# [postgis-users] Difficult Problem with Polygons

Ed Linde edolinde at gmail.com
Mon Oct 29 03:03:34 PDT 2012

```Hi All,
Thanks for the suggestions.
For 1) I will look into how ST_touches works and see if it can pick up all
the one I have. And also look into Mike's suggestion on ST_relate...though
more complex.
For 2) I will try to clarify it a bit more... its harder to do without a
figure :) but here goes.

Lets say we have a point Q which is the generator of a voronoi cell. Now I
compute the
intersection between the voronoi cell boundaries and my triangulation (Set
of polygons)
using ST_intersect. Once I have these triangles.. I say pick one triangle T
that is
intersecting the voronoi cell boundary of Q.
For all the triangles adjacent to T, I need to know which triangles are
INSIDE the voronoi
boundary (closer to Q) and which adjacent triangles are just OUTSIDE the
voronoi
boundary (farther from Q). I am basically testing for a certain property by
"shrinking" the
voronoi cell (closer to Q) and another property when "expanding" the
voronoi cell (away from Q).
Just need to make this division of triangles. Haven't thought of a nice way
to do this in postgis 2.0
So any suggestions would greatly help.

Thanks,
Ed

On Mon, Oct 29, 2012 at 10:15 AM, Mike Toews <mwtoews at gmail.com> wrote:

> On 29 October 2012 21:33, Ed Linde <edolinde at gmail.com> wrote:
> > Hi All,
> > I need help with 2 hard problems. I store triangles in a table as
> POLYGON.
> >
> > 1. I want to know for a given triangle, which triangles share an edge
> > (adjacent) with this triangle.
>
> Sounds like you have a finite element mesh with nodes and elements.
> You can use ST_Relate with pattern 'FF2F11212' to pick out elements
> that share the same edge. This DE-9-IM is sort-of a custom ST_Touches,
> but only takes linear boundary overlaps. So if you have a table
> "elements", and you want to find ones that touch ID 567:
>
> SELECT elements.*
> FROM elements, elements as e
> WHERE e.id = 567 AND
>     ST_Relate(elements.geom, e.geom, 'FF2F11212');
>
>
> -Mike
> _______________________________________________
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>
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