A question about the etrex vista HCx barometric altimeter..
  • The spec states the vista HCx has a barometric altimeter. Does this mean it computes altitude by actual atmospheric pressure, or by using GPS satellites, or can it do both?
  • Tim
    Posts: 19,398
    It can do both.
  • Boyd
    Posts: 11,625
    For it to be of any real value, I believe you need to first calibrate it at a location with a known elevation. You should be able to download the owners manual on Garmin's site (link on the product page) and find out more about this.
  • Marc
    Posts: 1,241
    Having had a barometric altimeter a long time ago, I never could understand the use for these during casual hiking. I would calibrate it, but by the time I wanted to use it to get some idea of my current altitude usually the barometric conditions had often shifted enough to make it off by hundreds of feet. I much prefer to have a GPS with Topo maps where I can read my altitude from the map.

    Also altitude estimation by use of satellite signals is not a particular strong point of hobbyist GPSRs. They are much more accurate in the horizontal directions than in altitude.
  • Marc
    Posts: 1,241
    So thinking about this, I did a little research and am now more confused than ever by something I read. This is from :
    http://wiki.motionbased.com/mb/Barometric_Altimeter and it reads like a Garmin advertisement
    Barometric Altimeter
    Check out the definitions at Wikipedia for an altimeter and barometer.
    GPS provides Altitude information when it is able to communicate with 4 or more satellites. This is referred to as Three Dimensional (3D) signal. Latitude, Longitude and Altitude are the three dimensions. If you want to get technical then the fourth dimension would be time.
    The big question most people ask is "Why do I need a barometric altimeter when I have GPS altitude?" A barometric altimeter uses the atmospheric pressure to determine your altitude above sea level. It turns out they are able to determine altitude within +/- 3 vertical meters or so. The altimeter will detect your change in altitude based on the change in the atmospheric pressure (the higher you go the less pressure).
    The problem with a barometric altimeter is the requirement to calibrate it every time you start your activity. If you don't the altimeter will still work and relative measurements will be accurate, but absolute measurements will be off. This means that your cumulative elevation gain/loss will probably be correct, but all of the elevations along the way will be offset by the difference of the actual altitude and the altitude on the device.
    GPS altitude doesn't need any calibration, but for complex reasons the GPS unit is not able to determine elevation as accurately as a barometric altimeter.
    Combining both GPS and barometric altimeters, Garmin GPS units are able to provide the most accurate altitude readings of any handheld device. Absolute location is provided originally by the satellite to help auto-calibrate the barometric altimeter, then the barometric altimeter is used to provide a more stable elevation change. The GPS device will constantly calibrate the barometric altimeter throughout an activity because the pressure may change due to weather conditions. This is a great advantage during long days of hiking, biking or running.


    So by continuously calibrating the barometric altimeter against something that is admittedly less accurate they achieve something more accurate? I think this deserves a better explanation Also I am not sure that over the course of many hours the barometric altimeters change in attitude is all that accurate. As a weather front moves across your location I've seen them shift hundreds of feet.
  • Marc
    Posts: 1,241
    So on my quest for useless knowledge as a means of avoiding the work I need to do I found this answer to my question:


    Lucky7777 How does the altimeter work?
    I set precise level of level. Altimeter->Calibrate->Set level.
    In 24 hours I came to the same spot and difference was 57 ft.

    How does the altimeter work, how precise is it? What factors can influence the measurement?
    Posted Jan 15, 2009 8:41 pm

    GPSFix re: How does the altimeter work?
    The barometric altimeter is very precise but keep in mind there are two factors that can influence the readings:

    - Elevation (obviously!)
    - Changes in atmospheric pressure (caused by weather changes)

    I find that over relatively short time periods (several hours) that if I manually calibrate the altimeter AND turn off auto-calibration that the barometer is very accurate (within 5-10')

    That brings up auto-calibration. Auto-calibration uses GPS elevation to calibrate the barometric altimeter. GPS elevation is visible on the satellite page. The issue with GPS elevation is that isn't very accurate even under the best satellite conditions with WAAS turned on. You still will probably have 20-30' error and in the woods 50-70' is more typical.

    What does this all mean? If you have auto-calibration turned on (which is the case by default) you'll see errors in the 50-100' range depending on GPS signal quality and how fast the weather is changing over time. I'm guessing this is what you observed. The up side is that you don't need to remember to manually calibrate and your altimeter will always be somewhat accurate.

    If you really want accurate altimeter readings though you'll need to disable auto-calibration and use manual calibration to some known good reference (topo map, survey mark, etc). But after several hours, again depending on weather, your barometer can drift requiring manual calibration.




    So you can decide whether auto-calibration is a bug or a feature.
  • I think I spotted a couple errors in the Wikipedia article!
  • Marc
    Posts: 1,241
    The Wiki article reads like an ad from Garmin. Perhaps you should post something further there.
  • Barometric altitude information is in the short term very accurate (once calibrated) and in the long term inaccurate (because of weather changes).
    GPS altitude information is in the short term pretty inaccurate, but if you average the altitude over a long time, you can get a pretty accurate number.

    So we have two sources of altitude data: one short term accurate, and the other long term accurate. We can combine these two to produce more accurate elevation data than either alone.

    One way to combine them is to continuously compute the difference of these two and average that error signal over some period of time (say an hour) and then use this averaged error to correct the barometer. This appears to be what Garmin's autocalibrate is doing.

    If I was designing this, I would also adjust the time constant for averaging the error signal depending upon the relative estimated quality of the two signals: if the estimated accuracy from the satellites is very good, or the unit has been off for many hours (barometer poor), you could use a shorter time constant, and vice versa. I have no evidence the Garmin does this.
  • Marc
    Posts: 1,241
    I don't beleive that altitude from a consumer GPSr is accurate in the long term or short term. Vertical position on consumer GPSrs is inherently inaccurate. I wouldn't trust it to be better than plus or minus 50 feet at best. I think autocalibrate is an ill conceived idea, and you are better off calibrating your barometer from reading altitudes off a decent topo map.
  • My first outdoor gadget was an Outbreaker from Oregon Scientific, a wrist-watch with barometric altimeter, given as a gift before a snowboarding holiday. I have to say that it was impressively accurate over extended periods. Cable-cars and chair lifts are often labelled with altitude and the reading was always in close agreement over a week with thousands of metres of vertical travel. The literature claims that it is able to distinguish very slow pressure change as weather while stationary and faster change as motion-based height change. Whatever, it works well.
    I have yet to compare results against my new eTrex30 but when I do I'll start out trusting the Outbreaker first. New units seem to be now unavailable.
↑ Top

Howdy, Stranger!

It looks like you're new here. If you want to get involved, click one of these buttons!