How to Calculate TOPO Grade

A topographical map, known as a "topo," provides information about the mapped area. The map is to scale, along the bottom. The USGS is the governing body for United States topo maps and common scales are 1:24,000 up to 1:500,000 for many Alaska quadrangles. In addition to routes, roads, pathways and navigation directions, a topo map represents slopes and grades of hills and mountains. They show these via striation lines on the map.

Things You'll Need

  • Magnifying glass
  • Compass
  • Pencil and paper
  • Scientific calculator
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Instructions

    • 1

      Lay out your map and draw out a line over the slope angle you are trying to find. Locate the contour lines on the map route. These are brown lines that spread out between each other for low slopes and get denser to each other for steep slopes. Locate the contour interval for the map in the topo map legend at the bottom.

    • 2

      Find the lowest contour line and get its elevation. Find elevation as a number along the contour line. Do the same with the top contour line of your proposed route. Subtract the low elevation from the high, giving you the elevation difference for the route.

    • 3

      Use the map scale markings (found on the edges of the maps) to calculate the horizontal distance of the route. (Example, between point A and point B.)

    • 4

      Take the elevation difference number and divide it by the horizontal distance number. Take the result and multiply this by 100 for the percentage of the grade line.

    • 5

      Take the percent result from Step 4 and enter it into the scientific calculator. Use the "arc" tangent function to get the slope angle for your proposed route.