Standard LANDFIRE slope themes are represented in units of degrees (°). Many locally produced landscapes over the year’s stored slopes in percent (%). It is much easier to estimate slope in %, estimating the elevation change and the horizontal distance and calculating the ratio. BehavePlus, and BEHAVE tools before that, default to slope input in %.
To convert from slope in degrees (°) to slope in percent (%), a scientific calculator is needed.
The process for directly calculating slope with measurements from a contour map:
1. Determine the contour interval. This is the elevation change between adjacent contour lines.
Example: 40 ft.
2. Determine the map scale and conversion factor. The map scale must be found in terms of the number of feet that each inch on the map represents (ft/in).
a. Map scales are usually given as the number of inches per mile, such as 2 in/mi, or as a representative fraction such as 1:31,680.
b. Use the spacing of section lines to determine the map scale. Normally section line spacing is 1 mile; be careful of foreshortened sections; look around on the map and find square sections with equal spacing. Measure the distance with a ruler graduated in inches and tenths of inches. Divide 5,280 by the map distance between section lines.
Map dist. between section lines = 2.64 in
3. Determine rise in elevation by counting contour intervals and convert to feet.Example: 11 contour intervals
11 x 40 ft/contour interval = 440 ft.
4. Measure the horizontal distance with a ruler graduated in inches and tenths of inches, and convert to feet with the map scale from step 2.
Example: Map dist. of slope length = 1.2 in
1.2 in x 2,640 ft/in = 3,168 ft.
Divide the rise in elevation from step 3 by
the horizontal distance from step 4.
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