GPS track distances change when imported into Caltopo

Andrew's Avatar

Andrew

07 Jul, 2019 03:34 AM

I have a Garmin Oregon GPS device and I use it to track hikes. Sometimes I import the tracks into Caltopo but when I do, the distance changes.

Today I imported the attached track. The device and Garmin Basecamp show 9.8 miles but after the gpx is imported into Caltopo it shows 8.91 miles. Over the last year or so I've been getting a consistent 10% difference. Always with Caltopo showing less. Elevation gain between device/Caltopo is usually pretty close.

I'm trying to work out which is the correct distance. Thanks in advance if there's anyone out there with this issue.

  1. Support Staff 1 Posted by matt on 07 Jul, 2019 06:19 PM

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    I don't have an immediate answer for you. I tried importing the attached GPX file, and it has the same number of points after import - it's not getting cleaned up or truncated. I checked the CalTopo-computed leg distances for the first 10 points, and they match what Basecamp shows you 1:1. Basecamp doesn't look like it's using the GPS elevations in computing distance (normally too small to be relevant, but possibly a source of error with bad elevation data), since point #6 shows a 10 foot elevation gain but only a 3 foot leg distance.

    For what it's worth, I loaded the same track into both GPSVisualizer and trackreport.net, and both of those match the CalTopo-reported distance and not the Basecamp distance.

  2. 2 Posted by Andrew on 08 Jul, 2019 04:21 AM

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    matt,

    Thanks for the super fast follow up!

    I did the math for myself (without accounting for elevation change) and got 8.8996 miles. That included an adjustment of the radius of the earth used in the calculation for each data point.

    If I do it always using the radius at the equator I get 8.9128 miles, which matches Caltopo and the other sites you mentioned. It seems like Caltopo and the other sites do not adjust for the varying radius of the earth at sea level and just use the radius at the equator? The practical difference is meaningless though.

    It was interesting that when I did account for elevation change, it increased to 9.005 miles, which is about a 1% increase, more than I expected.

    It seems like Basecamp and the device are doing something odd. Not sure how they are getting 10% off on what should be a relatively simple calc.

  3. Support Staff 3 Posted by matt on 08 Jul, 2019 04:35 AM

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    Correct, just one radius - I think the practical difference for what people are using CalTopo for is nil.

    Might not have been so quick, except that when you say the difference is off compared to Garmin's software, my immediate suspicion is that CalTopo has the bug, not Garmin. Go figure . . .

  4. 4 Posted by Andrew on 08 Jul, 2019 04:54 AM

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    matt,

    OK, Basecamp is definitely wrong. I found a post on the Garmin site forums where someone was saying that the posted distance that the device and Basecamp shows is from the "summary" within the GPX file. You can force a recalculation of this "summary" by deleting an inconsequential point in the file. When I did that, everything matched Caltopo. I don't know enough about GPX files to understand what that summary is and at this point it's not Caltopo's problem! thanks again from the help!

    https://forums.garmin.com/apps-software/mac-windows-software/f/basecamp-windows/76580/track-distance-discrepancy-between-summary-and-graph

  5. matt closed this discussion on 11 Jul, 2019 03:51 PM.

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