? what to do ?
currently that is built from NASA's SRTM 90m resolution global
height data. Viewable on the CycleMap mapnik slippy tiles.

how to handle depressions?
see http://wiki.openstreetmap.org/wiki/Proposed_features/Terrain_accident--
RP:there are no contours in osm; they are maintained separately for DB size reasons, and also because they rarely change
see http://wiki.openstreetmap.org/wiki/Srtm2Osm