[postgis-users] Re: ST_Difference Perplexes Me!

[Obe, Regina]

Dylan,
 
So are you saying that with ST_Union you got topological intersection
errors but with ST_MemUnion you didn't?  This would be very intriguing
if that were the case
since in theory both should yield the same result albeit slightly
differently.
 
ST_Union does an array collect and then unions where as ST_MemUnion
keeps on unioning 2 geometries at a time.
 
Thanks,
Regina

________________________________

From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of
Dylan Lorimer
Sent: Monday, May 05, 2008 9:47 PM
To: postgis-users at postgis.refractions.net
Subject: [postgis-users] Re: ST_Difference Perplexes Me!


Wanted to follow-up on this with the SQL that worked in the end.
Firstly...thanks so much Regina! I've no confidence that I ever would
have figured out this sql on my own, and yet looking at it now it makes
perfect sense!

So here it is in case anyone else is interested:

- vmap_countries contains country boundaries
- published_both_prototype_union contains dissolved polygons all over
the world (millions of them.)

CREATE TABLE not_meter_or_less_prototype AS 
SELECT ST_Difference(vmap_countries.the_geom, C.the_sum_geom),
vmap_countries.country_na
FROM vmap_countries INNER JOIN (SELECT
ST_MemUnion(published_both_prototype_union.the_geom) As the_sum_geom,
vmap_countries.country_na
   FROM vmap_countries INNER JOIN published_both_prototype_union ON
ST_Intersects(vmap_countries.the_geom,
published_both_prototype_union.the_geom)
               WHERE vmap_countries.country_na IN ('China', 'India',
'Japan')
               GROUP BY vmap_countries.country_na ) As C
ON  vmap_countries.country_na = C.country_na;

Many many (many) thanks to those on the list that have helped me out!
Cheers,
dylan



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