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Unit 3: Topography & Fire Behavior

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Topography   ::   Canyons & Drainages   ::   Retarders   ::   Slopes   ::   Exercises

Determining Slope Percent From Topographic Maps

Earlier in the unit we learned that the slope percent has a direct effect on rate of spread. The steeper the slope, the faster the rate of spread. We generally measure slope in slope percent. Slope percent is considerably different from degrees of slope, and the two should not be confused. The graphic below gives an example of a 40-percent slope. This means 40 feet of rise in 100 feet of horizontal distance.

Slope percent vs degree slope

Field Measurements For Determining Slope Percent

We're going to discuss some ways in which you can determine the slope percent in order to perform rate-of-spread calculations. There are several ways of taking necessary measurements in the field to determine slope percents:

  1. Direct readings from an abney.
  2. Elevation readings with an altimeter.
  3. Distance measurements from a map.
  4. Mileage readings from vehicles.
  5. Sightings on power poles, trees, and axe handles.

The first method is preferred if an abney hand level is available. Items 2 through 4 provide measurements for inputs into a formula which will give you slope percent. The last method gives a perpendicular from which to quickly estimate slope degrees or percent.

Determining Slope Percent From Topographic Maps

A means for determining slope percent, without actually being in the field, is the use of a standard topographic map with contour lines. The most common are the USGS quadrangle maps. This page gives several ways that you can use to determine slope percents from topographic maps. The first is the use of the slope indicator, which matches contour lines to those on the map. The following two formulae can be used on any contour map:

Formula 1: Use on any contour map or in the field.

Formula 1

We do want you to learn to use the two formulas on calculating slope percent. The first can be used with data taken in the field or from a topographic map. It simply states that slope percent equals the rise in elevation between two points, divided by the horizontal distance between those points times 100.

Formula 2: Use on any contour map.

Formula 2 is for use with contour maps only. It states that the slope percent is equal to the contour interval in feet, times the number of contours from point to point, divided by the horizontal distance between those points in feet, times 100.

Formula 2

Steps to determining slope percents using formula 2:

  1. What are the contour intervals on the map (feet)?
  2. What is the map scale (inches per mile)?
  3. What are the elevations at points on a slope (use contour lines)?
  4. What is the difference in elevation between those points?
  5. What is the distance between points in feet (use distance scales)?
  6. Calculate slope percent using the formula.

Copyright 2008, by the Contributing Authors. Cite/attribute Resource . admin. (2005, October 20). Unit 3: Topography & Fire Behavior. Retrieved January 07, 2011, from Free Online Course Materials — USU OpenCourseWare Web site: This work is licensed under a Creative Commons License Creative Commons License