Avalanche Terrain

You need to be familiar with the terrain in your forecast area. Plotting avalanche paths and risk zones on aerial photos or topographic maps is very helpful.

Several instruments, such as a slopemeter and inclinometer, can be used to determine the angle of a slope in the field. If you’re not in the field, you can determine slope angle using a topographic map, ruler, and scientific calculator (select INV, then TAN).

  1. From the map, determine the elevation gain of the slope in question (the rise), then measure the horizontal distance (the run) using the map scale and a ruler
  2. Using the calculator, divide the rise by the run (in feet or meters) and take the arctangent of that value. If your calculator does not have an arctangent function, use the following table to get an approximate value:
    Table for getting an approximate arctangent value

As an example, we’ll determine the angle of the Glory Bowl area. The straight blue line on the topographic map below marks the approximate location of where Joel Roof was snowboarding. To determine the slope along the blue line, get the rise and the run from the map.

  • For the rise, the starting point (left end) is along the 9,880-ft contour, the ending point (right end) is along the 9,600-foot contour
  • Subtracting the two yields a rise of 280 feet (85 m)
  • To determine the run, the map scale shows that the blue line corresponds to a 400-ft long linear distance
  • Therefore, the calculation for the slope is: arctan(rise/run) = arctan(280/400) = arctan(0.70)=35 degrees

Topo map of the Glory Bowl area annotated with information for determining slope angle

Summary table:

Relationship betwen terrain and avalanche potential