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.