Originally Posted by
Doge
It is supposed to the the tangent of the angle * 100. So rise over run on the horizontal not the hypotenuse.
45 degree is 100% grade.
But...not everyone does it the way they are supposed to.
30% grade is arctan .3 = 16.7 degrees.
Yes, but when you are calculating the MAXIMUM grade over a multi mile climb, you have to pick a interval for intermediate measurements since you can't take the instantaneous grade at every point on the road. And most of the road I've climbed with > 10% grades usually are in crappy shape, so you can probably find different spots on the same parallel that have different grades.
Civil engineers probably figure the grade for some shorter segment like every 100 yards and then pick the steepest of those.