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[RC] Doesn't account for altitude (was: Pueblo day 2) - k s swigart

Bruce Weary said:

Except for the fact that most GPS units, I'm told
?aren't able to take into account changes in elevation,
?and as such, as much as 10% or more of the distance
?traveled over hilly country may not be accounted for,
?thus leaving a shortened distance showing on the GPS.
Let's say, for example, that a 55 mile ride has 10,000 ft of climbing (which 
most people would consider a pretty substantial amountof climbing)...even if 
ALL of that climbing were straight up (so there was NO forward motion during 
the climb, which is, of course, impossible, but it would count as a "worst 
case" scenario with respect to the fact that a GPS doesn't account for altitude 
change) then that would be 10,000 ft (~1.9 miles) of distance that wasn't 
accounted for by the GPS.? And 1.9 miles is ~3.5% of 55 and if the ride also 
went straight back down the 10,000 ft then that would be a total of 7.5% not 
accounted for.
For the types of slopes one usually finds at endurance rides (10,000 ft would 
be an average of about 7% but let's say that there IS some flat in the ride and 
the rest of it has a 10% slope) that would make the GPS off by ~0.5% (i.e. 
about 1/4 of a mile).? And an endurance ride that has about 19 miles of 
climbing at 10% grade and 19 miles of decent at a 10% grade does not just have 
"significant" climbing, it would, by virtually anybody's estimation be 
considered bloody steep.
So...a bloody steep 55 mile ride would be measured about 1/4 of a mile short 
due to the fact that a GPS doesn't measure altitude change along with forward 
If GPS's routinely measure endurance rides as being shorter than the sanctioned 
distance, I am willing to go out on a limb and say that it has pretty much 
NOTHING to do with the lack of measurement of altitude change.? Even if all 
10,000 ft of climbing in a ride were straight up and down (literally) it 
wouldn't make the thing off by 10%.
There may be some other technical explanations, I am not THAT familiar with GPS 
technology (unlike my familiarity with geometry and trigonometry which accounts 
for the calculations above) to say that the device cannot introduce 
inaccuracies.? However, I am a big believer in Occam's Razor and figure that 
the best explanation is the simplest one, and that the reason GPS's have a 
tendency to measure endurance rides to be shorter than their sanctioned 
distances is that many of them ARE shorter than their sanctioned distances.
Certainly, when I have compared the distances that my GPS (a Garmin 
205)?provides, they are almost always easily within about 1% of the other 
measurement techniques I have had access to, including surveyor's wheels, 
mountain bike odometers, OHV odometers, and officialand unofficial (usually the 
most inaccurate)?surveys as well as "odometer checks" on the side of roads.? I 
checked its accuracy against just about everything I could before I was willing 
to take its word for it...and I continue to do so (although I HAVE found that 
my GPS often measures things a little bit longer than some other people's; it 
routinely measures longer than a friend's Garmin 201).
Orange County, Calif.

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