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Introduction

Introduction. GIS is often used to model processes. Two main objectives: To understand how the processes operate; To predict the likely outcomes given particular conditions. Today we will do two things: Look at process models in general Discuss cartographic models. Process Models.

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Introduction

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  1. Introduction • GIS is often used to model processes. Two main objectives: • To understand how the processes operate; • To predict the likely outcomes given particular conditions. • Today we will do two things: • Look at process models in general • Discuss cartographic models

  2. Process Models • Process models attempt to model dynamic processes. May be: • A priori – driven by data rather than theory. Approach is basically inductive. • A posteriori – based on established theory. Approach is more deductive. • Objectives: • Systems analysis • Decision support

  3. Process Models(2) • Process models can be either: • Descriptive / predictive • Prescriptive • Can be thought of as forming a continuum: • Description -> Prediction -> Prescription

  4. Gravity Models • Gravity models provide a good example of a process model. • Basic form: • The model may be fitted to determine the r and k parameters (calibration). • May need to distinguish between source and destination using exponents. • Many uses: e.g. migration, shopping trips, etc.

  5. Cartographic Models • A cartographic model is basically a flow diagram identifying the various coverages required for a particular end result. • For example, suppose we wished to identify suitable sites for a mountain cabin (DeMers). • There are several steps: • Identify our criteria. • Identify the data required. • Identify the types of GIS operation required.

  6. Weighting And Automation • In this example we use simple Boolean overlays – i.e. each criterion exercises a ‘power of veto’. • May be preferable to use fuzzy membership functions and/or weight the various criteria - i.e. weighted linear combination (WLC). • Can produce fewer or more suitable sites by adjusting the weightings. • May be useful to automate the model to allow it test ‘what if’ scenarios more readily. Could write a script or use a tool such as ESRI’s Model Builder.

  7. Pilot Study • In a big study it may be useful to do a pilot study using a sample area: • Can be done rapidly and cheaply; • Results can be ground truthed; • Can identify data that may be discarded if space an issue. • Verification entails checks of: • Accuracy • Usefulness

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