[postgis-users] Approximate union of polygons?
dmj at nrtc.northrop.com
Wed Oct 8 13:41:27 PDT 2008
Thanks, Kevin, that's exactly what I want. I thought I'd scoured the
documentation but somehow I missed ST_Simplify.
Kevin Neufeld wrote:
> Have you tried putting your resultant multipolygons through
> ST_Simplify or ST_SimplifyPreserveTopology? Both use the
> Douglas-Peuker algorithm to weed out unwanted vertices.
> 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.)
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