[Tile-serving] [osm2pgsql] Multipolygon rendered using tags from outer way (#528)

[mboeringa]

Note that AFAIK osm2pgsql, and a lot of other software, do not (yet) process **type=building** relations in a (specialized) way. Most just interpret any valid multipolygon or closed way with a **building=x** tag as being a building.

Anyway, from a OpenStreetMap "Simple 3D Building" tagging perspective, there are probably a couple of issues with the object as you entered it:

- The **type=building** super-relation (https://www.openstreetmap.org/relation/5857090) that is supposed to tie all building:parts together, is carrying a tag of its own: **building=yes**, which duplicates the building=yes tag on the outline. As far as I can interpret the Simple 3D scheme, it is best not to add any tags to the **type=building** relation itself, but to only add them to the geometry representing the outline of building and using the **role=outline** role as part of the *type=building* super-relation. This is done to make it easier for software that can't or doesn't process super-relations, to still access all details of the building as stored on the outline. Of course, via a properly set part with **role=outline**, software that does have full understanding of all relation levels, can still access all of the same data as well. The **type=building** relation should consequently only be used to tie all parts together.

- The outline way (https://www.openstreetmap.org/relation/5856226) used as **role=outline** correctly has a building=yes tag and name=x tag, but *doesn't* include the inner courts of the building by cutting them from the feature (way set as "inner" for the multipolygon). Although the specification doesn't require it, I would recommend to exclude or cut the area of inner courts of 
https://github.com/openstreetmap/osm2pgsql/issues/528#buildings as part of a multipolygon outline, so as to better represent them as an outline feature. Inner courts can be very big, and not showing them gives a false impression of the true building's size and form or structure. I would suggest to interpret the "outline" more as a "footprint" of the building on ground level, instead of the most outer way.

- The outline way (https://www.openstreetmap.org/relation/5856226), and some other building:part(s), has a **building:min_level=1** tag set. According to this page: http://wiki.openstreetmap.org/wiki/Simple_3D_buildings, the **building:min_level=x** tag specifies how many levels are ***not*** present in a certain building part. I am pretty sure, looking at your F4Map example, you did not intend to have this part of the building to not have a street level storey (which seems unlikely and also not displayed in F4Map)?

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Reply to this email directly or view it on GitHub:
https://github.com/openstreetmap/osm2pgsql/issues/528#issuecomment-171432801
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