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  1. #1
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    How can I correct this elevation data?

    My GPS (Garmin Oregon 450) has a barometric altimeter. I did a long ride today, starting and ending up in the same parking lot. When I look at the elevation profile / data, I ended up ~50 ft higher at the end (in the same spot).

    It's not just a single point that was off. When I look at the elevation data from the trackpoints, the 10 or so points recorded in the lot at the end of the ride are consistently about 50 ft higher than the 10 or so from the lot at the beginning.

    I figure the atmospheric pressure changed over time.

    My question is whether there is some software that can correct the data? I figure that I just need to add 50 ft to the beginning or subtract 50 from the end, but I'd like the rest of the points to be gradually corrected over the course of the ride (I'll just assume the pressure change was a consistent rate).

    Is there some software that can do this?

    Also, along the same lines, if the actual elevation is off, is there some application that will let me go back and retroactively calibrate the elevation data in the gpx file?

    I am using Topofusion to map gpx data, is there a better program for cleaning up the elevation data first (removing outliers, calibrating, etc)?

    Thanks
    15mm is a second-best solution to a problem that was already solved.

  2. #2
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    I think Topofusion's tools are going to be the best for correcting elevation profiles like that where there's a pressure change over the course of the ride.

  3. #3
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    Quote Originally Posted by NateHawk View Post
    I think Topofusion's tools are going to be the best for correcting elevation profiles like that where there's a pressure change over the course of the ride.
    I was looking around but could not find that kind of tool in topofusion. Where would I find that tool?

    Thanks
    15mm is a second-best solution to a problem that was already solved.

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    Quote Originally Posted by kapusta View Post
    I was looking around but could not find that kind of tool in topofusion. Where would I find that tool?

    Thanks
    Analysis>Climbing Analysis and then at the bottom of the dialog, "Add Elevation to Track".

  5. #5
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    Quote Originally Posted by NateHawk View Post
    Analysis>Climbing Analysis and then at the bottom of the dialog, "Add Elevation to Track".
    Does that just replace my collected elevation data with elevation data derived from DEM based on position? Or does it use DEM in combination with my gathered data?

    I used that function once to get elevation data for a track that had none recorded.

    If that is the case, is there even a point in my collecting elevation data on my ride?

    Thanks, I'm just starting to figure my way out, here.
    15mm is a second-best solution to a problem that was already solved.

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    pretty much. there is more processing involved with generating the elevation profile, but I think when you add elevations, it's only putting the DEM values on the points.

  7. #7
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    Did you know the actual elevation above sea level of the parking lot? Because if you barometric altimeter was off at the start of the ride, the GPS elevation numbers will slowly correct it over time, which might also create the effect you are seeing.

    But on a really long ride, 50 feet isn't so terrible, IMHO. I'd think of it as the cost of doing business with an altimeter.

    I've had a summer thunderstorm create a "Category 3 Climb" on my Strava elevation profile.

  8. #8
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    Quote Originally Posted by ghettocruiser View Post
    Did you know the actual elevation above sea level of the parking lot? Because if you barometric altimeter was off at the start of the ride, the GPS elevation numbers will slowly correct it over time, which might also create the effect you are seeing.

    But on a really long ride, 50 feet isn't so terrible, IMHO. I'd think of it as the cost of doing business with an altimeter.

    I've had a summer thunderstorm create a "Category 3 Climb" on my Strava elevation profile.
    No, I did not calibrate at the beginning of the ride. From what you just told me, I think I was under a bad assumption: I figured that if the elevation was off 50 feet at the beginning, it would be 50 feet off at the end (unless weather screwed with it).

    So the gps correction of the altimeter data is a gradual thing? Interesting, I was not sure just how that worked.

    I think this could explain what happened, because when I checked the elevation data, it was close to dead on at the end of the ride.
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    The automatic calibration in the Oregon 450 is an option. It will only do that if the feature is on. It's under settings>altimeter. It does tend to take a little time to calibrate but IME if you are turning it on/off at about the same elevation all the time, it will remember and take less time to calibrate.

    I always give the gps a few minutes to warm up before I go anywhere and it seems to help.

  10. #10
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    Quote Originally Posted by NateHawk View Post
    The automatic calibration in the Oregon 450 is an option. It will only do that if the feature is on. It's under settings>altimeter. It does tend to take a little time to calibrate but IME if you are turning it on/off at about the same elevation all the time, it will remember and take less time to calibrate.

