[Tagging] site relations for city walls?

Volker Schmidt voschix at gmail.com
Tue Jul 14 16:33:13 UTC 2020

Sorry to keep riding this horse, but many of my examples have areas, ways
and nodes as members, so they cannot be described by any kind of polygon.
Lets take my favourite example of a dismantled railway.
It contains:

   - nodes: tourist information tables
   - ways: embankments, all kinds of highways
   - areas: former railway buildings, bridge structures, vegetation areas
   (that correspond to the former rail bed)

On Tue, 14 Jul 2020 at 18:17, Peter Elderson <pelderson at gmail.com> wrote:

> https://wiki.openstreetmap.org/wiki/Multipolygon_Examples  example 1.7
> describes disjunct outers.
> Too bad you have to wrestle through some very complicated examples to get
> there if you start at the beginning. And, these complex examples should not
> be followed, because they advocate tying landuse to ways, borders to ways
> and other stuff you really should not do if you want to keep the map
> unbroken.
> Best, Peter Elderson
> Op di 14 jul. 2020 om 18:05 schreef Peter Elderson <pelderson at gmail.com>:
>> 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
>> multipolygon.
>> 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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