GIS and Modeling Overview

GIS and Modeling Overview MICHAEL F. GOODCHILD NATIONAL CENTER FOR GEOGRAPHIC INFORMATION AND ANALYSIS UNIVERSITY OF CALIFORNIA SANTA BARBARA, CALIFORNIA Modeling can be defined in the context of geographic information systems (GIS) as occurring whenever operations of the GIS attempt to emulate processes in the real world, at one point in time or over an extended period. Models are useful and used in a vast array of GIS applications, from simple evaluation to the prediction of future landscapes. In the past it has often been necessary to couple GIS with special software designed for high performance in dynamic modeling. But with the increasing power of GIS hardware and software, it is now possible to reconsider this relationship. Modeling in GIS raises a number of important issues, including the question of validation, the roles of scale and accuracy, and the design of infrastructure to facilitate sharing of models. 2 1: GIS And Modeling Overview The term modeling is used in several different contexts in the world of GIS, so it would be wise to start with an effort to clarify its meaning, at least in the context of this book. There are two particularly important meanings. First, a data model is defined as a set of expectations about data—a template into which the data needed for a particular application can be fitted. For example, a table is a very simple example of a data model, and in the way in which tables are often used in GIS, the rows of the table correspond to a group or class of real-world features, such as counties, lakes, or trees, and the columns correspond to the various characteristics of the features, in other words, the attributes. This table template turns out to be very useful because it provides a good fit to the nature of data in many GIS applications. In essence, GIS data models allow the user to create a representation of how the world looks. A later section of the chapter provides a more extended discussion of data modeling in the particular context of dynamic models. Second, a model (without the data qualification) is a representation of one or more processes that are believed to occur in the real world—in other words, of how the world works. A model is a computer program that takes a digital INTRODUCTION Figure 1. The results of using the DRASTIC groundwater vulnerability model in an area of Ohio. The model combines GIS layers representing factors important in determining groundwater vulnerability and displays the results as a map of vulnerability ratings. (screen shot from http:// www.gwconsortium.org/DRASTIC.gif, needs permission)