    I always give the gps a few minutes to warm up before I go anywhere and it seems to help.
    OK, I checked, and auto calibrate was turned on. I think I may know what happened, please tell me if I am understanding this correctly:

    I did not turn on the unit until a few minutes before the ride. The unit had been turned off the night before at an elevation ~800 feet higher.

    So when I turned it on, I guess the baro altimeter still calibrated to the conditions from the night before, thought is was 50 feet lower, and it took a while to recalibrate.

    I was under the impression that the altimeter recalibrated every 15 minutes to the gps derived elevation, but I was a little puzzled by this because I thought GPS elevation was a bit less accurate. I think this is wrong, now.

    My understanding now is that auto-calibrate keeps comparing the altimeter data to the gps data over a period of time (15 minutes?) and looks for an overall difference in these data sets, and compensates for that. Barometer data is more sensitive and consistent, but it can "drift" away from the real elevation. GPS data is less accurate and more erratic, but it does not drift, and stays in the general vicinity of the true elevation. So if the baro elevation data is averaging 30 feet higher than the gps elevation data, the unit will gradually subtract 30 feet from the collected baro data.

    Am I understanding this right?

    Thanks.
    15mm is a second-best solution to a problem that was already solved.

  11. #11
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    Close.

    GPS elevation has the potential to be more accurate than barometer elevation. Airplanes use it so it can't be all bad. The problem with it is that when you're on the ground, you've got things like trees and hills for the signal to bounce off of. GPS elevation accuracy winds up being a function of your reception quality more than anything so its variation is more random, rather than a consistent drift as with barometric readings.

    I don't know the exact process used to do the auto calibration, but time absolutely is a factor.

    Your idea of how it worked is close, but the main reason the GPS thought the elevation was 50ft off was because the air pressure changed overnight because of a weather system, which threw the readings off. It then took some time for the auto calibration to account for that. You could have done a manual calibration at the trailhead if you knew the elevation there, and it would address the issue quicker.

  12. #12
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    Quote Originally Posted by NateHawk View Post
    You could have done a manual calibration at the trailhead if you knew the elevation there, and it would address the issue quicker.
    One of the biggest improvements in the edge 500 vs my old etrex is that you can put in pre-defined known elevation points, then just roll over them and press start to calibrate the altimeter at the beginning of a ride.

    Not sure what other units have this feature.

    I have one at the end of my driveway and one in the parking lot at work, as well as a few other common ride start locations.

  13. #13
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    the handhelds don't have that feature (yet).

    it would make things a little easier to deal with at common starting points of rides/hikes. but then again, handhelds have manual calibration while the bike/fitness models don't.

  14. #14
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    Let it equilibrate

    When I first starting using a GPS the beginning altitudes were often off a good bit, especially if I was doing rides at many different elevations. So I began starting the unit about a half hour before each ride to let it equilibrate. I even looked up the elevations at the start of every ride to make sure the unit was ready. I don't seem to have had that problem in quite a while though with my 800.

  15. #15
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    Quote Originally Posted by kapusta View Post
    My GPS (Garmin Oregon 450) has a barometric altimeter. I did a long ride today, starting and ending up in the same parking lot. When I look at the elevation profile / data, I ended up ~50 ft higher at the end (in the same spot).

    It's not just a single point that was off. When I look at the elevation data from the trackpoints, the 10 or so points recorded in the lot at the end of the ride are consistently about 50 ft higher than the 10 or so from the lot at the beginning.

    I figure the atmospheric pressure changed over time.

    My question is whether there is some software that can correct the data? I figure that I just need to add 50 ft to the beginning or subtract 50 from the end, but I'd like the rest of the points to be gradually corrected over the course of the ride (I'll just assume the pressure change was a consistent rate).

    Is there some software that can do this?

    Also, along the same lines, if the actual elevation is off, is there some application that will let me go back and retroactively calibrate the elevation data in the gpx file?

    Thanks
    Well, this thread has been super informative for me with regards of how to best use the altimeter

    However, I am still curious as to whether there is some way to add corrections to the elevation data in an existing file? E.g., add elevation across all the track points, or add them gradually across a file?
    15mm is a second-best solution to a problem that was already solved.

  16. #16
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    It's possible, but not exactly simple to do with consumer software. You would have to modify the xml manually.

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