GPS Compensate for elevation????

[14802]

Is there a GPS unit,or a setting to compensate for elevation change? Say you are tracking your current total miles,(by GPS) and you go from 1000 ft above sea level to a peak at 4500 ft ASL. Does it and/or is there a setting to correct your distance??? The only resaon is satalites only track from above, straight line, point A to B.Could be showing only 2 miles. But really during terrain changes you may be at 2.5 miles..Over a long race you could be off by alot of miles.. I will call Scott @ P.C.I. ,but thought I would ask here first..

[ntsqd]

The GPS Sat's don't track you, your GPS unit tracks the Sat signals. Other Sat's track you. :)

GPS works by checking the time difference btwn when the time-stamped signal was sent and the real time (all based on the atomic clock in Colorado). Once it has at least three of these time differences (it already knows where the Sat's are) it does a triangulation calc. From that it knows the true distance from the previous calc'd position. In other words, it knows the new 3 dimensional position and calcs the true distance from the last 3D position.

That being said, Elevation is the direction that GPS will likely have the most error in.

[ChuckH]

Accuracy is the same in X Y or Z planes, elevation has the same accuracy as the other planes, if it wasn't the FAA wouldn't allow aircraft to navigate in bad weather with GPS.

[Chase 2]

The precision of GPS elevation data is indeed the lowest of what your GPS unit calculates. Typical GPS accuracy is within 10 feet of where you are on a map. I guess my question is why would you need to be more accurate than that? If you are trying to use your GPS for some type of odometer, then you would be better off using some other means. But, if you are using odometer mileage for something like fuel consumption estimation, remember that wheel spin will throw off what you are trying to accurately calculate much more than errors introduced by elevation gain or loss with a GPS unit.

[ChuckH]

A GPS has to calculate where your at in 3D space, now a cheap unit might show elevation in say 100 foot units but it has know the true height or it wouldn't know the correct Latitude and Longitude, if anything thats dumbing down the data displayed so you have to buy a more expensive unit.

[14802]

It all is a make alot more sence now that, it has been pointed out. I thought that it(GPS) might not be able to track elevation changes. Now knowing that it is based off longtitude and latitude, it hit me in the head like a hammer. DUHHH.. Thank you for the replies, I appreciate how knowlegdable you guys are...As far as odometer accuracy how good can a GPS get? I understand wheel spin from your standard odometer is not accurate(not that I have one). I was just interested if GPS odometer is somewhat accurate to 1/10th mile+ or- VS. a standard odometer on highway conditions.

[ChuckH]

For better accuracy get a GPS antenna so you can receive more satellites, you want one that can recive more channels/satellites, a WASS enabled GPS is even more accurate, im not sure how much WASS would help in mexico its supposed to just be for the USA.

[Chase 2]

A typical GPS unit available to the public can get you within 10 feet, which is better than the 528 feet or 1/10 miile you seem to be looking for. WASS correction increases that slightly but is only found in the states. If you are looking for a cheep odometer you might try a wireless bicycle computer with the pickup mounted to the front wheel which would eliminate the tire spin issue.

[ntsqd]

A GPS has to calculate where your at in 3D space, now a cheap unit might show elevation in say 100 foot units but it has know the true height or it wouldn't know the correct Latitude and Longitude, if anything thats dumbing down the data displayed so you have to buy a more expensive unit.

A man who is a position to be intimately familiar with GPS & it's military applications is the source of my info on Altitude being the least accurate calc. Until then it seemed logical that each coord's relative accuracy should be the same. He assured me it is not. Something to do with X & Y being in the same "plane" as the satellites, while Z is not.

We used a bicycle computer in CSU, Chico's HEV, that worked extremely well.

EDIT (3/13/06): I was thinking about this this last weekend and it occured to me that the info I was given is coming up on 2 years old. It is entirely possible that someone has devised a method to reduce the error in these calcs.

[Jack]

I always wonder how do they mark miles with them? Maybe it is a setting I don't have or don'r know how to use, but I can only see the distance from "home" in a straight line not how far I have traveled from point to point. This is on a lorance i-finder.

[Junior]

Jack I suggest spending some time reading the manual = boring and difficult due to booklet type manual. I downloaded the manual for my ifinder H2O from the net and printed it on normal paper and put in in a 3 ring binder for better reference. It can be done. I was amazed at the capabilities of the unit.

Also I believe it is called WAAS. According to the manual it increases accuracy a great deal.

Jr

[poolman]

You don't need no stinking directions!Ask the co-dog,pm