Spatial Thinking in Planning Practice: An Introduction to GIS

3 Even with the advent of computers, GIS applications to several decades to transform to the personal computer that we use today. Originally, the largest and most powerful computers were mainframes that were available to some academics and government o&cials. In the 1980s, most GIS applications ran on workstation computers tied to mainframe computers because the early microcomputers (IBM, Apple, etc.) did not have enough mem- ory, storage capacity, or processing ability. Today’s personal computers, however, are fast, capable of storing and processing large datasets, and can process multiple tasks simultaneously. "is enables many academics, govern- ment agencies (from local to federal), organizations, and small and large businesses to use GIS. Computer-based GIS has its advantages, but requires trained users. GIS DATA MODELS In order to visualize natural phenomena, one must !rst determine how to best represent geographic space. Data models are a set of rules and/or constructs used to describe and represent aspects of the real world in a computer. Two primary data models are available to complete this task: raster data models and vector data models. VECTOR DATA MODEL An introductory GIS course o$en emphasizes the vector data model, since it is the more commonly used in the planning professions. Vector data models use points and their associated [X and Y] coordinate pairs to represent the vertices of spatial features, much as if they were being drawn on a map by hand (Arono# 1989)1. "e data attributes of these features are then stored in a separate database management system. "e spatial information and the attribute information for these models are linked via a simple identi!cation number that is given to each feature on a map. "ree fundamental vector types exist in GIS: points, lines, and polygons, each of which we de!ne below (and illustrate in Figure 1.2): % Points are zero-dimensional objects that contain only a single coordinate pair. Points are typically used to model singular, discrete features such as buildings, wells, power poles, sample locations, and so forth. Points have only the property of location. Other types of point features include the node and the vertex. Speci!cally, a point is a stand-alone feature, while a node is a topological junction representing a common X, Y coordinate pair between intersecting lines and/or polygons. Vertices are de!ned as each bend along a line or polygon feature that is not the intersection of lines or polygons. Points can be spatially linked to form more complex features. % Lines are one-dimensional features composed of multiple, explicitly connected points. Lines are used to represent linear features such as roads, streams, faults, boundaries, and so forth. Lines have the property of length. Lines that directly connect two nodes are sometimes referred to as chains, edges, segments, or arcs. % Polygons are two-dimensional features created by multiple lines that loop back to create a “closed” feature. In the case of polygons, the !rst coordinate pair (point) on the !rst line segment is the same as the last coordinate pair on the last line segment. Polygons are used to represent features such as city boundar- ies, geologic formations, lakes, soil associations, vegetation communities, and so forth. Polygons have the properties of area and perimeter. Polygons are also called areas. 1 Aronoff, S. 1989. Geographic Information Systems: A Management Perspective . Ottawa, Canada: WDL Publications. Chapter 1: De!ning a Geographic Information System