Altitude calculation on GaiaGps

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  • Avatar
    Nathan

    @sjmj3us
    Thanks for posting.

    Currently, Gaia GPS take in all of the GPS data for your hike and get rid of any points that seem erratic (too high or too low). Then, the data is averaged by every 10 points to achieve a smooth and accurate change in elevation.

  • Avatar
    sjmj3us

    The issue is that the rule for what is erratic is a judgement. Also the averaging window size is also judgement. Every GPS device characteristics is likely to be different. 

    I think a better way would be to parameterize the algorithm and allow the user more flexibility to customize the algorithm based on their specific experience.

    It's pretty clear that Gaia produces altitudes that deviate significantly from surveyed altitudes and also has a lot of jitter. Given there is no perfect answer why not delegate to the user?

  • Avatar
    william101.chen

    I find this is still a big problem. The smoothing is far too aggressive. It doesn't make sense that Gaia is calculating elevation gains that are 1,000 ft+ below surveyed stats. And even if you just count the contour lines on topo maps along a track, ignoring big jumps in the track and whatnot, Gaia's number still doesn't make sense.

  • Avatar
    sjmj3us

    They don't appear to smooth the actual data just ascent, low and high. Therefore, I would suggested downloading Google Earth Pro desktop (not the web version). Then export the track as KML. Open the KML with Google Earth, select Edit->Show Elevation Profile. In addition to elevation profile it gives the gain.

    I'm not sure that Google Earth uses the elevation points. What I think it does is use digital terrain elevation data (DTED)  (or perhaps it uses both). I added a requested for the gaia cloud to add a DTED server to resolve the issue you might look at.

     

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    bcorwin

    I feel this problem might actually be getting worse?  I made a recent track public so you can see:

    https://www.gaiagps.com/datasummary/track/5141c5982dca7b1bd68b27f9a91c142746f0e041/?layer=GaiaTopoRasterFeet

    On this track, gaia android is showing 713 feet of gain.  Caltopo shows 1,405 for the gpx file.  The gaia route created ahead of time showed 1,038 and we did more up and down than my pre-planned route.  I have noticed in past that gain is understated in the gaia android app and I reported this to gaia support.  I've gotten used to adding a couple hundred feet to the gain based on my reading of the map and Caltopo usually confirms this.  But I think this is the biggest discrepancy I ever recorded-- the gain is understated by almost half???  I'm mostly satisfied with the gaia app but I need better gain numbers!

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    sjmj3us

    It's certainly not getting better.

    Consider this track:

    https://www.gaiagps.com/datasummary/track/bfe1f6cc06bc8f0af7d5cdbf5b875b04be01c236/?lat=34.1014&lon=-118.2293&zoom=8.0&layer=OSMCycleMapHD

    The more bumpy the track is the more they will underestimate because of their smoothing. Your track looks to be very bumpy. I think it's just a function of the track track. Find a track which is mostly monotonic even significant over smoothing will still report accurate ascent.

    I've talked to support and pointed out that other sources and even just adding the crudest peak and valley readings show clear underestimation and they consistently disagree.

    I've tried to get them to look at DTED and also include barometric altitude so they could have some independent source. To me it's pretty clear they have no intention of fixing this and I've given up. I'm just glad they provide the raw data so I can use other sources like Google Earth or my own program to calculate ascent.

    You can download the KML or GPX and analyze the elevation numbers your self.

     

     

  • Avatar
    bcorwin

    sjmj3us, your track really demonstrates the problem!  If I rely on Caltopo's numbers (which I have generally found to be reasonable based on my examination of the map), Gaia's gain numbers were understated by 48% on my bumpy (undulating) hike above.  Today I did another hike that was monotonic (mostly all up, then mostly all down).  Gaia showed 2,536 feet of gain and Caltopo 2,860, a difference of 11%.  I know Caltopo's numbers are closer because Gaia's number is actually less than net gain (difference between highest and lowest points).  So I think there is some understatement even on monotonic tracks, but not nearly so much.   Thanks for your comments about the smoothing, which explains why the discrepancy is so much larger on some hikes.  Like you, I'm glad we have the raw data.

  • Avatar
    Robyn

    @bcorwin
    I'm sorry you are seeing such a discrepancy between with your track recording and route.

    Was the area you were recording in heavily wooded? Tree coverage or other obstructions to your device's view of the sky can increase GPS drift and cause the app to need to filter your elevation data more.

    Can you send you activity logs to support and we can also check if there are any errors that would cause for this?

    - Open the app> Settings> Other> Send Logs

  • Avatar
    bcorwin

    Thanks, Robyn.  When I tap "Send Logs" a window comes up that says "Share with" and then a choice between Gmail, Save to Drive, and Whats/App.  Which one of these do I click in order to send the logs to support? 

    This was the one that had the biggest discrepancy:

    https://www.gaiagps.com/datasummary/track/5141c5982dca7b1bd68b27f9a91c142746f0e041/?layer=GaiaTopoRasterFeet

    There was tree cover on much of this route.

  • Avatar
    Robyn

    You can select Gmail and send your logs to support@gaiagps.com.

    I took a look at your track and from your notes, it sounds like your comparing your track recording stats to the track imported into CalTopo.

    CalTopo filters track information in GPX files differently then Gaia GPS which results in different elevation and distance stats.

    Seeing different stats between programs is common as all software handle track information differently.

  • Avatar
    bcorwin

    I'm also comparing it to the route I created in Gaia ahead of time.  The route shows 1,038 feet of gain and the track shows 713 feet.  Here's the link for that route:

    https://www.gaiagps.com/datasummary/route/e7d56877-aa14-46c8-abe3-c1eb5567059a/?layer=GaiaTopoRasterFeet

    The two are almost identical on the map-- we followed the route pretty closely.  Yet the track shows 31% less gain than the route, and these are both Gaia.  When I count the lines on the map, not taking into account any changes less than 40 feet so it's a conservative estimate, I get about 1,000 feet of gain for this hike.  This is typical with Gaia, that the track shows a few hundred feet less gain than the route, even when the exact route was followed.  A review of the map usually shows that the track was more accurate than the route.

    I sent the logs to support and received an acknowlegement.  Thanks for your help.

  • Avatar
    sjmj3us

    I looked a the first route @bcowin posted and there were holes of up to I think two minutes. With that in mind I'd like to address the comment Nathan made: "Then, the data is averaged by every 10 points to achieve a smooth and accurate change in elevation."

    At work we heard a lecture on cleaning data and the point was made that you should not smooth by sample that you should smooth over time, just because of drop outs. When you smooth you are assuming a) that the data is approximately constant over the smoothing range and b) that your sensor has jitter. If you are getting a sample a second then this makes sense. However if there is a two minute gap then this doesn't make sense. Therefore I think if there is drop outs it might make the ascent problem even worse.

    Gaia might consider what Strava does. What they appear to do is a) add barometric altitude and b) they average multiple runs from each person that's recorded a track over time. Since Gaia has a bunch of recorded track data they could start averaging the points recorded from a hikes to get more accurate data.

    Then again I've suggested they could also use the topo data from the open cycle map or a digital terrain elevation data (DTED) server to supplement once synced with the cloud. 

     

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