Network analysis - Spatial Data Analysis: An Introduction for GIS Users - page 75

6

Network analysis

Introduction

6.1

h e previous chapter dealt with combining information from dif erent data layers,
with one objective being to dei ne connections between objects. h is chapter is con-
cerned with connections between places within networks. In particular, methods for
the characterization of network complexity and for ascertaining shortest paths
between start and end points are detailed. In Section 4.3, approaches for measurement
of line lengths were discussed while information on connectivity of vector features
was considered in Section 2.2.3. h ese can be put to use in the analysis of networks.
For instance, i nding the shortest route between one place and another through a road
network is an example of a network analysis approach that necessitates measurement
of distance and information on the connections between arcs (representing, for exam-
ple, roads). Many real-world applications make use of such approaches—the determi-
nation of optimal routes for emergency vehicles is one such case. In this chapter some
key ways to explore networks are detailed. h e account is selective, but will provide
a basis on which readers can build.

Networks

6.2

Figure 6.1 shows the constituent parts of a simple network. h ese include arcs, vertices
(representing a change in direction of the arc), and nodes at the end of each arc and
connecting dif erent arcs.
Each segment of a network is associated with an impedance factor. In many cases
this is simply the distance between nodes, but some alternative factor such as travel
cost or time might be used instead. Before network analysis can proceed it is necessary
to determine the impedance factors. For a road network, penalties on let turns or
right turns (depending on the side of the road on which vehicles drive) might be
imposed and u-turns, for example, may or may not be allowed.