Incorrectly calculated contour lines

[qugebert]

Hello everyone,

I stumbled across an area on OSM where completely misleading contour lines are displayed on the cyclosm layer.
https://www.openstreetmap.org/#map=17/47.512389/12.995979&layers=YN

In comparison, the Tracestack Topo Layer show the correct contour lines:
https://www.openstreetmap.org/#map=17/47.512389/12.995979&layers=PN

Opentopomap has the same issue but as the contour lines are rendered on the lake too its a bit more obvious that there cannot be a height difference of 1000m within a lake https://opentopomap.org/#map=16/47.51501/12.99608

Since the contour lines are not calculated from OSM data, I think that the error must lie somewhere in the renderer.
Unfortunately, I have little idea of the algorithms used to calculate these contour lines, but after it was pointed out to me (here) that they are calculated from SRTM data from viewfinderpanorama, I noticed that the affected area lies pretty much exactly on the border between two tiles from this data source (L33/N47E012.hgt and L33/N47E013.hgt). Perhaps this will help. In any case, the elevation data from the SRTM seems to be correct.

If the calculation of the contour lines is not done in this repo, I would be grateful if you could point me to the right place.