Now you have the raw material (those speeds at which cars going to the park may travel) for building a cost surface. What numbers do you put in the cost surface cells? You cannot put miles per hour (mph), because then higher speeds would imply greater resistance to flow—the opposite of what you want. You could use the reciprocal, hours per mile. To get this reciprocal, you simply divide “1” by miles per hour, which gives you hours per mile. For example, 50 mph is 0.02 hours per mile; 25 mph is 0.04 hours per mile. Since 0.04 is a larger amount than 0.02, and that larger amount represents a slower speed, 0.04 is what you want to see in your cost raster.
But there is another consideration: It is important to express the cell costs in terms of the units of the raster and the problem statement. You are interested in drive times in minutes. And the raster is measured in feet. The numbers you have been given to work with are miles and hours, but what you want, strange as it may seem, is minutes per foot! How do you convert hours per mile to minutes per foot? Multiply hours per mile by a conversion factor: 0.0113636, which is the number of minutes it takes to travel a foot at the rate of one hour per mile.12
____ 17. Create the travel cost surface:
Find and start the Raster Calculator. Enter the expression13
that will convert mph into minutes per foot for every cell in the raster:
1.0 / “Speeds” ∗ 0.0113636
Call the output raster Cost_Surface, heading it into ...