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Subject: [ggl] Snapping
From: Mateusz Loskot (mateusz)
Date: 2009-11-30 17:59:11

Stephen Leary wrote:
> 3. Yes and No.

So, it's an acceptable deviation of something else or
number of properties.

> A geometric tolerance is used when for example two vertices are less
> than the geometric tolerance a comparison operator will return true.

Yes, it's been clear since the beginning, but...

> This is not always the numerical precision so much as the limit of
> what we care about in real life. i.e. less than 1mm on a countrywide
> map might be the geometric tolerance. Snapping would be 1cm in this
> case.

It seems to be confirming my assumption that geometric tolerance will be
based on actual size and coordinate of geometry.
...bug it is not clear to me how the snapping would work:
- for non-GIS geometries (no CRS, no projection)
- no precision model
- the only information we have is: type of coordinate (float, integral),
  dimension, size of geometry (minimal bounding rectangle)

What would be geometric tolerance in that case?
A fraction of a size of geometry (or MBR)?

I imagine, that for vertex-to-vertex snapping this kind of issues could
be solved relatively easy by generating buffers of vertices using
snapping tolerance as buffer distance and finding if buffers intersect.
IOW, point equlity is more fragile than intersection of larger
geometries. However, that's the simplest case of snpaping :-P

> 6. I'm happy to once I can include everyones comments.

Sure. Though, my suggestion was that Wiki is an alive and growing
animal, no need to wait for all details first :-)

Best regards,

Mateusz Loskot,
Charter Member of OSGeo,

Geometry list run by mateusz at