[OSM-dev] What exactly is a polygon?

[Stefan Keller]

> So the whole thing is really messy.

Indeed... That was an issue many tried to resolve recently - including me:
c.f. this thread with a proposal:
http://lists.openstreetmap.org/pipermail/dev/2008-July/010937.html
I'm still convinved, that OSM deserves a better handling of real polygons... :->

Stefan

2008/11/6 Jochen Topf <jochen at remote.org>:
> On Thu, Nov 06, 2008 at 09:04:01AM +0100, Till Harbaum wrote:
>> i am still not sure i fully understand when exactly a way
>> a polygon is. What i've learned so far:
>>
>> If the first and the last node of a way are identical then the
>> way is a polygon, except it's tagged highway, waterway or
>> railway.
>>
>> What's the purpose of the area tag? Does it make a way
>> a polygon, even if the first and last node are not identical?
>> Is it used to make highways/waterways or railways a polygon?
>>
>> Should my tool automatically tag a way an area if the first
>> and last node are identical?
>>
>> What about more complex shapes? What if e.g. the first and
>> a node in the middle are identical? Is this is polygon with a "tail"?
>
> You can't say that a way is an area without looking und understanding
> the combination of all the tags.
>
> The only thing you know for sure is: If the first and last node are not
> identical, its not an area. And even that is debatable as you mention in
> your "polygon with a tail" example.
>
> And it gets even more complicated with polygons with holes or
> multipolygons which need additional relations to model them. So you can
> never only look at a way, you also have to check whether there is a
> 'multipolygon' relation that includes this way. (And, btw. the
> 'multipolygon' relation is misnamed, because it only models polygons
> with holes, not multipolygons.)
>
> So the whole thing is really messy. Everybody is using different ad-hoc
> methods to deal with this problem, thats why the renderings of areas
> often look different with different renderers.
>
> Jochen
> --
> Jochen Topf  jochen at remote.org  http://www.remote.org/jochen/  +49-721-388298
>
>
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