Talk:OSM-4D/3D building

Building definition and outline

Hi, it looks like a lot of work has been put into this page/concept thus far, but using a different definition of the building outline immediately stood out to me as potentially causing significant problems. This definition may make it easier to map some 3D building roofs without having to employ a multipolygon, and may appear okay when only considering buildings, however the existing definitions of a building footprint are valid; your's is not. You are effectively saying building footprint=roof, but in reality this is simply not the case. Roofs often overhang the building footprint and/or appear larger than it. This overhang is not typically represnted in simplified diagrams, though it is present in your chicken-house picture.

Consider a dense urban area where there are small alleyways and footpaths between buildings and going under roofs. Mapping in the way I understand you're suggesting would not allow enough room for all features on the ground level to be mapped without overlapps. Roofs may make buildings appear to touch in aerial imagery, when they in fact do not. How would this issue be solved? Would it not be better to add a functionality to the 3D renderer to show some overhang to buildings if you tag them in some way and don't want to use multipolygons to represent this? Or perhaps to find some other solution? What problem(s) are you trying to solve by employing a different definition? It seems to me that you're likely to create more. What does this allow us to do which https://wiki.openstreetmap.org/wiki/Simple_3D_Buildings does not? --Gregory Peony (talk) 16:43, 23 October 2025 (UTC)