[postgis-users] Difficult Problem with Polygons
Ed Linde
edolinde at gmail.com
Mon Oct 29 09:34:50 PDT 2012
I tried something like the following without removing the same points ad it
seems to work:
SELECT st_astext(ST_ConvexHull(ST_GeomFromText('MULTIPOINT(593901 5219610
814,593901 5219610 814,593899 5219610 814,593899 5219610 814,593901 5219610
814)')));
"LINESTRING Z (593901 5219610 814,593899 5219610 814)"
Ed
On Mon, Oct 29, 2012 at 5:23 PM, Nicolas Ribot <nicolas.ribot at gmail.com>wrote:
> You should filter out the consecutive points. If there are more than 2
> points, you can then call st_convexHull() on the point set:
>
> with points as (
> select 'POINT (0 0)'::geometry as geom
> UNION
> select 'POINT (1 0)'::geometry as geom
> UNION
> select 'POINT (1 1)'::geometry as geom
> ) select st_convexHull(st_collect(geom))
> from points;
>
> If the points are aligned, convexHull will be a linestring, that you
> can discard or further process according to your needs.
>
> Nicolas
>
> On 29 October 2012 16:51, Ed Linde <edolinde at gmail.com> wrote:
> > Hi Nicolas,
> > It seems like sometimes the points are one and the same and I get
> > linestrings and not
> > actual polygons. I pass these points in from a perl script, so is there a
> > way I can just
> > give the raw x,y,z coordinates of these points and do a st_convexhull
> > function on them?
> > Could you please give me a small example?
> >
> > I tried something like:
> > The two coordinates marked in red and marked in orange are the same.
> >
> > bounds=# insert into vfaces values (1,
> > bounds(# ST_GeomFromText('POLYGON((593901 5219610 814,593901 5219610
> > 814,593899 5219610 814,593899 5219610 814,593901 5219610 814))') );
> > INSERT 0 1
> > bounds=# select st_isvalid(geomtext) from vfaces;
> > NOTICE: Too few points in geometry component at or near point 593901
> > 5219610 814
> >
> > st_isvalid
> > ------------
> > f
> > (1 row)
> >
> > Thanks,
> > Ed
> >
> >
> >
> >
> > On Mon, Oct 29, 2012 at 4:26 PM, Ed Linde <edolinde at gmail.com> wrote:
> >>
> >> Ok thanks, will look into that function. Because I wonder if the s/w I
> am
> >> using is
> >> actually outputting the vertices of each face in a cyclical fashion or
> >> just
> >> arbitrarily.
> >>
> >> Cheers,
> >> Ed
> >>
> >>
> >> On Mon, Oct 29, 2012 at 4:23 PM, Nicolas Ribot <nicolas.ribot at gmail.com
> >
> >> wrote:
> >>>
> >>> Yes, it looks like some points were not put in the right order before
> >>> building a polygon, thus these "butterfly" polygons you generated.
> >>> If the formed polygon are expected to be convex, you could use
> >>> st_convexhull on the point cloud to generate the polygons.
> >>>
> >>> On 29 October 2012 16:09, Ed Linde <edolinde at gmail.com> wrote:
> >>> > Thanks Nicolas! Will look at the script that generated the polygon
> >>> > text..
> >>> > must have goofed something up there.
> >>> >
> >>> >
> >>> > On Mon, Oct 29, 2012 at 4:05 PM, Nicolas Ribot
> >>> > <nicolas.ribot at gmail.com>
> >>> > wrote:
> >>> >>
> >>> >> Hi,
> >>> >>
> >>> >> No. this is because some of your polygons are not valid:
> >>> >> for instance:
> >>> >> select st_isvalid('POLYGON ((593921 5219610 803,593921 5219610
> >>> >> 818,593921 5219620 818,593921 5219620 803,593921 5219610
> >>> >> 803))'::geometry);
> >>> >> NOTICE: Too few points in geometry component at or near point
> 593921
> >>> >> 5219610 803
> >>> >> st_isvalid
> >>> >> ------------
> >>> >> f
> >>> >> (1 row)
> >>> >>
> >>> >> you can control this with st_isvalid, st_isvalidReason and correct
> >>> >> them with st_makeValid.
> >>> >>
> >>> >> Use only valid objects before processing them with Postgis
> functions.
> >>> >>
> >>> >> Nicolas
> >>> >>
> >>> >> On 29 October 2012 16:03, Ed Linde <edolinde at gmail.com> wrote:
> >>> >> > Thanks Nicolas. Just about the error, is this because the line
> >>> >> > segments
> >>> >> > are
> >>> >> > too close
> >>> >> > and postgis 2.0 could not handle this? If so is there a
> workaround,
> >>> >> > even
> >>> >> > if
> >>> >> > it means
> >>> >> > slightly having to "perturb" each point's position to not run into
> >>> >> > this
> >>> >> > bug.
