Elevation gain on an imported GPX track

Andrew's Avatar

Andrew

14 Jul, 2019 06:33 AM

I have a GPX track that I made on a hike today (attached) that has 2323 points. I uploaded it to Caltopo to get the stats (distance and elevation gain). The elevation gain on Caltopo came out to be 5031 feet, which was less than I thought it would be. So I did the math myself by importing the GPX to excel, finding the difference between each successive point, and summing all the positive numbers (also attached). I got 5608 feet, which is 577 feet more than what Caltopo shows.

How does Caltopo determine elevation gain for an imported track and which is correct based purely on the GPX?
(I know that GPX tracks wander a bit based on accuracy, conditions, and stationary times but considering only the data available, which is correct?)

  1. Support Staff 1 Posted by rmarcucci on 16 Jul, 2019 08:53 PM

    rmarcucci's Avatar

    Andrew, I'm not 100% sure what's going on with the sampling here, so forwarded this along to the developer. He'll be available again starting on Thursday.
    Thank you for the patience,
    Rom

  2. Support Staff 2 Posted by rmarcucci on 19 Jul, 2019 03:36 PM

    rmarcucci's Avatar

    Andrew,

    The short answer to your question is neither your excellent Excel work, nor the CalTopo sampling is going to be correct. CalTopo is defaulting to 300 points, which for a line this long is likely too few. By using every point, you're picking up a bunch of noise from the data (this gets particularly noticeable on sidehills). Additionally, your Excel numbers are using GPS-reported elevations from the GPX file itself, while CalTopo is using the location and looking up the elevation on a digital elevation model. In my experience normally GPS-reported elevations are going to have more up-down fluctuations, but yours look really accurate, or at least smooth.

    If you look at all the little twists and turns in his GPS track, a 50' interval is going to overstate gain/loss, 100' or 200' is more accurate. For what it's worth, with a pro level subscription, you can resample your lines. Here's the data for various intervals on this track:

    Default (308' interval in this case): 5031'
    200' interval: 5561'
    100' interval: 6446'
    50' interval: 7035'
    40' interval (each point): 7521'
    25' interval: 7532'

    Thank you for using CalTopo,
    Rom

  3. rmarcucci closed this discussion on 19 Jul, 2019 03:36 PM.

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