FYI OSM doesn't (really) store elevation data. Individual mountain peaks are often mapped, and tagged with the elevation. But OSM doesn't store land contours like that.

There are some free, open, datasources for that, like SRTM[1], ASTER[2] (and maybe OpenDEM[3], but I don't know what that's like)

[1] https://wiki.openstreetmap.org/wiki/SRTM [2] https://wiki.openstreetmap.org/wiki/ASTER [3] http://www.opendem.info/

So how are these contour lines being generated?

My hike of a few months ago:

https://www.gaiagps.com/datasummary/route/67dcc2001bea9e9e39...

Also, if OSM doesn't store elevation data, then why do you need a GPS tracker with a barometric sensor in order to map for OSM?

> So how are these contour lines being generated?

It depends on the people make the map.

That map uses Thunderforest's Outdoor style[1]. The person running that company is a long time OSM contributor, but that isn't "from the OpenStreetMap project". I suspect they are using SRTM data to generate countour lines in addition to OSM to create the map

> why do you need a GPS tracker with a barometric sensor in order to map for OSM?

You don't? You can map for OSM without needed a GPS tracker. You can just trace things from the aerial imagery, or add local features from memory ("There's a post office at that road junction", "the speed limit of this road is 40 kmph").

[1] https://www.thunderforest.com/maps/outdoors/

Here: http://www.udeuschle.selfhost.pro/panoramas/makepanoramas.ht...