The altitude data is from NASA’s SRTM mission which you can include in the map, but it requires some work from you.
There is a tool called srtm2osm which produces osm files which you can merge with regular osm files and feed those to Mkgmap or use xinclude as described below:
Could you give some details on how to do it? My problem right now is that srtm2osm does not seem to work right because NASA did changed the servers (at least that is my explanation for the errors). Even when I put all the relevant srtm-data in a cache-folder, it does not work. Becauce srtm2osm is not longer supported I do not know how to get it running.
I am looking for contour data for northern sweden and because I could not find those I would like to make them myself.
SRTM data is only available to 60 degrees north, so Northern Sweden is not covered by the SRTM data afaik. Regarding the inner workings of strm2osm: i don’t know if it works fine or that it needs adjusting to nasa server changes…