The grade of a road is expressed as a percent, and defined as the vertical gain or loss
divided by the horizontal distance times 100. Even though you don't know the horizontal
distance, you can still calculate the grade. This application first finds the horizontal
distance using the elevation and distance along the road, then uses the elevation and the
horizontal distance to calculate the grade. The solution is exact, not an approximation.
A flat road is 0% grade, while 100% grade is a 45 degree angle.
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