Doing some quick calculation on the GPS (horizontal) speed vs. the true down hill speed, I found it to be somewhat interesting. And thought I would pass it along to anyone who may be interest in it.
If my trigonometry and vector analysis is correct (it has been a long time), to get your true ground speed going down a slope, you would divide the horizontal (GPS) speed by cosine A (A is the angle measure between the horizontal line and the downward line of the slope). Or stated another way: true slope speed = GPS speed / cos A.
Another way of looking at this, is that cos A x 100 will give you what the percentage of your GPS speed is of your true slope speed.
For example, take the following three slope angles:
10 degrees (probably a moderately steep road grade)
30 degrees
45 degrees (probably a moderately steep ski slope grade)
cos 10 = .984 or 98.4 % of your true slope speed
cos 30 = .866 or 86.6 % of your true slope speed
cos 45 = .707 or 70.7 % of your true slope speed
At 50 mph GPS speed the differences are:
10 slope -- true slope speed is 50.8 mph
30 slope -- true slope speed is 57.7 mph
45 slope -- true slope speed is 70.7 mph
