[postgis-users] Geometric precision

[strk at refractions.net]

On Thu, Feb 02, 2006 at 02:56:44PM +0100, Tim Baumgartner wrote:
> Hi,
> 
> I have two questions. Perhaps somebody can help
> 
> 1.
> I am porting a GIS-related application from Oracle to PostGIS. In 
> Oracle, It was possible to define the precision of spatial data, so that 
> two points x(0.001, 3) and y(-0.001, 3) might be considered equal by the 
> database. Is something like that possible in PostGIS as well?

You can filter your geoms using SnapToGrid.

> 2.
> I noticed something that I don't understand when testing spatial queries:
> 
> insert into testtable values(GeomFromText('POINT(10.3 10.3)'));
> SELECT * FROM testtable where g && SetSRID('BOX3D(10.300001    10,    
> 11     11)'::box3d,-1)
> 
> => found something
> 
> SELECT * FROM testtable where g && SetSRID('BOX3D(10.30001    10,    
> 11     11)'::box3d,-1)
> 
> => found nothing
> 
> Is there a rule for this behaviour? Does it have something to do with 
> the fact that I am using -1 for SRID?

Nothing to do with SRID, precision should be the problem.
SnapToGrid both inputs (g and BOX) and see what happen.

BTW, there was a bug in SnapToGrid that would likely give
you problems, better always dropbbox() unless you upgrade to
postgis head (1.1.2CVS):

	// this should work:
	DropBBOX(SnapToGrid(g)) ....

--strk;