[postgis-users] Combining / agglomerating / clustering numbers ofadjacent polygons

[Matias Massigoge]

Derek,

I work with a "use case" similar for Site Specific Farming.
Right now I'm using statistical clustering without spatial relations.
After the clustering i use spatial relationship just to adjust the
result.
I'm interested in using spatial factor in the clustering, I'm not
experts in statistics but perhaps I can help you.

Good Luck.

Matías


On Wed, Nov 4, 2009 at 5:02 AM, Paragon Corporation <lr at pcorp.us> wrote:
> Derek,
>
> Which version of PostgreSQL are you using?
>
> Interestingly we are demonstrating some of these kinds of exercises in our
> upcoming book "PostGIS in Action" book well actually we are in the middle of
> writing those chapters now -- and cleaning up some of our examples - so they
> aren't quite ready to disseminate without a lot of explanation.  But all
> these tricks require the new PostgreSQL 8.4 window function and/or recursive
> query (common table expression) functionality.
>
> Sound like the recursive query might be closer to what you want  (where you
> keep on accumulating based on some condition and then start a new bucket).
> There are also several techniques for creating on the fly grids in PostGIS
> which would help out here.
>
> Hope that helps,
> Regina
>
>
> -----Original Message-----
> From: postgis-users-bounces at postgis.refractions.net
> [mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Derek
> Jones
> Sent: Tuesday, November 03, 2009 5:27 PM
> To: postgis-users at postgis.refractions.net
> Subject: [postgis-users] Combining / agglomerating / clustering numbers
> ofadjacent polygons
>
> Dear list,
>
> I have been using postgres/postgis for some years, and written a fair amount
> of spatial SQL and pgsql and R and Perl based queries.
>
> However, I've hit a brick wall in my thinking (November time change ;-)
> perhaps? ) and wondered if someone could suggest a method to help out.
>
> I have a spatial data set that varies in density across a region.
>
> I have constructed a grid (did it in qgis) and overlayed on the region, and
> then derived the density of items per grid area.
>
> Where the density is >= 3 per area, I retain the grid square in a new table.
>
> Then I want to combine the grid squars to form a larger region and (then
> incidentally to this question) find the centroid of that region where the
> grid areas touch. Singletons are rejected, and, if, say, I had 10000
> original grid squares, I might end up with 1000 clusterings of squares
> unioned together of arbitrary eventual shapes.
>
> Touching here - adjacency - could mean colinear or diagonal with 2 minimum.
> The maximum # of adjacent grid squares could run to 1000s per unioned area.
>
> But, I'm having a hard time to think through how to write the query.
>
> There are no columns in common across the grid squares, otherwise it'd be an
> easy st_union where col_xyz = 'PQR' kind of select, but all I have is the
> spatial adjacency.
>
> I've looked through the archives for the past couple of days, and perhaps
> I'm just using the wrong search criteria but I haven't seen anything that
> helps.
>
> I can see how to do this programmatically by agglomerating clusters and
> deleting from the remaining set as I go (and if I did that I would probably
> export the data and do it externally because it's quite a large data set),
> but I was really wanting to write a query for it and let it chug.
>
> Any thoughts?
>
>
> Kind regards
>
> Derek Jones.
>
>
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-- 
Lic. Matías Massigoge
Cel: 54-11-62047686
www.agerpro.com.ar