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

[Toby Rainthorpe]

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>-0.574252637693809,53.2359726746639  
-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></ 
outerBoundaryIs></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
> )
> _______________________________________________
> postgis-users mailing list postgis-users at postgis.refractions.net
> http://postgis.refractions.net/mailman/listinfo/postgis-users
>
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