Topographical maps include lines that denote elevations of land.
Maps are used to depict an area of land in two dimensions. Because land is rarely flat and often has various changes in elevation, topographical maps have been designed to show the rise and fall in the contour of the land. This is depicted with contour lines, which represent specific elevations of the land. Topographical maps include a key that tells the difference in elevation between each contour line.
These lines can be used to calculate the slope of the land, which is the rise or fall in elevation compared to the horizontal distance of the land, also known as the run.
Instructions
1. Choose an area on the map for which you want to calculate slope. Be sure to note how much the elevation changes between lines on the map. For example, a map may have a 10-foot change in elevation for every contour line.
2. Use a ruler to measure the distance across a series of concentric contour lines. Use the map's key to determine how far of a distance this is. For example, if you measure 3.25 inches across four contour lines, and according to the key, 1 inch equals 400 feet, your horizontal distance is 1,300 feet.
3. Determine the change in elevation by counting the contour lines and subtracting one. Then multiply that number by the change in elevation between lines. For example, if you measured across four lines, there were only three increases in elevation. If the land rises 10 feet between each contour line, the rise of the land is 30 feet.
4. Divide the rise of the land (30 feet) by the horizontal distance, or run of the land (1,300 feet). The result will be a decimal between 0 and 1. In the example, the slope is 0.023, which rounds to 0.02.
5. Multiply your answer by 100 to express the number in a percent. In the example, the slope is a 2 percent grade.
Tags: contour lines, change elevation, contour line, horizontal distance, land feet
