Determine the area that you want to calculate the slope for. Draw a straight line perpendicular to the shape of the slope and be sure to draw the line so that it starts and ends at the top and bottom of the map or graph you are using. For the best results, choose an area to measure where the slope direction does not change (i.e., avoid hills and valleys).
Measure the line that was drawn and use the map to convert the length of the line into feet. Determine the total elevation change along the line you drew by subtracting the elevation of the lowest point on the map from the highest point. This number will be the slope.
Divide the elevation change that is found between the highest and lowest point by the distance of the line that was drawn (in feet). This will give you the tangent value of the slope angle. To obtain the angle of the slope in degrees, enter the tangent value and hit the "Inv" button followed by the "Tan" button on any scientific calculator. The calculator will now display the angle in degrees.