? what to do ?
http://wiki.openstreetmap.org/wiki/Contours#The_PostGIS_approach
http://www.openstreetmap.org/?lat=-43.9989&lon=-176.5611&zoom=13&layers=00B0FTF
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--
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RP:there are no contours in osm; they are maintained separately for DB size reasons, and also because they rarely change
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see http://wiki.openstreetmap.org/wiki/Srtm2Osm