Ok, first of all GPS uses trilateration, which is the measurment of the rate of change between two point or more know points. This is why the clocks on GPS satellites are so important, you have to have exact time to do this. Triangulation is the measurement of the difference in angles, not the same and isn't used in GPS no matter what most people think. Now that being said, GPS is very inaccurate in the Z plane, which is elevation. Particularity in the MOUNTAINS. Now the reasons for this has to do with the fact that there is a model of the earth that was created for GPS. This model of the earth doesn't, nor can it take into account anomolies in the earth's surface; ie, mountain ranges, deep canyons. The closer you are to sea level the more accurate the GPS is in the Z plane. Also there is huge difference in accuracy when it come to sign processors, the more you pay the more accurate they are. We are talking differences of up to 10s of meters. The signal processors in planes and for surveying equipment, is high dollar and very accurate, not so with the GPS on your mountain bike, snowmobile, car or you are holding in your hand, they are bare bones and spit out the basics, now are they better then 5 years ago, very much so. However they don't have to be super accurate, just good enough. Also ski areas are generally located on the sides of mountains and down in canyons another issue with GPS, they operate line of sight, if you are locked into three satellites and go behind a ridge and block one out, the processors tend to make for the differences by basically faking it. Now I know some of you will argue with me on this, but rest assured I know what I'm talking about. Now I also didn't get too technical on purpose.
My point of this being, yeah you may get speeds from those these gadgets and it is fun, but I won't hang my hat on saying that I went 73 because it says so on my pocket GPS. Or that you skied 14,420 vertical in 3 hrs. You may have got 14000 ish vertical, but I won't get too fired up about the last 400 ft. If you want to know how fast you can ski, find a timed course. If you are hung up on how much vert you got, buy a topo map. These things are toys and are meant to be fun, but they aren't that accurate. My friend has one and he broke it out and was showing me, I laughed and went into my speech, while riding Headwall at Squaw. He called me a buzz kill, LOL. I told him I'm sorry that the facts got in the way of his toy.
Originally Posted by MojoMan
The only difference between military and consumer GPS unit is the sophistication of the maps and the durability of the device itself. The military also uses DOD satellites systems that are carefully monitred for reliability, signal strength, and there are numerous backup systems.
Having served in Kuwait I can assure you that military issued GPS can suck about as badly as the consumer devices. GPS relies on passive signals that use triangulation. Any GPS unit can lose coverage during atmohepheric distrubances. They are also prone to sunspots or other fits. They also can spit out bogus data. No special magic occurs just because the GPS unit is in use by a member of the military. The DOD cannot alter the laws of physics.