5.5: Connectivity Analyses

Connectivity analyses use functions that accumulate values over an area traveled. Most often, these include the analysis of surfaces and networks. Connectivity analyses include network analysis, spread functions, and visibility analysis. This group of analytical functions is the least developed in commercial GIS software, but this situation is changing as commercial demand for these functions is increasing.

Vector-based systems generally focus on network analysis capabilities. Raster-based systems provide visibility analysis and sophisticated spread function capabilities.

Spread Functions (Surface Analysis)

Spread functions are raster analysis techniques that determine paths through space by considering how phenomena (including features) spread over an area in all directions but with different resistances. You begin with an origin or starting layer (a point where the path begins) and a friction layer, which represents how difficult—how much resistance—it is for the phenomenon to pass through each cell. From these two layers, a new layer is formed that indicates how much resistance the phenomenon encounters as it spreads in all directions (see Figure 5.19).

Add a destination layer, and you can determine the “least cost” path between the origin and the destination. “Least cost” can be a monetary cost, but it can also represent the time it takes to go from one point to another, the environmental cost of using a route, or even the amount of effort (calories) that is spent.

Figure 5.19: Spread Functions. This example shows that the shortest distance is not always the least cost distance.

Viewshed Modeling (Intervisibility Analysis)

Viewshed modeling uses elevation layers to indicate areas on the map that can and cannot be viewed from a specific vantage point. The non-obscured area is the viewshed. Viewsheds are developed from DEMs in raster-based systems and from TINs in vector systems. The ability to determine viewshed (and how they can be altered) is particularly useful to national and state park planners and landscape architects. Figure 5.20 depicts the areas within a park where a proposed radio antenna can be seen.

Figure 5.20: Viewshed Analysis. Map courtesy of the National Park Service, Department of Interior, 2007.

Network Analysis

Network analyses involve analyzing the flow of networks—a connected set of lines and point nodes (sometimes called centers or hubs). These linear networks most often represent features such as rivers, transportation corridors (roads, railroads, and even flight paths), and utilities (electric, telephone, television, sewer, water, gas). Point nodes usually represent pickup or destination sites, clients, transformers, valves, and intersections. People, water, consumer packages, kilowatts, and many other resources flow to and from nodes along linear features.

Each linear feature affects the resource flow. For example, a street segment might only provide flow in one direction (a one-way street) and at a certain speed. Nodes can also affect flow. A stuck valve might allow too much of a resource to stream out and away from its intended destination. Network analysis tools help you analyze the “cost” of moving through the network. Like spread functions, “cost” can represent money, time, distance, or effort. Network analyses are vector-based applications, but there are similarities with raster-based spread functions.

The three major types of network analyses include route selection (optimal path or shortest path), resource allocation, and network modeling.

• Route Selection attempts to identify the least “cost” route. As described above, cost can be defined a number of ways. You might want to find the shortest path between your home and a weekend destination or the least costly route that delivers UPS packages to their recipients. In any route selection routine, two or more nodes, including an origin and a destination point, must be identified and be able to be visited on the network. Sometimes there are a large number of possible routes. It is the job of the network analysis algorithm to determine the least cost route. Multiple paths are tested until the least cost path connects the starting and destination points.
• Resource Allocation, the second major type of network analysis, involves the apportionment of a network to nodes. To do this, you define one or more allocation nodes on the network. Territories of linear features, like streets, are defined around each of these allocation nodes. The linear features are usually assigned to the nearest node, where distance is measured in time, length, money, or effort. Figure 5.21 depicts 4-minute response times from six fire stations and three potential fire station locations. The polygon that is drawn around each station (triangle) represents the area that can be covered in 4 minutes.

Figure 5.21: Resource allocation. Map is courtesy of Tyler Schrag, Bellingham Fire Department, 2006.

• Network Modeling uses interconnected linear features and point nodes to analyze how resources travel through networks. The linear features, like streets or river channels, have attributes that might define travel speed, number of lanes, and volume of flow. Nodes also have attributes that might identify vehicular turns and the time or cost required for each turn. Resources like water or traffic are placed in the network and their movement modeled. This way, problems with the network load can be identified.