[Tagging] site relations for city walls?

Peter Elderson pelderson at gmail.com
Tue Jul 14 16:05:54 UTC 2020

Just two outers is a regular use of multipolygon.
If the tags of two areas are the same, you can represent two or more
distinct areas as a multipolygon

If you have one area as a multipolygon with an inner, a separate closed way
can be used as an extra outer, it will then get the attributes of the

Major renderers support this.

One parking lot on two sides of a road is perfect for this method.

Best, Peter Elderson

Op di 14 jul. 2020 om 16:55 schreef Lionel Giard <lionel.giard at gmail.com>:

> Wouldn't a multipolygon with just two outers solve that parking case?
>> Best Peter Elderson
> That's a bit of a stretch of the multipolygon definition as there is no
> inner ring.  I never used multipolygon for anything else than complex
> geometry (with inner ring(s)) and that seems to be what the feature is for.
> As we already have the site relation for grouping features that are part
> of the same thing, but disjoint, i think that it is good to use it. It also
> solves the problem when mappers use multipolygon for two polygons sharing
> the same edge (it is forming an invalid geometry), while with site relation
> it is not a problem. Another advantage is that it is quite easy to edit.
> You just need to add or remove a feature : no specific roles (yet) or order
> needed.
> Le lun. 13 juil. 2020 à 23:29, Volker Schmidt <voschix at gmail.com> a
> écrit :
>> On Mon, 13 Jul 2020 at 22:56, Martin Koppenhoefer <dieterdreist at gmail.com>
>> wrote:
>>> actually all of these could be „grouped“ with tags alone, e.g
>>> distributed museums could have an identifying „network“ tag (or sth
>>> similar).
>> But why invent a new network tag, if we have  a site relation, waiting to
>> be used. (I was thinking of open air museums, where the various exhibits
>> are spread over the landscape)
>>> For power plants a site might be appropriate, if an area does not do it
>>> and you don’t want to rely on only tags.
>> If you have ever looked at the complexities of a hydro-power-plant with
>> dams, lakes, pipes, turbines deep in the mountains or in dedicated
>> buildings . they are really complex, and only parts of it are visible on
>> the surface.
>>> In theory objects like the Great Wall in China can and should be modeled
>>> as areas, although they seem to be linear in nature, they are also thick
>>> enough to „require“ an area representation in order to be well mapped in
>>> the scale of OpenStreetMap (you can walk on it).
>> That's not true - you can walk on parts of it, other parts are completely
>> missing, others are heaps of stones.
>>> In practice we would also want a way to have preliminary mapping as a
>>> line, and mixed geometry relations. A multipolygon relation for all parts
>>> of the great wall would likely be broken every day, and would be over the
>>> member limits for relations.
>> It's not a multipolygon - it is bits and pieces, some connected, same
>> not. Some may be linear (in first approximation).
>>> Would those that are in favour of using a site relation for a linear,
>>> circular, interrupted structure, 19km long and some meters wide, also see
>>> it as a good relation type for the Chinese Great Wall?
>> You lost me with your question here.
>> Volker
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