Mapping oddity (Strava question!)

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larkim Mar 2016 5:13pm, 7 Mar 2016 481 posts	0 larkim I ran probably the flattest run I've ever done last Sunday along the Wirral Way at the top of the Wirral, following the largely concrete coastal path from Leasowe to New Brighton. As is my wont, uploading just to Fetch doesn't suffice, so I upload also to ~~strava~~ (and various other sites too). ~~strava~~ corrects altitude data on devices without a barometric altimeter by reference (I think) to a topographic map it has stored in its backend (don't know who provides it, possibly Google, possible OS, not sure). Anyway, partly along the route looking on ~~strava~~ there are these odd elevation changes, which I've converted into a segment for isolation. Does anyone here have access to topographic data that might show how ~~strava~~ gets this sooooo wrong?! strava.com Just curious; a mild Monday pm diversion!
cackleberry Mar 2016 6:00pm, 7 Mar 2016 1,016 posts	0 cackleberry I don't have any data, but I find the ~~strava~~ elevation is often rubbish! Fetch is much more accurate. (By that I mean I have got my paper map out and counted contour lines. Fetch matches much more closely than Strava)
larkim Mar 2016 6:19pm, 7 Mar 2016 482 posts	0 larkim Does fetch actually do anything with the elevation data, or did it just take it add recorded by the watch?
cackleberry Mar 2016 6:21pm, 7 Mar 2016 1,017 posts	0 cackleberry No idea! Hopefully someone more technical will join the thread...
K5 Gus Mar 2016 6:46pm, 7 Mar 2016 1,698 posts	0 K5 Gus Yep, pretty sure Fetch will use your GPS points to go and reference some backend map database and work out the altitude gain. If you just load your data to Fetch, and don't do a Glasses View on it ( either you, or anyone else ) then the altitude gain shows as 0, he only works it out when someone does a Glasses View. No idea why he doesn't work it out when loading the activity - sure I've asked on threads and not had an answer, although to be fair I've never sent a Feedback to ask. I don't use ~~strava~~ but used to use mapmyrun, and Fetch was more accurate that mmr. So there are obviously different reference maps being used by different companies, or perhaps the same reference maps, but different sampling rates ?
Badger Mar 2016 7:07pm, 7 Mar 2016 6,981 posts	0 Badger My guess there would be that ~~strava~~ is using a simple rectangular grid of elevations, and you've got a shoreline running north-east and sloping up steeply, so the track keeps switching between squares which are at sea level in Strava's head and at 25m in Strava's head. You can see there's a bit of smoothing in the slopes, but not smart enough to take care of that kind of variation. I think most of the online data come from the Shuttle SRTM map, but different companies have cleaned it up in very different ways - Google's is the best one I've played with. And Gus is right about how FE works. I can tell because I live in Cambridge, and my Fenix 3's track shows the railway bridges, but they all vanish when Fetch processes them and there is nothing left to see on the elevation plot