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Mapping oddity (Strava question!)

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larkim
larkim
Mar 2016
5:13pm, 7 Mar 2016
481 posts
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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
cackleberry
Mar 2016
6:00pm, 7 Mar 2016
1016 posts
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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
larkim
Mar 2016
6:19pm, 7 Mar 2016
482 posts
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larkim
Does fetch actually do anything with the elevation data, or did it just take it add recorded by the watch?
cackleberry
cackleberry
Mar 2016
6:21pm, 7 Mar 2016
1017 posts
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cackleberry
No idea! Hopefully someone more technical will join the thread...
K5 Gus
K5 Gus
Mar 2016
6:46pm, 7 Mar 2016
1698 posts
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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
Badger
Mar 2016
7:07pm, 7 Mar 2016
6981 posts
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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 :)

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Maintained by larkim
I ran probably the flattest run I've ever done last Sunday along the Wirral Way at the top of the W...

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