Introducing Geographic Information Systems with ArcGIS: A Workbook Approach to Learning GIS, 3rd Edition

Analysis of Networks

OVERVIEW

In ArcGIS Version 9.1, Esri introduced the Network Analyst in ArcGIS Desktop. The capability to do network analysis has been present in ArcInfo for years, but it had to be accessed through typed commands, rather than by point-and-click activities.

The sorts of networks that Network Analyst deals with are primarily composed of roads. You might think of network analysis as sort of a simulation of a vehicle, confined to streets, whose driver wants to get from A to B by the shortest route or in the least time—staying on the roads, of course. Or network analysis can develop service areas, considering the network, such as assigning voters to the nearest polling place.

To analyze a network, one needs a network to analyze—and this is no mean feat. First, you have to have all the sort of data that is, very roughly, supplied by TIGER-like files. Then the actual lengths, in linear units—not degrees of latitude and longitude—of segments must be developed. The connectivity of segments must be ensured. Since time is usually as important as distance, the time to traverse each segment needs to be produced. While traffic signals and turns from one segment to the next aren’t reflected in distance calculations, they very much affect time. So tables, for each intersection, that describe averages of how long it takes to turn right, turn left, or go straight are needed. Also, the 99 percent of you who have driven a car know that the time it takes to drive a particular block ...