[postgis-users] Difficult Problem with Polygons

[Ed Linde]

Hi All,
Wondering if that diagram made things any simpler or is it still not clear
what the problem
is?

Thanks,
Ed

On Mon, Oct 29, 2012 at 11:57 AM, Ed Linde <edolinde at gmail.com> wrote:

> Attached is a figure. Where the dotted line is the boundary of the voronoi
> cell whose
> generator is point P1. So triangle "4" intersects with the voronoi
> boundary, but we are
> interested in the adjacent triangles of triangle 4, which are closer to
> point P1.
> For example, triangle 5.
>
> Hope this helps.
> Cheers,
> Ed
>
>
> On Mon, Oct 29, 2012 at 11:50 AM, Nicolas Ribot <nicolas.ribot at gmail.com>wrote:
>
>> Could you draw a figure ?
>>
>> Nicolas
>>
>> On 29 October 2012 11:03, Ed Linde <edolinde at gmail.com> wrote:
>> > 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 adjacent polygons to
>> > the one I have. And also look into Mike's suggestion on
>> ST_relate...though I
>> > must admit it looks
>> > 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');
>> >>
>> >> I'm not certain about your second question.
>> >>
>> >> -Mike
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>> >
>> >
>> >
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