graphic representation of the spatial data, operations and their sequence of use in a cartographic model

Cartographic models may be represented by Flowcharts:Near road, near lake, not wetland.Data layers represented by rectangles, operations by ellipses, and sequences by arrow.

8ApplicationLand use planningTransportation route and corridor studiesDesign and development of water distribution systemsHuman disease spreadSite selectionPollution response planningEndangered species preservationUseful to managers, public and policy makers. 9Designing a Cartographic ModelSpatial functions and operations are mixed and matched in cartographic modelsVariation in sequence of same operations will result in entirely different outputsWith a small set of data layers and tools, a huge number of models can be createdUsually produces a large number of intermediate or temporary data layers that are not needed in final output or decision making10Designing the BEST Cartographic ModelSelection of appropriate spatial tools and specification of their sequenceMost important and often the most difficult process in ~. 11Conti.. Designing a Cartographic ModelBased on a set of criteriaThese criteria are usually defined in qualitative termsThe slope must not be too steepInterpretation /translation of criteria into selection and sequence of spatial operationsWhat is meant by too steepNeed to be converted to specific, quantitative measuresSometime conversion from qualitative to quantitative criteria involves iterative process with repeated interaction between the analyst and manager/decision maker. 12Problem: Ranking sites by suitability for home constructionCriteria:Slopes should not be too steepSouthern aspect is preferred to enhance solar warmingSoils suitable for on-site septic systemsSites should be far enough from a main road to offer some privacy but not so far as to be isolatedExample: Home Site SelectionThe area to be analyzed has steep terrain and is in a seasonally cold climate.Steep slopes may substantially increase costsThere is a range of soil types in the study area, with a range of suitabilities for septic system installation.These criteria must be converted to more specific restrictions prior to the development and application in a cartographic model.Discussion between the analyst and decision maker: what type of classification required, what range of classes, what type of ranking, ordinal, ratio/interval? Measurement scale of results? Slope threshold beyond which construction is infeasible can be defined by studying how slopes affect construction costs. 13First convert these criteria into more specific quantitative termsCheck availability and quality of dataDo the required data layers exist for the study area?Are spatial accuracies, spatial resolution and attributes appropriate for analysisWhat level of map generalization?If required data is not availableObtain or develop the required data ORModify the goalsExplicit ranking of the relative importance of different classes or types of criteria

Map generalization: will inclusion of different soil types in a soil polygon lead to inappropriate results? Is the minimum mapping unit appropriate?

14General CriteriaRefined CriteriaSlopes Not Too SteepSlope < 30 degreesSouthern Aspect Preferred90 < aspect < 270Soils Suitable for Septic System Specified list of septic-suitable soil unitsFar enough from Road to Provide Privacy, But Not Isolated300 meters < distance to road < 2000 metersRanking and WeightingRanking: Assignment of relative values within the same layerHow we rank a sandy soil vs. a silty soil in a soil layerWeighting assigning the relative values of different layersHow we weight the values in an elevation layer vs. the values in a land use layerRanking Within Criteria Each Criterion in cartographic model is usually expressed by a data layer or criterion layerEach criterion layer is a spatial representation of some constraint or selection conditionSelect site outside floodplain: Floodplain sites = 0, Upland sites = 1select site outside floodplain may consist of a set of numbers in a layer identifying flood plain locations. Obtain floodplain mapsInterpret codes in the map to delineate the most flood-prone areas.Rank areas based on the likelihood of flooding.

17Discrete vs. Continuous RankingDiscrete: when input data are interpreted such that criterion data layer is a map of discrete valueSoil = Good, BadSlope = steep or acceptableContinuous: ranks vary along a scaleSoils: Rated from 1 to 100 for construction suitability When there are clear discrete classes to be represented in criteria.Continuous: ranks or scores range over real or large integer interval e.g. 1 to 100 or 1 to 1000. Not necessarily a linear relationship but may be complex relationship between an input value and output ranking scores.

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Discrete and Continuous ranking

Non-linear relationship. Mercury concentration in drinking water20

Criterion: desirable sites are greater than 300 but less than 2000 meters from roadAt 1150 meters = rank 121

Weighting Among CriteriaCriteria combined in spatial analysis in overlay or addition processHow 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 resultDifferent weights are likely to result in different suitability rankingsEasy to define when importance of the various criteria expressed on a common scale moneyReducing all criteria to a common scale removes differential weighting among criteriaContinuous ranking may be combined, often through a weighted addition process to generate a combined suitability score.

Assign slopes a monetary values as it affects construction cost. Costs can be calculated Nuisance cost for noise and distance cost in lost time or travel might be quantifiable23

Assigning Weights based on Importance Ranking

In our example all criteria are equally weighted and all criteria are binary. 3 base layers are needed Elevation, Soils and Roads. 3 branches in the flowchart.Reclassify: areas that do and don't meet the respective criteria. 25

Left most branch of the flowchart.Low elevation = blackLow slopes = light shadesAspect= light to dark from 0 360Reclassify based on the threshold values. Two main river systems in the study area: Running from west to eastRunning from south to north26

Major roads: all divided and multilane roads. 29

Spatio-temporal ModelsIncludes time driven processes within the framework of modelFeature boundaries, point feature locations, and attribute variables may change within model runComplicated modelsInvolves lots of computer coding. 31