[Tagging] site relations for city walls?

Tue Jul 14 18:24:33 UTC 2020

Sure! I was just sidestepping about the parking lot example.

Best, Peter Elderson

Op di 14 jul. 2020 om 18:34 schreef Volker Schmidt <voschix at gmail.com>:

> 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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