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Re: [RC] Old Pueblo Day 2 - Truman PrevattActually GPS does a wonderful job with altitude - why do you think the military uses it in aircraft. The issue with GPS and altitude is blockage. In order to calculate three dimensions effectively it needs visibility to five to six birds. When you are flying in an aircraft you see probably 7 depending on latitude and altitude - no problem. When you are on flat ground with no blockage on the horizon you can see six. Now throw in blockage, that damn mountain over there, if not for these two canyon walls, that blooming stand of trees, etc. you are down to about four. At that point it defaults to use the WGS 84 geoid and you location is laid over onto the geoid. That not only doesn't account for altitude but your altitude above the geoid (which is more or less mean sea level but not really since mean sea level is not constant over the earth) produces an off set in your location.Probably a bigger issue is your visibility will cause large errors when you get significant blockage which can happen in the above conditions. I've measured a know trail that was 2 miles with heavy tree cover and it came out 3/4's of a mile. I would be very skeptical of any trail claimed to be measured with the distance calculation function in a GPS unit. If you are going to use GPS, use a unit that will save the data say every second or 5 seconds and download that to a GIS mapping package and calculate the distance in the GIS package. Truman Bruce Weary DC wrote: 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 -- "There is always a well-known solution to every human problem--neat, plausible, and wrong." H. L. Mencken =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Ridecamp is a service of Endurance Net, http://www.endurance.net. Information, Policy, Disclaimer: http://www.endurance.net/Ridecamp Subscribe/Unsubscribe http://www.endurance.net/ridecamp/logon.asp Ride Long and Ride Safe!! =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
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