Cost surfaces

A cost surface represents some factor or combination of factors that affect travel across an area. For example, steep terrain can increase road construction costs, so the slope of the terrain is a cost factor.

Percentage slope values do not, in themselves, indicate whether costs are high or low. To reflect the cost, or to create the cost surface, you must transform the slope values to cost values, such as dollars, or rank the slope values using a common scale.

One way to reflect the cost of traveling through a cell is to rank the cell values using a common scale, (e.g., numbers from 1 through 9). A ranking of 1 might indicate that the cost of traveling through the cell is very low, while a ranking of 9 may mean that the cost of traveling through the cell is very high.

A particular area can have many cost surfaces: one for each factor. For example, if an area has slope and snow depth as factors affecting travel across it, then the area has two cost surfaces.

In order to combine two cost surfaces, the values of both cost surfaces need to be ranked based on a common scale.

When you use ranked values in a cost surface, you are creating a manageable range of values from which you can distinguish the good from the bad. In other words, the value 9 is not 9 times more costly than the value 1. It is simply the most costly value.

Determining which factors affect costs and ranking cost surface values can be a very time-consuming process. You may need to consult experts about what the costs might be, or form a consensus among your peers. Keep in mind that a single, final cost surface is required input for the Cost Weighted Distance function.