[postgis-users] Difficult Problem with Polygons
Ed Linde
edolinde at gmail.com
Mon Oct 29 07:37:58 PDT 2012
Hi All,
Thanks for the tips! Just another thing, when I compute the "transition
points" on each edge (shown as red points in my pdf).
I need to join them to make a polygon. Wondering how I can connect them
together so that I start with a point and end on it
to form a closed polygon?
Cheers,
Ed
On Mon, Oct 29, 2012 at 3:03 PM, Stephen Woodbridge <woodbri at swoodbridge.com
> wrote:
> Hi Ed,
>
> Well if clarifies one thing at least, you can ignore Mike's st_Relate
> because 5 is not adjacent to a linear edge, it is only adjacent to a
> vertex, so st_touches should work fine.
>
> so you are looking for:
>
> 1. a specific triangle by id
> 2. that intersects triangle VC
> 3. and the triangles adjacent to triangle by id
> 4. and those that are contained in VC
> 5. sorted by distance to P1 limit 1
>
> so something like:
>
> select id, the_geom
> from (select id, the_geom as adjacent
> from triangles
> where st_touches(the_geom,
> (select the_geom
> from triangles a,
> (select the_geom as vc
> from vc_table
> where id='p1') b
> where a.id=4)) c
> order by st_distance(c.the_geom, (select the_geom
> from points
> where id='p1')) asc limit 1;
>
> Untested, but should give you a model to work with.
>
> -Steve W
>
>
>
> On 10/29/2012 6:57 AM, Ed Linde 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
>> <mailto:nicolas.ribot at gmail.**com <nicolas.ribot at gmail.com>>> wrote:
>>
>> Could you draw a figure ?
>>
>> Nicolas
>>
>> On 29 October 2012 11:03, Ed Linde <edolinde at gmail.com
>> <mailto: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
>> <mailto:mwtoews at gmail.com>> wrote:
>> >>
>> >> On 29 October 2012 21:33, Ed Linde <edolinde at gmail.com
>> <mailto: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 <http://e.id> = 567 AND
>>
>> >> ST_Relate(elements.geom, e.geom, 'FF2F11212');
>> >>
>> >> I'm not certain about your second question.
>> >>
>> >> -Mike
>> >> ______________________________**_________________
>> >> postgis-users mailing list
>> >> postgis-users at postgis.**refractions.net<postgis-users at postgis.refractions.net>
>> <mailto:postgis-users at postgis.**refractions.net<postgis-users at postgis.refractions.net>
>> >
>>
>> >> http://postgis.refractions.**net/mailman/listinfo/postgis-**users<http://postgis.refractions.net/mailman/listinfo/postgis-users>
>> >
>> >
>> >
>> > ______________________________**_________________
>> > postgis-users mailing list
>> > postgis-users at postgis.**refractions.net<postgis-users at postgis.refractions.net>
>> <mailto:postgis-users at postgis.**refractions.net<postgis-users at postgis.refractions.net>
>> >
>>
>> > http://postgis.refractions.**net/mailman/listinfo/postgis-**users<http://postgis.refractions.net/mailman/listinfo/postgis-users>
>> >
>> ______________________________**_________________
>> postgis-users mailing list
>> postgis-users at postgis.**refractions.net<postgis-users at postgis.refractions.net>
>> <mailto:postgis-users at postgis.**refractions.net<postgis-users at postgis.refractions.net>
>> >
>>
>> http://postgis.refractions.**net/mailman/listinfo/postgis-**users<http://postgis.refractions.net/mailman/listinfo/postgis-users>
>>
>>
>>
>>
>> ______________________________**_________________
>> postgis-users mailing list
>> postgis-users at postgis.**refractions.net<postgis-users at postgis.refractions.net>
>> http://postgis.refractions.**net/mailman/listinfo/postgis-**users<http://postgis.refractions.net/mailman/listinfo/postgis-users>
>>
>>
> ______________________________**_________________
> postgis-users mailing list
> postgis-users at postgis.**refractions.net<postgis-users at postgis.refractions.net>
> http://postgis.refractions.**net/mailman/listinfo/postgis-**users<http://postgis.refractions.net/mailman/listinfo/postgis-users>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/postgis-users/attachments/20121029/a16396bf/attachment.html>
More information about the postgis-users
mailing list