Calculating the slope on a topographic map is similar to calculating the slope of a line on a graph (rise/run). When we look at contour lines, the rise is the difference in elevation between two contour lines. The run is the horizontal distance between those same two contour lines.
To calculate the slope on a topographic map, select two contour lines that you want to calculate the slope between. Determine the vertical distance by identifying the elevation values for each contour line you have selected. Then measure the horizontal distance between these two contour lines by using a ruler. We want to make sure the measurement is in the same unit as the elevation distance. We now have enough information to calculate the slope. Use the following formula:
Slope = change in elevation between two points / horizontal distance between those same two points
We can also calculate the percent slope by multiplying the slope ratio (determined above) by 100.