[geos-devel] createEmptyGeometry() creates instance ofGeometryCollection

[strk at refractions.net]

On Fri, Mar 31, 2006 at 12:04:39AM +0200, Mateusz £oskot wrote:
> Martin Davis wrote:
> > Yes.  The spec doesn't cover this, but I chose to use an empty 
> > GeometryCollection as a "typeless" empty geometry, for methods which 
> > need to return one.  The createEmptyGeometry method is just a 
> > convenience method for this convention.
> 
> OK, I understant it.
> 
> > Anyway, what else could that method return that would make sense 
> > (given that it has no type information)?
> 
> It depends on the idea behind "empty geometry" or "null geometry" [1].
> As I know, OGC Simple Feature Spec. explains that empty geometry
> represents empty point set (empty set of coordinates).
> According to my understanding:
> Point is empty if it contains "uninitialized" coordinates
> MultiPoint is empty if it contains no points in its set.
> etc.
> 
> There is also a distinction between empty and null.
> NULL means something undefined, uninitialized - something unsafe.
> EMPTY means something well constructor but with empty set of points.
> In some case, NULL object can be compared to EMPTY object with true result.

But what's defined in an EMPTY set of points ?
It's the points themself giving semantic of a geometry type.
What distinguish an EMPTY point from an EMPTY line from an
EMPTY collection ? It's the exactly same topological space:
the NO space at all. How can you say that NOTHING is a point
or a line or an area ?

--strk;