Problems with GPS altimeter

[Rover Nick]

I recently got a Garmin GPSmap 60CS, and it has a barometric altimiter. The problem is that the altimiter varies alot. In the same place it will sometimes show 20ft, other times it will show 142ft, another time I got -3ft. Does anybody know whats wrong with it? How do I fix it?

[TerryS]

I have never seen a GPS that gave altitude based upon barometric pressure, rather than 3 (or more ) satelite fix. But there is so much new stuff out there, I gave up trying to keep up with it.

So, back to the barometric altimeter; It sounds like there is nothing wrong with it. As the barometric pressure changes, the corresponding altitude changes. In order for a barometric altimeter to be of any use, in terms of altitude above sea level, it must be adjusted to read a known elevation. Then, as you change altitude, up or down, it is reading this change also as a change in barometric pressure.
If you're at the beach on a beautiful day, and you adjust your barometer,showing a pressure of 31" (of mercury), to read 0' above sea level, and If you don't move an inch, while a storm comes in, and the barometer drops to 29", your barometer will tell you you are now at 1880' above sea level.
Barometric altimeters are extremely precise, but for very short periods of time. That is why the control tower always tells a pilot what the barometer is at that airport, so the pilot can adjust his altimeter.
I'm sure I haven't explained it well, but I hope you get the idea.

[Adam in NYC USA]

Originally Posted by Rover Nick

I recently got a Garmin GPSmap 60CS, and it has a barometric altimiter. The problem is that the altimiter varies alot. In the same place it will sometimes show 20ft, other times it will show 142ft, another time I got -3ft. Does anybody know whats wrong with it? How do I fix it?

Some of the bobble will be based on the barametric sensor, and some with be based on the GPS system.

As for the former, there will be a 20 to 50 foot variation due to changes in altitude and the usual barametric changes. I have a Timex watch and its sensor generates a lot of crap.

As for the latter, remember we are in two conflicts in the Middle East. The GPS system has two transmitted components, one for civilian use and one for military platform. Occasionally, there is quite a bit of error put into the civilian sensor. You can see it whenever there is significant military activity.

With those two inputs, no wonder you see a lot of crud.

If you can, run the GPS for quite awhile. Make sure it sees a lot of sky with a top-mounted antenna. If you can, turn off the barometer. The GPS altmeter along generated from the satelite is accurate enough.

Hope I was help.

Adam in NYC