[Tagging] Simplify building:part areas

"Christian Müller" cmue81 at gmx.de
Fri Aug 18 18:00:01 UTC 2017

> Sent: Fri, 18 Aug 2017 09:47:04 +0100
> From: "Javier Sánchez Portero" <javiersanp at gmail.com>
> To: "Tag discussion, strategy and related tools" <tagging at openstreetmap.org>
> Subject: Re: [Tagging] Simplify building:part areas
> Thank you. This clarifies me a lot because I had not thought to use both building=* and building:part=* in the footprint.

While you can do this, I do not suggest this.  It is prone to the same problems we had with closed ways for a long time, which is how to decide which tags refer to the outline only and which to the area encased.

OSMs good practices [1] states to create one osm object for each feature in reality, so if we describe different parts of one building these are all features different from the outline of the building (which may be imagined as a rubber band around all of the parts).

Whether you create overlapping ways or multipolygon relations to represent each part and the outline, is to some extent up to you.  You will find both methods in OSM currently and the community works with and handles both.
Try out kendzi3d plugin if you have not already, to find manifestation of this in tools.

However, I personally recommend to stick to a multipolygon approach.  While it may not ''seem'' as simple as the closed way approach, it has several advantages:

- overlapping ways step back to be used if and only if features truely overlap in reality (e.g. a retaining wall below a building wall)
- all ways used in defining multipolygon parts may take tags describing the linear feature, which again sticks to the recommendation: "one feature, one element"  (e.g. you have a building part with four sides, one made mainly from glass, the others with differently coloured plaster, you'd use an mp relation to describe the volume as an extruded area, and the individual bounding ways will hold wall and material tags)
- neighboring building parts are easily identifiable: the dividing wall (or equivalent) will take part in all the relations it divides


[1] http://wiki.openstreetmap.org/wiki/Good_practice#One_feature.2C_one_OSM_element

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