[postgis-users] merging geometries of buffer on large data set

[Sitansu pattnaik]

Have you thought of using rasters ?
Sitansu
@ kCube


On Sun, Jun 30, 2013 at 10:33 PM, Martin Davis <mtnclimb at telus.net> wrote:

>  Hi, Karsten.
>
> One potential problem with merging all the buffers for a given value is
> that you may wind up with a very large polygon that creates its own
> handling issues.
>
> If you only need this for analytical purposes, not for display, then it
> might work better to aim for a middle ground.  You could merge the buffers
> in "clumps".  To do this, you need to partition the buffers into
> spatially-coherent groups.   A simple way to do this is to create a grid
> over the area (say based on the coordinate system you are using).  Create a
> reprsentative point for each buffer (e.g. the interior point or centroid).
> The buffers can then be assigned to the grid cell their point lies in.
> Then group the buffers by their grid cell, and union each group.
>
> Actually, if you want to carry on and create single polygons, then the
> clumped buffer polygons give a better basis to work from (since they should
> have many fewer points than the source polygons).
>
> If there are still memory issues with the clumped unions, then you can
> carry out this process in an iterated fashion, starting with small grid
> cells and then repeating with a larger size.  (At this point you will have
> basically reimplemented the GEOS CascadedUnion algorithm.  But that's ok -
> performing the algorithm at the SQL level allows more control over memory
> and processing usage).
>
> I'd be interesting to hear if this approach works out.
>
> Martin
>
>
> On 6/28/2013 5:11 PM, karsten vennemann wrote:
>
>  I was wondering if there is a good (or best practice) approach on how to
> merge geometry features that are touching or overlapping and  have one
> common value in one table field.****
>
> ** **
>
> Here is what I was trying to do: given a large dataset such as the
> (detailed NHD data layer) of rivers in California I created multiple
> buffers and inserted the results into a new table with one geometry column
> adding a score value to each of the same buffers distances used. Thus the
> buffer polygon layer has a score with a value of 10,100,500 and 1000 m
> corresponding to the buffer distance used. Given the approach I used  to
> create the buffers those are often spatially overlapping (because there was
> no merge operation of the buffers and because the rivers are split along
> the flow line in multiple segments by node in the source shape file). The
> resulting layer works ok for my purposes (which is to retrieve information
> in which buffers a certain location is intersecting it with the river
> buffer (results can be 10,100,500 and 1000 or no intersect with the
> buffers).****
>
> Now the layers is about 20 GB big disk size having a lot of unnecessary
> geometries with are overlapping.****
>
> How can I go about merging all the existing geometries on this huge data
> set into a result layer that has (optimally ) only 4 polygons with the
> result scores to find my intersects. ****
>
> When I tried some of my own approaches (e.g. using  st_collect and such to
> do this) so far whenever I started  these sever resource intensive
> operations soon these where aborted by the system because i got some kind
> of out of memeonry errosr on my server (an ubuntu achjien). Is there a good
> way to optimize this kind of query operation without using 100% of my
> server ram so that I will not run out of memory or resulting in a lengthy
> query that would be running for 6 weeks or so J ?****
>
> Any query examples or general  insight are  greatly appreciated .****
>
> ** **
>
> Cheers****
>
> Karsten****
>
> ** **
>
> Karsten Vennemann****
>
>  ****
>
> Terra GIS Ltd****
>
> 2119 Boyer Ave E****
>
> Seattle, WA 98112****
>
> USA****
>
> tel ++ 206 905 1711****
>
> fax ++ 925 905 1711****
>
> ** **
>
>
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