[postgis-users] Attributes update

[Obe, Regina]

Markus,

Rule of thumb for relation operations - I think you always read it  A
function_name B

So A within B -> Within(A,B)

I think that holds true for all the spatial relation functions in
PostGIS. 

Lucas - Markus gave you the correlated sub query approach.  You can also
do it without a subquery.  Which one is faster depends on how big your
data sets are I think.  The non-correlated I think generally works
better (especially for large data sets) (make sure to have indexes on
both geom fields).

I think the correlated sub query always forces a nested loop join.


Below is a non-sub query way of doing it.  For the new versions of
PostGIS you can also replace the && Within with just ST_Within.

UPDATE land_use SET type = parcels.landuse 
FROM parcels 
WHERE 
ST_Within(centroid(parcels.the_geom),land_use.the_geom);

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
Markus Schaber
Sent: Friday, May 30, 2008 7:26 AM
To: postgis-users at postgis.refractions.net
Subject: Re: [postgis-users] Attributes update

Hi, Lucas,

"Lucas Mueller" <lucas.mueller at gmail.com> wrote:


> I have two polygon layers (let's say Parcels and Land_use). I would
like to
> assign the attribute Land_use.type to the Parcels.landuse column by a
query
> that assigns a certain land use type to a parcel if the centroid of
the
> parcel is within a certain land use polygon. I hope to be clear
enough...
> Any idea on how to proceed?

Try something like:

update land_use set type=(select parcels.landuse FROM parcels where
land_use.the_geom && parcels.the_geom AND
within(centroid(parcels.the_geom),land_use.the_geom));

Btw, I tend to mix up the argument order of within and contains, so
please double-check.


Regards,
Markus

-- 
Markus Schaber | Logical Tracking&Tracing International AG
Dipl. Inf.     | Software Development GIS

Fight against software patents in Europe! www.ffii.org
www.nosoftwarepatents.org
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