> >>> >> > I was really hoping that the intersection of two polygonal shapes
> in
> >>> >> > 3D
> >>> >> > would be fairly
> >>> >> > simple in postgis 2.0 :(
> >>> >> >
> >>> >> > Ed
> >>> >> >
> >>> >> >
> >>> >> > On Mon, Oct 29, 2012 at 4:00 PM, Nicolas Ribot
> >>> >> > <nicolas.ribot at gmail.com>
> >>> >> > wrote:
> >>> >> >>
> >>> >> >> If the points are computed in the right order, you can store them
> >>> >> >> and
> >>> >> >> pass them to st_makeLine and st_makePolygon.
> >>> >> >> If not, you can form a segment between 2 closest points and
> connect
> >>> >> >> it
> >>> >> >> to the closest points.
> >>> >> >>
> >>> >> >> On 29 October 2012 15:37, Ed Linde <edolinde at gmail.com> wrote:
> >>> >> >> > 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>> 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
> >>> >> >> >>> <mailto:postgis-users at postgis.refractions.net>
> >>> >> >> >>>
> >>> >> >> >>> >>
> >>> >> >> >>>
> http://postgis.refractions.net/mailman/listinfo/postgis-users
> >>> >> >> >>> >
> >>> >> >> >>> >
> >>> >> >> >>> >
> >>> >> >> >>> > _______________________________________________
> >>> >> >> >>> > postgis-users mailing list
> >>> >> >> >>> > postgis-users at postgis.refractions.net
> >>> >> >> >>> <mailto:postgis-users at postgis.refractions.net>
> >>> >> >> >>>
> >>> >> >> >>> >
> >>> >> >> >>>
> http://postgis.refractions.net/mailman/listinfo/postgis-users
> >>> >> >> >>> >
> >>> >> >> >>> _______________________________________________
> >>> >> >> >>> postgis-users mailing list
> >>> >> >> >>> postgis-users at postgis.refractions.net
> >>> >> >> >>> <mailto:postgis-users at postgis.refractions.net>
> >>> >> >> >>>
> >>> >> >> >>>
> >>> >> >> >>>
> http://postgis.refractions.net/mailman/listinfo/postgis-users
> >>> >> >> >>>
> >>> >> >> >>>
> >>> >> >> >>>
> >>> >> >> >>>
> >>> >> >> >>> _______________________________________________
> >>> >> >> >>> postgis-users mailing list
> >>> >> >> >>> postgis-users at postgis.refractions.net
> >>> >> >> >>>
> http://postgis.refractions.net/mailman/listinfo/postgis-users
> >>> >> >> >>>
> >>> >> >> >>
> >>> >> >> >> _______________________________________________
> >>> >> >> >> postgis-users mailing list
> >>> >> >> >> postgis-users at postgis.refractions.net
> >>> >> >> >> http://postgis.refractions.net/mailman/listinfo/postgis-users
> >>> >> >> >
> >>> >> >> >
> >>> >> >> >
> >>> >> >> > _______________________________________________
> >>> >> >> > postgis-users mailing list
> >>> >> >> > postgis-users at postgis.refractions.net
> >>> >> >> > http://postgis.refractions.net/mailman/listinfo/postgis-users
> >>> >> >> >
> >>> >> >> _______________________________________________
> >>> >> >> postgis-users mailing list
> >>> >> >> postgis-users at postgis.refractions.net
> >>> >> >> http://postgis.refractions.net/mailman/listinfo/postgis-users
> >>> >> >
> >>> >> >
> >>> >> >
> >>> >> > _______________________________________________
> >>> >> > postgis-users mailing list
> >>> >> > postgis-users at postgis.refractions.net
> >>> >> > http://postgis.refractions.net/mailman/listinfo/postgis-users
> >>> >> >
> >>> >> _______________________________________________
> >>> >> postgis-users mailing list
> >>> >> postgis-users at postgis.refractions.net
> >>> >> http://postgis.refractions.net/mailman/listinfo/postgis-users
> >>> >
> >>> >
> >>> >
> >>> > _______________________________________________
> >>> > postgis-users mailing list
> >>> > postgis-users at postgis.refractions.net
> >>> > http://postgis.refractions.net/mailman/listinfo/postgis-users
> >>> >
> >>> _______________________________________________
> >>> postgis-users mailing list
> >>> postgis-users at postgis.refractions.net
> >>> http://postgis.refractions.net/mailman/listinfo/postgis-users
> >>
> >>
> >
> >
> > _______________________________________________
> > postgis-users mailing list
> > postgis-users at postgis.refractions.net
> > http://postgis.refractions.net/mailman/listinfo/postgis-users
> >
> _______________________________________________
> postgis-users mailing list
> postgis-users at postgis.refractions.net
> http://postgis.refractions.net/mailman/listinfo/postgis-users
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/postgis-users/attachments/20121029/511b22ad/attachment.html>
More information about the postgis-users
mailing list