[postgis-devel] closed surfaces, volum or just closed surface

[Nicklas Avén]

Great, 
just wanted to calibrate the thinking :-)/Nicklas

2010-08-17 Olivier Courtin wrote:


>On Aug 17, 2010, at 1:35 PM, Nicklas Avén wrote:
>
>Nicklas,
>
>> in #568 you write:
>> "As a consequence closed surface (so volume) have a 0 perimeter 
>> value return"
>>
>> Is it so. Isn't it nessecary to be able to describe a closed survace 
>> without handling it as a volum or polyhedron?
>> I mean like a closed linestring. Sometimes you just want a closed 
>> linestring not a polygon.
>>
>> I thought this was to opff topic in #568 so I put it in a separate 
>> post.
>>
>> Does the spec say anything about this?
>
>Well in fact in ISO 19107 we have all the kind of geometries we could
>dream in GIS field fully describded.
>And so we've got points, curves, surfaces, solids, conic and so on...
>
>Right now this full model is really ambitous.
>(This model is also used in GML 3, and so in CityGML)
>
>Current spatial databases standard only take a part of it.
>(Even if we have strong clues that the next one will take in count
>the full ISO 19107 model.)
>
>And in fact PolyhedralSurface or TIN are initialy surfaces.
>So by analogy they behave more like your linestring (who could be 
>closed).
>
>And still by analogy your polygon equivalent in 3D is a solid
>in ISO 19107, but there's still no database standard to describe it.
>
>So the best we have right now (in spatial database standards) is 3D 
>closed surface...
>
>
>> The difference shows for example when we measure distance from a 
>> closed survace/volum
>> to a point inside it. If it is a volum the distance is 0 if it is 
>> just a closed surface there is a distance
>> to calculate to the closest facet.
>
>I'm agree with you that's a really good idea to have
>a flag to indicate on the geometry if it's closed or not (volume or 
>surface)
>
>I've just began to think on a topology serialization,
>need to work on it far more...
>
>
>--
>Olivier
>
>
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