Import GPX elevation data

Tom Brown's Avatar

Tom Brown

24 Mar, 2017 03:25 PM

I record GPS tracks using an etrex-30. I think the elevation data it records is pretty accurate (or at least locally consistent, avoiding the problem of small GPS errors creating large gross elevation errors as Matt described in "Terrain Statistics vs. Profile" Oct 2016). I noticed when viewing profiles in CalTopo that the GPX ele data attached to every trkpt in my file is not used. This difference is particularly noticeable when crossing the Golden Gate Bridge, compare https://caltopo.com/m/1PLQ which shows a track ranging from -100m to +76 to the attached file directly generated from the GPX going from +66m to +87m.
I wouldn't be surprised to hear that it is somewhat complex to plumb the elevation through and then decide what to use but no harm asking. Overall I'm a huge fan of CalTopo, my friends may be tired of hearing me rave about it.

  1. Support Staff 1 Posted by matt on 24 Mar, 2017 05:40 PM

    matt's Avatar

    GPS-recorded elevation data is discarded on import.

    The fundamental issue is that GPS tracks are editable. It would be pretty easy to hang onto the elevation data and use it for profiles if they were fixed, but the moment you drag even a single point, I'd need to do a bunch of calculations to figure out what which points were edited, and which corresponding elevation points should be dropped, And then, when you run a profile, I'd be trying to use a mish-mash of GPS-recorded and DEM-provided point elevations, which would be both inaccurate and difficult to cobble together.

    I plan to support GPS-recorded elevations at some point, but I'm still trying to figure out whether it makes more sense to introduce a new, object type with limited editing options, or work them into the existing line object but drop all recorded elevation data when the line is first edited.

    Further complicating all of this is the fact that it doesn't make sense to track the elevation at each point on a hand-drawn line. On a recorded GPS track you can just look at each point elevation, but I want to be able to draw a straight line up a mountain and look at its profile, which requires getting elevations on a bunch of intermediate points as well.

  2. matt closed this discussion on 24 Mar, 2017 05:40 PM.

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