How to Determine Slope From a Long Section Plan

Determine the highest elevation of the slope on the long section. Elevation tags or dimension strings mark the height of the elevations. Note the elevation and find the lowest elevation of the slope. Subtract the lowest elevation from the highest elevation to find the elevation change.

If there is not an elevation tag or dimension string providing the elevation, trace a horizontal line from the highest point over the lowest point and measure the dimension between the two elevations using an architectural or engineering scale. Ensure you use the correct drawing scale with the architectural or engineering scale - the drawing scale will be noted in a corner of the long section drawing.

Determine the horizontal distance between the highest and lowest elevations in the section plan by drawing a vertical line through both the highest elevation and the lowest elevation. Find and note the horizontal distance between the two lines reading the dimension strings or using the architectural or engineering scale.

Some long sections use a different scale for the horizontal run from the vertical run because of the size of the land relative to the drawing. Make sure you use the proper scale for the horizontal run, which will be given with the vertical scale. If the drawing does not mention a horizontal and vertical scale change, assume the horizontal and vertical scales are the same.

Divide the vertical elevation change by the horizontal distance. Ensure both the vertical and horizontal dimensions are in the same unit of measurement. The quotient is the percentage of slope. To find the slope angle in degrees, calculate the inverse tangent of the vertical elevation change divided by the horizontal distance.