Tuesday 5 July 2011

GPS track errors

Another small feature is rolled out; if your track has GPS measurement errors they may be elevation or latitude/longitude.

For elevations it was easy -- uplad your track, go to "Elevation smooth" tab, click "Remove extreemes" and voila -- all elevation errors are gone. But for position (i.e. latitude/longitude) there wasn't this option. At least till now.

And, by the way, the algorithm for GPS elevation errors detection is now much better.


  1. Where can I find the algorithms used for elevation and GPS smoothing and error removal? Without knowing the algorithms the functions are basically useless, as you don't know what you are getting. As an example, I have run GPS smoothing on a hiking track and the resulting distance was shorter. This appears to be wrong, as you don't walk from point to point but in a curve which is generally longer than the sum of the line segments.

    Is there a way to download the corrected track?

  2. Hi GnC,

    The result of "smoothing" will always be a shorter track. That's normal, because no GPS error will shorten your track and when you (at least try) to remove error -- the result must be a shorter track. Even if you take in accout the curves between points this is what will happen. Look, for example, the images here.

    But, at the moment I do compute only distances as lines from point to point. From my own experience the error is very small. At least if your GPS is set to record points no longer than cca 50m between them. I may decide to compute the distance using curves between points, but this will be much slower and more CPU intensive. I haven't tried yet so don't know how much slower. Do you have any idea of an fast algorithm to use?

    All the algorithms can be found here: https://github.com/tkrajina/gpxpy The library is open source so, if you find any way to improve it -- you're welcome!

    About the edited GPX. I can add a link to the original file and the manipulated file on every page. What do you think?


    PS. You took some great photos from your "Cruz del Carmen" trip!

    PPS. Trackprofiler is still a hobby project of mine, and still in beta. So every feedback is welcome. Do feel free to ask/suggest anything you would like to see on the site.

  3. Hi tk,

    I haven't had time to look at the algorithms yet. Can you briefly explain what the GPS Error>Smooth function does? And what is the difference between Smooth and Remove Errors? I was thinking that "Smooth" would add points similar to the Topofusion splining function (see http://www.topofusion.com/spline.php).

    The latter definitely makes the track longer, which you would expect from a smoothening function. I am now thinking that your "Smooth" function may not be smoothening the track but just removing errors. But then I don't understand why you have two functions/links?

    Yes, it would be nice to have a download link for the edited track to allow a detailed investigation of the changes.

    I will look into algorithms for track smoothening. They will most likely be somewhat CPU intensive.


  4. By the way, I featured Trackprofiler as "website of the week" on my Danish page at www.netkvik.dk.

    (no need to publish this comment)