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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