GPS vs. barometric altitude

[+imajeep]

Reading another post on the forum brought up an old question: If a GPS unit can get an altitude fix by triangulating from the satellites in 3D, then why would I want a GPS unit with a barometric altimeter? We use a Garmin 60 CSx, but I haven't paid too much attention to the barometric altimeter since I got it. Does the barometric altimeter add something to what the GPS receiver can do on its own? Thanks.

Edited August 15, 2007 by imajeep

[Hertzog]

Reading another post on the forum brought up an old question: If a GPS unit can get an altitude fix by triangulating from the satellites in 3D, then why would I want a GPS unit with a barometric altimeter? We use a Garmin 60 CSx, but I haven't paid too much attention to the barometric altimeter since I got it. Does the barometric altimeter add something to what the GPS receiver can do on its own? Thanks.

Each approach has its own advantages an disadvantages. The gps calculation tends to have large instantaneous errors (typically about twice the horizontal error), but averaged over time gives good results. The barometric altitude is accurate over short times (once calibrated) but tends to drift over longer periods due primarily to barometric pressure changes. Using the automatic calibration feature combines the best of both (IMHO) under most circumstances by continually updating the barometric altitude based on the gps calculation. I normally just use this setting and forget about it, doing a manual calibration whenever things look strange or if it's important to have accurate readings. Other people prefer to do frequent manual calibrations; it's mostly a matter of preference and what you feel comfortable with.

[+geo climber]

Reading another post on the forum brought up an old question: If a GPS unit can get an altitude fix by triangulating from the satellites in 3D, then why would I want a GPS unit with a barometric altimeter? We use a Garmin 60 CSx, but I haven't paid too much attention to the barometric altimeter since I got it. Does the barometric altimeter add something to what the GPS receiver can do on its own? Thanks.

Using the automatic calibration feature combines the best of both (IMHO) under most circumstances by continually updating the barometric altitude based on the gps calculation. I normally just use this setting and forget about it, doing a manual calibration whenever things look strange or if it's important to have accurate readings. Other people prefer to do frequent manual calibrations; it's mostly a matter of preference and what you feel comfortable with.

 

I can't find an option for "automatic calibration" on my Vista HCx. Is that a feature specific to the 60CSx?

[dogwalkers2]

On the Vista HCx:

 

Main Menu>Setup>Altimeter

 

Then there's a dropdown box for Auto Calibration (On/Off)

[+geo climber]

On the Vista HCx:

 

Main Menu>Setup>Altimeter

 

Then there's a dropdown box for Auto Calibration (On/Off)

Got it, thanks.

 

So what does "Barometer Mode" do for Fixed vs Variable Elevation, on that page?

[+allory]

If you're using your gps on a boat on a lake or ocean then the elevation can be set to fixed. At that point the altimeter functions as a barometer and can be used in weather forcasts. If you leave the elevation as variable then it will continue to view pressure changes as possibly influenced by changes in elevation.

[DogFleazJR]

Reading another post on the forum brought up an old question: If a GPS unit can get an altitude fix by triangulating from the satellites in 3D, then why would I want a GPS unit with a barometric altimeter? We use a Garmin 60 CSx, but I haven't paid too much attention to the barometric altimeter since I got it. Does the barometric altimeter add something to what the GPS receiver can do on its own? Thanks.

 

Barometric altimeters also provide much smoother track data than the GPS altimeter. As Hertzog mentioned, the vertical accuracy of GPS units is much lower than the horizontal, so vertical positional errors are magnified and produce a lot more "noise" in the reported elevation. The pressure signal has a lot of dampening behind it so a lot less noise.

 

If smooth track data is not important to you, or you don't plan to also use your GPS as a portable weather station, then a GPS without the barometric altimeter is perfectly acceptable.

 

Another complaint of barometric altimeters is you can't use them to determine altitude in a pressurized airplane cabin. You will have to trust what the captain tells you.

[newellj]

I've never used a GPS with an altimeter, but my separate altimeter is way more accurate on elevations (if set correctly each morning) than the GPS, even on WAAS.

[+Alan2]

Barometric altimeter readings were even more important with older models of GPS's without the "x" or Sirf Star III chip. Older receivers often lose the sats in foliage, but the barometric capability would allow them to continue to provide data.

 

That and the ability to predict weather.

 

Also, people using Garmin's Jumpmaster program perfer the quicker updates of altitude with barometric units when they jump out of airplanes for obvious reasons.