3D operations (Re: [geos-devel] C API coordinate sequencedimensions)

[Chris Hodgson]

AFAIK, the "3-d" functions were never truly 3-d, more what is sometimes 
called 2.5-d. That is, all of the operations return the same result, in 
2-d, that they would return if the inputs had been 2-d - but then some 
heuristics are used to come up with a reasonable value for the 3rd 
coordinate.

So in your example, Regina, the two polygons DO intersect in 2-d. In 
2.5-d, there is no way for something to be "above" something else. Think 
of the z value as the height of the line - not the elevation, but think 
of the line as having a height, like a ribbon on it's side.

These 2.5-d operations are very useful, particularly the cases which 
have well-defined results. They are also not compatible with true 3-d 
operations, but since GEOS doesn't support volumes, (polyhedrons and 
spheres and the like) true 3-d operations aren't something I think we 
need to worry about yet.

Chris

Obe, Regina wrote:
> Well from my observation it looks like most of those Ops functions (not
> sure about Union and Intersection) -- first do a 2D operation and then
> do some sort of apply the Z back on.  So the Z is never considered in
> the basic operation -- thus the cause of a lot of the goofiness.
>
>  Take this example.  It seems clear to me that these polygon's are not
> within each other so it should be a multipolygon not a Polygon with a
> hole. One lies above the other.
>
> SELECT ST_AsEWKT(ST_BuildArea('MULTILINESTRING((0 0 1,20 0 1,20 20 1,0
> 20 1,0 0 1),(10 10 2,10 11 2,11 11 2,11 10 2,10 10 2))'))
>
> "POLYGON((0 0 1,0 20 1,20 20 1,20 0 1,0 0 1),(10 10 2,11 10 2,11 11 2,10
> 11 2,10 10 2))"
>
>
>
>
>
> -----Original Message-----
> From: geos-devel-bounces at lists.osgeo.org
> [mailto:geos-devel-bounces at lists.osgeo.org] On Behalf Of strk
> Sent: Wednesday, January 14, 2009 4:04 AM
> To: GEOS Development List
> Subject: Re: 3D operations (Re: [geos-devel] C API coordinate
> sequencedimensions)
>
> On Wed, Jan 14, 2009 at 07:54:21AM +0000, Mark Cave-Ayland wrote:
>
>   
>> Can anyone with more experience with BuildArea (like Martin!) comment
>>     
> on
>   
>> whether this is a GEOS bug or simply a problem with behaviour > 2
>> dimensions being undefined?
>>     
>
> JTS didn't support 3d when I added it to GEOS, so can't really
> serve as a source of information. The 3d task was added driven by
> customer needs, so the only working and tested functions was the one
> the customer needed. If I recall correctly it was mainly Intersection
> and probably Union.
>
> Since other functions are based on the former, there may be side-effects
> you wouldn't care about if you aren't using 3d features.
>
> Paul draw the specs about expected behaviour. Dunno if there's a doc
> around and I likely lost mine (Oops).
>
> --strk; 
>
>  Free GIS & Flash consultant/developer      ()  ASCII Ribbon Campaign
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