[postgis-users] Advice on Unioning Polygons

[Nicklas Avén]

Hallo


As said before you will gain a lot if upgrading since PostGIS has implemented cascading union since 1.3.


You say that you only want to union intersecting polygons.


Easier is if it is ok to union all polygons with common attribute to multipolygons.


ST_Union is an aggregate function which means it can be used like this:


SELECT ST_Union(geom) as new_geom , group_id FROM my_polygons GROUP BY group_id;


Then if you want polygons taht doesn't intersect on different rows you can dump them, then it could look like this:


SELECT (ST_Dump(new_geom)).geom as newest_geom, id FROM
(SELECT ST_Union(geom) as new_geom , group_id FROM my_polygons GROUP BY group_id) a;

There is many more possibilities, but the point is that you are in the SQL-world now. iterations is done by the database in a very efficient way.


HTH


Nicklas



2013-02-01 Rebecca Clarke  wrote:

>
Hi there>

>>
Looking for some advice.>

>>
I have a table with thousands of polygon records.>

>>
I want to union all intersecting polygons that have the same attributes into one polygon (So one record, rather than 10 or 20 etc.). >

>>
Can anyone recommend the best way to do this.>

>>
I can do a loop which goes through each record, unions its geometry with the geometry's of the other matching records that intersect with it, then move to the next record. Problem with this is that it creates duplicate polygons when it comes to a record that has already been unioned with a previous one record. I can easily delete any duplicate records generated, but I'm wondering if there's a less long winded way.>

>>
I hope I'm explaining myself correctly.>

>>
My details are:>
postgis 1.3.5>
PostgreSQL 8.3.8>

>>
Many thanks in advance.>

>>
Rebecca>

>
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