[postgis-users] Approximate union of polygons?
Kevin Neufeld
kneufeld at refractions.net
Tue Oct 7 18:41:16 PDT 2008
Have you tried putting your resultant multipolygons through ST_Simplify
or ST_SimplifyPreserveTopology? Both use the Douglas-Peuker algorithm
to weed out unwanted vertices.
Cheers,
Kevin
Dan Johnson wrote:
> I have an application that keeps a database of video footprints, one
> quadrilateral per frame of video. I consolidate each 5 seconds of
> video footprints into a single multipolygon in order to speed up
> queries for video of a specified area. That's typically a union of 150
> (30fps) polygons with 4-vertexes. I build this multipolygon with
> ST_GeomUnion, and it's acceptably fast. So far, so good.
>
> I've been quite surprised by the variance in the number of vertices in
> the resulting multipolygon - sometimes it's as high as 1200. Usually
> it's lower than the equvalent set of frame footprints, but not always.
> This hasn't been a problem for database queries, but now it's time to
> build a user interface and of course the UI has problems displaying
> that many vertices. (Actually, the UI is getting the polygons via a
> SOAP message, and our web server's SOAP infrastructure often just dies
> when the result is so long.) Eyeballing these consolidated polygons,
> it looks like keeping only 20-30 vertices would provide an excellent
> approximation. (It's quite common for the camera to stare at a single
> point for a long time, in which case all the frames are in about the
> same location.) This seems like the sort of thing PostGIS would be
> able to do but I haven't been able to figure it out from the docs.
> Does anyone know if this is possible?
>
> (I've been told the footprints have to consider every frame, so
> building the union with one frame per second or similar is out.)
>
> Thanks,
> Dan
>
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