Estimating a slope is important. Whether the area under consideration is a smaller, front yard pathway or a large expanse (and everything in between!); consideration given to percentage of slope will be used to properly irrigate an area (drip or spray), calculate fire behavior for areas prone to fire hazard/rate of spread, pump pressure to reach a location, proper plant material or pathway material, and how to navigate the slope.

To calculate a slope, you will need the following:

• Vertical Distance (also referred to as Rise) – This is the difference in elevation between two points; it is calculated by subtracting the elevation of one point from the elevation of another point.
• Horizontal Distance (also referred to a Run) – This is the distance from one point to the other and is calculated by measuring distance with ruler, using a map or landscape concept scale. For example, if the concept scale is 1:24,000 and the distance between the two points when measured with a ruler is 1/2 inch, the horizontal distance would be 12,000 inches or 1,000 feet.

Slope can then be calculated using the slope formula:

Vertical Distance/Horizontal Distance x 100 = % of slope

Another way to write the slope formula is:

Rise/Run x 100 = % of slope

There are a number of slope calculation aids in the form of tables that show the relationship between map scale and counter level. Be careful when using slope calculation aids because they are most often tailored to specific map scales and counter intervals.