[postgis-users] nearest polygon

[Paragon Corporation]

Charles,
 
If you just need the 1 nearest neighbor for each result.  Use distinct ON as
described here
http://www.bostongis.com/PrinterFriendly.aspx?content_name=postgis_nearest_n
eighbor
 
SELECT DISTINCT ON(g1.gid)  g1.gid As gref_gid, g1.description As
gref_description, g2.gid As gnn_gid, 

        g2.description As gnn_description  

    FROM sometable As g1, sometable As g2   

    WHERE g1.gid <> g2.gid AND ST_DWithin(g1.the_geom, g2.the_geom, 300)   

    ORDER BY g1.gid, ST_Distance(g1.the_geom,g2.the_geom) 
For the 300 you want to change that to the max distance you expect the
nearest neighbor for any record will be.  If you are using census data, you
wil also want to transform that from long lat 4269 to some other projection
like 2163 (National Atlas US).
 
You can also use the array approach we describe in the above article to have
nearest neighbors in columns instead of as separate rows (a sort of distance
cross tab if you will).
 
The generic solution we described below is more for returning multiple near
neighbors per record, and there is a bit of a bug in it that it doesn't
handle non-point geomtries quite accurately.  We are working on fixing this
and still maintaining performance and also updating it to use the new
PostGIS functions.
 
 
Leo

  _____  

From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of
charles.blankenship at gmail.com
Sent: Monday, June 08, 2009 9:02 PM
To: postgis-users at postgis.refractions.net
Subject: [postgis-users] nearest polygon


Hi,

I'm  very new to GIS and SQL so this is a naive question.

I have some boundary files from the US census containing school districts
and urban areas.

I would like to find the nearest, or if possible several nearest, urban
areas for each district.

I know this type of problem can get really complicated, but I need a fairly
quick and easy solution if one exists.  I could limit this to centroid to
centroid distances if that makes a big difference.


I found the following link which seems like it might do what I need.
http://www.bostongis.com/?content_name=postgis_nearest_neighbor_generic#130


Can anyone tell me if the solution above is the best way to go about this?
Or, if there's a simpler "quick and easy" way, that would be helpful as
well.


Thanks,
Charles

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/postgis-users/attachments/20090609/6078a4f3/attachment.html>