[postgis-users] Approximate union of polygons?

[Dan Johnson]

Thanks, Kevin, that's exactly what I want. I thought I'd scoured the 
documentation but somehow I missed ST_Simplify.

- Dan

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.
>
> 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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