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Ways to Determine Slope of Land

Any piece of land with a slope poses potential problems and challenges. Construction crews need the right equipment to clear trees and level the land prior to construction, and water drainage needs to be factored in. Slopes present challenges for getting fire crews to bring in equipment, calculate pump pressure for extinguishing fires, and assess a fire's behavior. Geologically, slopes influence rainstorm erosion and potential contamination of the water table and water sources.

Slope Defined
- A slope can be defined as a grade, slant, inclination, pitch or tilt. It is the degree of incline or steepness to a piece of land, usually stated as a percentage. A calculation of "rise over run" --- or rise divided by run, muliplied by 100 --- might produce a finding for a 1-percent slope, which has a 1-foot rise for every 100 feet. Slope can be determined using a topographic map or a clinometer. A slope of 11 percent is equivalent to about 6 degrees, a 20-percent slope equals an 11-degree rise, a 50-percent slope roughly correlates to a 26.6-degree rise, and a slope of 100 percent compares to a rise of 45 degrees.
Determining Slope with a Clinometer
- Clinometers are a component of many compasses and can be used to estimate slope and measure elevation. To use the clinometer, first determine the clinometer's scale. Face the slope while holding the clinometer vertically. Align the clinometer's horizontal band with the slope and, keeping both eyes open, read the scale indicated by the horizontal band, either in percentages or degrees.
Determining Slope with a Topographic Map
- Topographic maps offer two main pieces of information to help determine slope: the contour interval and the map scale. Using the contour interval, determine the rise --- also known as the vertical distance or difference in elevation --- by first counting the number contour lines (e.g., 6 lines) between the bottom and top of the slope. Then multiply the figure by the contour interval value (e.g.,100 feet) to find an elevation change of 600 feet. Next calculate the slope's run --- or vertical distance --- by using a ruler to measure the distance from the bottom to the top of the slope in inches. Compare that measurement to the map scale. If the map scale is 1:24,0000, and the run of the slope is 1 inch, then the run is 24,000 inches, or 2,000 feet.
Calculating Slope Percentage
- The topographic map example above produced a rise of 600 feet and a run of 2,000 feet. Determine the slope in percentages with the following equation: vertical distance (rise) / horizontal distance (run) x 100 = slope percentage; so, 600 feet / 2,000 feet x 100 = 30 percent slope.

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