Skewed areal data: mapping roof or base?

Hi all,

I’m busy mapping some buildings in Belgium, and some Bing imagery is a bit skewed: it seems that the plane didn’t fly straight over the building when taking pictures. Now, as I want to map the building nodes as accurately as possible, do I need to position them at the base of the building, or rather at the roof? I’d think that the former one is more accurate, but I’ve spotted quite some roof-oriented buildings (I’m guessing because it’s far easier to do so).

An example where the roof has been mapped:


You are right, orthophotos are usually produced according to the terrain surface and the aim is that the basements of the buildings will project correctly.

Do both! Trace the building around the roof, that’s the easiest. Then drag the outline so it aligns with the building base.

This gets a bit tricky with multilevel buildings, but what’s important is you should put the base outline in OSM.

Moving the outline of the building (roof) to the base of the building will also reflect the distance from building to ways in the surrounding.

All right, clear answers. Thanks!