I’m drawing building footprints and I’m starting to wonder if I should clip them to pertain to just exterior walls. My rationale is if you’re walking and looking at a map, you could be under an awning, but you don’t want to be shown as within the footprint.
Is this too niche of a question? Most maps are probably not being drawn this detailed at this time?? Thanks for any input.
For building footprints/outlines, yes, you only include footprint (area at the ground).
Sometimes, some building are more detailed. Maybe not for awnings.
A more advanced tagging scheme for buildings is currently: https://wiki.openstreetmap.org/wiki/Simple_3D_buildings
Actually it is more usual on OSM to include the area of the building visible from above, including covered walkways, canopies etc (but obviously not movable fixtures). The reason for this is that in many cases when buildings are mapped from aerial images the mapper may not know the actual arrangement on the ground. Similarly even if the situation on the ground is known by the original mapper, it might not be by subsequent mappers who may perceive a building as being inaccurately mapped. I recall Marek emphasising this point in a talk at a SotM conference: i.e., there is a difference between a building outline (seen from above) and its footprint (on the ground)
The most usual approach is to map these areas with building:part and wall=no (largely because a large number of such outlines have been imported from the French Cadastre).
Note that Simple 3D Buildings has this to say about the topic: “The building outline represents the area of land covered by the union of all parts of the building”. To give an example, the part above the motorway for this building would be included in the outline according to that principle – not just the building’s ground-level footprint. So depending on your current elevation, you definitely can be outside a building despite being inside its 2D outline.
Awnings are a bit tricky, as I don’t believe there’s a good approach for mapping them yet, and they often aren’t permanent, are they? (Although some variants seem to be permanent and could be mapped as building parts using S3DB.)
Judging from the distribution map, the wall=no tag seems to have relatively limited use outside of the French Cadastre import. To me, it doesn’t seem very useful in practice because the import used it for such a broad range of things – from roofs to sheds.
For manual mapping with S3DB, I would rather recommend using building:part=roof (or building=roof for stand-alone roofs that aren’t part of a larger building).
