[postgis-users] Union, but need to exclude any void

[Paragon Corporation]

Toby,
I agree with Stanley that it is probably not a ST_Union problem.  I'm
guessing there is something wrong with your WHERE clause.  Keep in mind
PostgreSQL is case sensitive so if by chance
you had something like ln1  instead of LN1 in the com_dist field, then the
ln1s would be picked up as NOT ....
 
To rule out ST_Union as the culprit, can you do a simple attribute query
 
SELECT postarea, com_dist, gid
FROM postcode_sectors
WHERE  postarea = 'LN' AND NOT com_dist = 'LN1'
ORDER BY postarea, com_dist
 
Also I tend to do com_dist <> 'LN1' , but that shouldn't matter.
 
Hope that helps,
Regina

  _____  

From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Toby
Rainthorpe
Sent: Saturday, November 01, 2008 2:19 PM
To: PostGIS Users Discussion
Subject: Re: [postgis-users] Union, but need to exclude any void


Sadly it doesn't work, here is a search for the post code area LN, but
excluding LN1. This should give the polygon of lincoln, with the exception
of the town centre district. Instead it creates a polygon for the whole LN
post area.  

Just to clarify, I do want the hole in a polygon.


SELECT ST_AsKML(ST_Union(the_geom)) as the_geom
FROM postcode_sectors 
WHERE postarea = 'LN' AND NOT com_dist = 'LN1'


<?xml version="1.0" encoding="UTF-8"?>
<kml xmlns="http://www.opengis.net/kml/2.2">
  <Placemark>

<Polygon><outerBoundaryIs><LinearRing><coordinates>-0.574252637693809,53.235
9726746639 -0.570638045173646,53.2342660459289
-0.567956817933028,53.233334871246 -0.542511758566372,53.2276409117944
-0.539261287526225,53.2285001962972 -0.537587144360509,53.23365835124
-0.536621729986015,53.2443903523506 -0.538372820842824,53.2545802514289
-0.53867243559146,53.2902048795337 -0.541250872030082,53.3316560575988
-0.567075865458079,53.3529438083222 -0.549064460589285,53.4257467252708
-0.461252440918993,53.4290079476242 -0.459280037729661,53.4356804144195
-0.491910432874966,53.497600359203 -0.41389940584748,53.5500565803702
-0.310449877650045,53.5267476031091 -0.219352544114183,53.5272954168687
-0.215026390840363,53.5063814691382 -0.203265094143773,53.466744705434
-0.122544300870874,53.4548941206983 0.0264268890418949,53.4620080919002
0.083849617015283,53.496544395631 0.228359151391426,53.408400422663
0.268028563946318,53.342544544341 0.302708719734213,53.2940795243504
0.321850405882593,53.2715163271867 0.234840151689661,53.1942224753385
0.194430850648443,53.213308161178 0.169714736355569,53.2124120839752
0.0834492984598471,53.2348844372266 0.0208307835734331,53.2478135975227
-0.0126938531249155,53.1710150547887 -0.13716904881661,53.1418544045366
-0.150749526290076,53.1243892073786 -0.130710137006856,53.0239835120991
-0.229285061035197,53.0038950480853 -0.338497757654447,53.0609112327498
-0.408675738441825,53.055621865059 -0.452241950934627,53.0579499714055
-0.52047527802287,53.0487392813369 -0.660214167307015,53.0546658032031
-0.714807399468163,53.0584148660486 -0.699290317426583,53.0756434022022
-0.716927624544292,53.1105928908201 -0.674018446595594,53.2187206490539
-0.668182682830745,53.2552110975952 -0.640114978977996,53.2538783112452
-0.574252637693809,53.2359726746639</coordinates></LinearRing></outerBoundar
yIs></Polygon>
  </Placemark>
</kml>






From: ssufficool at rov.sbcounty.gov

Subject: RE: [postgis-users] Union, but need to exclude any void

Date: 1 November 2008 14:58:26 GMT

To: postgis-users at postgis.refractions.net
Reply-To: postgis-users at postgis.refractions.net

I've never had a problem with st_union including holes when it
shouldn't. Your query syntax is a bit odd though. Maybe try the
following:

SELECT ST_Union(the_geom) as the_geom
FROM territories 
WHERE is_an_outer_territory = 1




-----Original Message-----


From: postgis-users-bounces at postgis.refractions.net 


[mailto:postgis-users-bounces at postgis.refractions.net] On 


Behalf Of Toby Rainthorpe


Sent: Saturday, November 01, 2008 1:37 AM


To: PostGIS Users Discussion


Subject: [postgis-users] Union, but need to exclude any void








I have a list of territories, say 9 represented by a tic, tac, toe   


board (3 x 3 grid ). A query selects all the outer perimeter (8  


territories), but omits the central block. If i were then to union  


this group the resulting polygon would fill, or include this central  


block, even though I have not exclusively said that that 


territory has  


been selected.





Is there a way of doing a union, or another method on a set of  


polygons that will result in a polygon being created, but allow for  


the void not to be included.





Thanks for your help,





Toby








SELECT


GeomUnion(geom) as the_geom,


FROM (


SELECT (dump(the_geom)).geom FROM territories WHERE  


is_an_outer_territory = 1


) 


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