Analysis of Networks - Introducing Geographic Information Systems with ArcGIS - page 587

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 can depend on what time of
day it is, whether there is construction ahead, and whether there is an accident. In other words, a truly
effective network needs to be sensitive to the time of day, and, better yet, sensitive to traffic conditions.
The need for “dynamism” of this sort is well beyond our scope here. What follows in the next section are
the basics of network analysis, given that the network is already built.