[postgis-users] 3d semantic / binary predicates

[strk at refractions.net]

I realized I was confused about the topic (dumb me!).

The problem is that postgis does not really support 3d,
but just a z value on each point.

This means that any postgis (and OGC) areas is a plane,
possibly NOT parallel to the ground but still a plane.
An exception is using polygon holes, which make
a collection of trapezoids (dunno the word in english)
formed by connecting segments of the holes and the outer
ring.

This means you cannot have a BOX3D as a 3-dimensional
polygon geometry. So envelope3d(geometry) is IMPOSSIBLE
to encode !

Would GEOS do better ? Thats a good question !
(I don't think so)

--strk;

On Wed, Oct 06, 2004 at 12:43:46PM +0200, strk at refractions.net wrote:
> On Wed, Oct 06, 2004 at 03:37:28AM -0700, Ron Mayer wrote:
> > 
> > 
> > On Wed, 6 Oct 2004 strk at refractions.net wrote:
> > > mmm.. check how those BOX3d gets casted to a geometry.
> > > The problem might be in the cast only using 2d.
> > 
> > Seems so...
> > 
> > fl=# select envelope('BOX3D(-1 -1 -1, 1 1 -2)'), 
> >             envelope('BOX3D(-1 -1  1, 1 1  2)');
> >                    envelope                   |                   envelope                   
> > ----------------------------------------------+----------------------------------------------
> >  SRID=-1;POLYGON((-1 -1,-1 1,1 1,1 -1,-1 -1)) | SRID=-1;POLYGON((-1 -1,-1 1,1 1,1 -1,-1 -1))
> > 
> > I'll check with the latest versions and if it's still happening I'll 
> > mention it (on the geos list?).
> 
> Don't bother... the envelope(geometry) thing is in my todo list,
> not implemented yet.
> Probably will never get into the 0_9_0 branch...
> (unless you make enough noise).
> 
> (cuts)
> > > > On the windows machine I used the pre-compiled binary form dcmms
> > > > that was called "postgis-setup-0.9.0-1.exe" yet 
> > > > postgis_full_version() tells me it's "POSTGIS="0.8.2".   Hmm.
> > > > Guess I'll need to learning how to compile something with windows.
> > > > If I figure out how to use a complie on wndows, I'll try GEOS2.0.1 
> > > > there as well.
> > > 
> > > That's very bad :(
> > > Try also postgis_lib_version() but that should ont change.
> > > Are you sure the library you're using (check probin from pg_proc
> > > where proname = 'postgis_lib_version') is the 0.9.0 one ?
> > 
> > All it really says is '$libdir/libpostgis.dll', and when I do a strings of 
> > it I see 0.8.2 and no 0.9.0.   However the only installer .exe currently 
> > on the machine (according to find files) is called postgis-setup-0.9.0-1.exe.   
> > I'll ask around work tomorrow (ugh, later today) to see if anyone else 
> > might have touched it.
> > 
> > Thanks for your pointers so far.  I'll re-do the whole experiment being 
> > carful to note the things you mentioned before taking more of your time.
> 
> Thank you.
> I don't think this is a waste of time.
> It makes readers (and writers) in sync..
> --strk;
> 
> > 
> >    Ron
> > 
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