[geos-devel] supporting filters

[Kralidis,Tom [Burlington]]

> > Tom,
> > 
> > I'm also interested in some info about internals of DWithin 
> and Beyond 
> > operations.
> > These both operations are supported by FDO
> > (http://fdo.osgeo.org) but unfortunately I've not found much 
> > information about their specification.
> >
> 
> True.  The only info I found was from FES 1.0.0, section 8.2:
> 
> "The spatial operators DWithin and Beyond test whether the value of a
> geometric property is within or beyond a specified distance of the
> specified literal geometric value."
> 
> So both DWithin and Beyond take as inputs a geometry and 
> distance value.
>  
> > I also need to learn about it soon, so if I will find 
> > anything I let you know.

Further info from Peter Vretanos (FES 1.0.0 spec editor):

"According to the BNF in the Cat 1.0 specification DWithin tests to see
whether geometry A is completely within a certain distance of geometry
B.

I interpret this to mean a "buffer" query.  That is you create a buffer
around A at the specified distance and then do a WITHIN (as defined in
SFSQL) between this new buffer-expanded geometry (A') and the test
geometry B.  If B is completely within A' then the operator evaluates to
true

BEYOND is the opposite of DWITHIN.   If B is completely outside of A'
then the operator evaluates to true." 

Cheers

..Tom