GIS For Dummies®

Chapter 15

Working with Networks

In This Chapter

Figuring out how connected a network is

Working with impedance values

Accounting for directions of movement in linear features

Modeling circuits, turns, and intersections

Putting it all together to model and route traffic

The Earth is filled with linear objects. You can find treelines, fault zones, shorelines, fences, rows of houses — the list goes on. Some linear objects are special not so much because of what they are, but how they work. These special linear objects are called networks. Networks are collections of connected linear objects such as roads, railroads, or rivers that branch from place to place. They come in different sizes, numbers of branchings, and angular configurations. One classic type of network is the stream network, which allows movement of water along its length. A network that has this movement along its length is called a corridor. This chapter covers the different properties of corridors and shows you how to measure them.

Measuring Connectivity

You can measure connectivity in a network by comparing the number of actual node-to-node links that exist in a given network to the maximum number of nodes that are possible. This measure of connectivity is called the gamma index. Usually, the index ranges from a value of 0 (which indicates no connected links at all) to 1 (where all possible links are connected). Both general and transportation-specific GIS software packages contain algorithms that allow you to ...