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Sunday, March 18, 2012

All about barometric altimeters

MT 600 altimeter screen

Barometric altimeter screen on the Garmin Montana 600

For folks who hike and bike in mountainous terrain, tracking elevation gain can be as (or even more) important than trail miles. But while GPS is getting very accurate in terms of horizontal positioning, it is surprisingly weak at determining elevation. While it’s a bit of an oversimplification, let’s just say that it’s primarily due to the geometry of the satellite constellation and any GPS receiver involved. Add in mountainous terrain and the situation can get worse.

Enter the barometric altimeter, which is supposed to solve this. Atmospheric pressure drops as you gain altitude, allowing the change in elevation to be calculated. Unsurprisingly though, barometric altimeters are affected by changes in the weather, such as when a low or high pressure system moves in. Duh!

So is a GPS with a barometric altimeter better for tracking elevation gain? Generally yes, but it is not perfect, especially when those fronts are moving through. The manufacturers are doing a much better job these days though, with firmware that takes into account both barometric and GPS-derived elevation readings, comparing them and trying to figure out just where in the z-axis you are. Still, don’t expect perfection.

Best practices

You can improve accuracy of barometric altimeters by doing the following, which comes from an email exchange with a Garmin rep:

Make sure the unit has a calibrated barometer.  You can do this by letting the unit sit at a fixed elevation for at least half an hour with a clear view of the sky to let the elevation trend to the correct number, or manually calibrating the barometer so that the elevation is correct.  If weather is adversely affecting your altimeter (pressure is changing due to weather instead of elevation changes), you may disable it by setting the device to “fixed elevation” mode which will use GPS only for elevation data.

If you want to check the Total Ascent/Descent data fields set the track log record interval to once per second.  This records all of the elevation data used to calculate the Ascent/Descent data fields in the track log which can then be used as a good reference.

Using it for weather data

I’m not a weather expert, so I won’t go into a lot of details here, but I will say that many of the models below have the ability to record and graph ambient or barometric pressure, and some can even record pressure data with the unit turned off. UPDATE: Here’s a good post on using your altimeter to monitor weather changes.

Garmin barometric altimeter FAQs

UPDATE: I came across a few FAQs which may prove useful, especially the first one…

Models with barometric altimeters

Here is a list of current handheld GPS receivers sporting a barometric altimeter (most also offer an electronic compass):

About Rich Owings

Rich is the owner, editor and chief bottle-washer for GPS Tracklog. Connect with him on Twitter, Facebook or Google Plus.

Comments

  1. Hello!

    Nice article, my new Galaxy Note II has a barometer, and I’ve just started using and looking for apps that take advantage of it.
    Have you had an opportunity to use an android that has a barometer built in?

    Off topic of sorts, my new phone using the gps feature has 24 (?) bars that used to be associated with gps satellites. What up with that? It does get a lock wicket fast.

    I’ve been following you since your mapping book, and have appreciated your columns.

    thanks!

    dave

  2. Does the Garmin Edge 800 cycle computer have a Barometric Altimeter?

  3. I am a fan of altimeters. I usually take thommen , doesn’t require batteries and is quite reliable.
    It is quicker to use , if I am turning my GPS on/off to save batteries etc.

    I do have an altimeter on my older Etrex Vista HCx, while older has a fairly good chip (I don’t think these have been improved much since then?). But I have found it rather useless, since I turn the unit on and off. For some reason it assumes the elevation is what ever it was when I last turned it off. (For instance after hiking in the Wind RIver Mtns, it said I was at 10,000 feet rather than at Sea level back in Portland OR) Worse if I haven’t moved , like say a lunch stop it sometimes gets an incorrect location when I turn it back on, then after getting a better satellite fix / location it changes the altitude by the difference between the bad fix and the good one, all while I am still at the same place. Once it jumped the elevation when I turned it back on, even though it showed the correct location for all my tracking points. Since I turn the unit on and off allot to save batteries this makes it nearly useless. . I don’t think mine will gradually adjust to the correct elevation either. If I have a topo map loaded my location will show the elevation however.
    Which makes one wonder why one needs an altimeter , at least on a GPS? (I trust the location more, and hence the elevation on the topo map more etc.)

    I have used the tracking feature a bit more for workouts with a heart monitor. It is probably more useful there, though if the track is loaded onto a topo map software, it will calculate elevation change based on these points as well. Just be careful to note its initial reading EACH time you turn it on!.

    I haven’t noted the pressure readings, it would interesting to see if at least those aren’t based on the readings when it was last turned on.

    • You might want to check your altimeter settings.

      • I see only two setting for the altimeter:

        Variable elevation seems like the only choice , except for when one stops for the night, though if you are turning it off I am not sure that matters, except if you are moving. Garmin told me to use Variable.

        The other option is auto calibration, which I do have turned off. Since it takes 15 minutes for this to work it would not do much good if one turns the unit on/off? The other problem with it is if you calibrate the altimeter with the GPS it would seem to me that you would eliminate the accuracy of a barometric altimeter? It would be interesting to compare if it gives the better of the two “takes into account both barometric and GPS-derived elevation readings”, when on. I am not sure this would solve the problem of the skipping it does when I turn the unit back on after a lunch stop. I think one would have to turn tracking off before turning the unit off (so it isn’t on when you turn it back on). Then after say lunch turn it on. Wait for it to get a good satellite fix, enter the elevation when you turned it off (or known elevation if you have that) and then turn tracking on. Or when you get home delete the erroneous tracking points. With auto calibration on, it might adjust an erroneous initial input like my 10,000 feet while in Portland, but the tracking would be off and the CHANGE in elevation would be much worse. Once adjusted from 10,000 feet to say 100 feet, this would be solely GPS elevation anyway?

        I too asked Garmin about this:

        “You are correct, Sir. Every time you turn your device back on, it may
        take about 15 minutes for the auto-calibration to get a correct reading.
        If you do decide to turn it off while hiking and then you don’t know the
        elevation, you can use the current pressure if you know that or you can
        use the current GPS altitude to calibrate the device again. You may need
        to calibrate it manually each time you turn it on, if you do not want to
        wait about 10-15 minutes for the auto-calibrate to work. Variable
        Elevation is to be used when you are moving, while Fixed Elevation will
        be used when you are stationary. I would recommend leaving that set to
        Variable as it should give you a slightly more accurate reading.”

        Part of the problem is this works so differently than my thommen, and it is not mentioned in the manual. Even now that I known what it is doing, I don’t use it much, except just recently to monitor fitness workouts.

        • Larry, is your firmware up to date?

          Auto-calibration may take UP TO 15 minutes, but my units seem to be pretty accurate almost immediately.

          Calibration doesn’t eliminate the accuracy of the barometric altimeter, it merely fine tunes it. I often calibrate if I’m at an elevation benchmark.

  4. Joel Reeves says:

    If you need to set the altitude of your start point for calibration, this web site uses Google Maps to give you an altitude

    http://www.daftlogic.com/sandbox-google-maps-find-altitude.htm

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