[postgis-users] geos result sets

[Wood Brent]

> 
> JTS/GEOS is not in fact "mathematically correct" (as you use the term) -
> it is bound by the same precision limitations as any system that uses
> floating point numbers to represent continuous quantities. 

Agreed, but it does not (as I understand it) have an internal storage system
where points have values changed arbitrarily to fit an internal
construct/representation (except for the necessary precision limitations). If
you insert a point at a location & retrieve it, you will get the same value
back within the system precision tolerances. This does NOT happen with all GIS
systems. Mathematically correct was poor way to express this, (computationally
correct is meaningless)- what is a tidy way to express it?

> As you say,
> when it computes an intersection point of two line segments (which is
> THE fundamental computation in constructive spatial functions) it can
> only compute an approximation to the actual value.  This is unavoidable.
> The best you can hope for is that the computation is robust (i.e. does
> not fail unexpectedly due to topological inconsistencies caused by the
> inaccurate intersection points) and that the answers produced are
> topologically correct and "close" to the right answer (according to some
> measure).  

And also consistent. Which GEOS is. A point inserted will not have a slightly
different value if inserted in maps of different scales. A computed
intersection won't either. At least in GEOS "maps" don't change with scale.


> 
> This is a HARD problem to do in general.

No argument from me there. I don't wanna do it. But some nice person is
providing a tool I can use :-)



Cheers (& thanks for work you guys put in)

 Brent Wood