Most of the road courses in this area are very close to what could be called "pancake flat." I tend to use my wrist-mounted GPS unit as an early-warning system of sorts when measuring.
During a recent (10km) project, on a course with a start/end elevation of 11ft, max of 84ft, and min of 7ft - total elevation gain/loss of 323ft, I was occasionally surprised by the GPS (mile) split chime going off past the intermediate split count. "No worries," said I, as I walked my bike back to the intermediate split, set my mark, and measured my relative landmarks.
Looking at my GPS data later it didn't surprise me much to see my total distance at ~6.18. You see, the (figure-of-eight-shaped)course has tight turns and about 40% tree coverage, on top of the elevation changes, too.
Two weekends after the race, a young lady who ran the course mentioned that her GPS unit showed a total distance of 6.2 miles. I began scratching my head about this and did not know whether this could be caused by the typical technological limitations of consumer/sport GPS units (single point-horizontal error) or if it's also because of vertical error.
I read a brief review on DC Rainmaker's site about elevation issues (https://www.dcrainmaker.com/2010/05/understanding-sport-device-gps.html) ...naturally, most "typical runners" don't have units with barometric altimeters.
For giggles, I pulled a Google Earth measure of a section of road used for at least two of the most-popular races here in town. One end is at an elevation of 69 feet, the other end at 56. "Point-to-point" distance was 2270 feet, but when elevation - all that Pythagorean theorem stuff, which I took the time to do for each intermediate point Google gave me - was taken into account, the "point-down-the-hill-up-the-hill-to-point" distance was an additional 3.4 feet. Yeah, that's an added .15%.
Maybe I'm simply barking at airplanes because this is a first-seen for me. Do folks who actually have elevation changes deal with this sort of thing when measuring?
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