knox harrington wrote:
It doesn't make much difference until the grade gets really steep. For 1mi at a steady 15% (crazy steep for a paved road btw):
vert/terrain distance -> 792' gain
vert/horizontal -> 783' gain
People often use terrain distance instead of horizontal distance because it's easier to measure. In cases where the difference between the two methods is unacceptable, I believe rise/run is used.
Maybe I'm misinterpreting your post, but it looks like you've got this backwards.
If you cover one horizontal mile up a 15% grade, you've gained 792' elevation. (15% rise/run * 5280' run = 792' rise)
If you cover one mile along the hypotenuse (aka the road) you've gained 783' elevation. (15% rise/run * cos(atan(0.15 rise/run)) * 5280 rise/run = 783' rise)