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Hi, Karsten.<br>
<br>
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.<br>
<br>
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.<br>
<br>
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).<br>
<br>
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).<br>
<br>
I'd be interesting to hear if this approach works out. <br>
<br>
Martin<br>
<br>
<div class="moz-cite-prefix">On 6/28/2013 5:11 PM, karsten vennemann
wrote:<br>
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<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:10.0pt;
font-family:Arial">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.<o:p></o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:10.0pt;
font-family:Arial"><o:p> </o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:10.0pt;
font-family:Arial">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).<o:p></o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:10.0pt;
font-family:Arial">Now the layers is about 20 GB big disk
size having a lot of unnecessary
geometries with are overlapping.<o:p></o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:10.0pt;
font-family:Arial">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. <o:p></o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:10.0pt;
font-family:Arial">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 </span></font><font
face="Wingdings" size="2"><span
style="font-size:10.0pt;font-family:Wingdings">J</span></font><font
face="Arial" size="2"><span
style="font-size:10.0pt;font-family:Arial"> ?<o:p></o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:10.0pt;
font-family:Arial">Any query examples or general insight
are greatly
appreciated .<o:p></o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:10.0pt;
font-family:Arial"><o:p> </o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:10.0pt;
font-family:Arial">Cheers<o:p></o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:10.0pt;
font-family:Arial">Karsten<o:p></o:p></span></font></p>
<p class="MsoNormal"><font color="navy" face="Verdana" size="3"><span
style="font-size:12.0pt;font-family:Verdana;color:navy"
lang="DE"><o:p> </o:p></span></font></p>
<p class="MsoNormal"><font color="navy" face="Verdana" size="3"><span
style="font-size:12.0pt;font-family:Verdana;color:navy"
lang="DE">Karsten Vennemann</span></font><span lang="DE"><o:p></o:p></span></p>
<p class="MsoNormal"><font face="Times New Roman" size="3"><span
style="font-size:12.0pt" lang="DE"> <o:p></o:p></span></font></p>
<p class="MsoNormal"><font color="navy" face="Verdana" size="2"><span
style="font-size:10.0pt;font-family:Verdana;color:navy"
lang="DE">Terra GIS Ltd</span></font><span lang="DE"><o:p></o:p></span></p>
<p class="MsoNormal"><font color="navy" face="Verdana" size="2"><span
style="font-size:10.0pt;font-family:Verdana;color:navy"
lang="DE">2119 Boyer Ave E</span></font><span lang="DE"><o:p></o:p></span></p>
<p class="MsoNormal"><font color="navy" face="Verdana" size="2"><span
style="font-size:
10.0pt;font-family:Verdana;color:navy">Seattle, WA 98112</span></font><o:p></o:p></p>
<p class="MsoNormal"><font color="navy" face="Verdana" size="2"><span
style="font-size:
10.0pt;font-family:Verdana;color:navy">USA</span></font><o:p></o:p></p>
<p class="MsoNormal"><font color="navy" face="Verdana" size="2"><span
style="font-size:
10.0pt;font-family:Verdana;color:navy">tel ++ 206 905 1711</span></font><o:p></o:p></p>
<p class="MsoNormal"><font color="navy" face="Verdana" size="2"><span
style="font-size:
10.0pt;font-family:Verdana;color:navy">fax ++ 925 905 1711</span></font><o:p></o:p></p>
<p class="MsoNormal"><font face="Times New Roman" size="3"><span
style="font-size:
12.0pt"><o:p> </o:p></span></font></p>
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