[postgis-users] Adjecent Polygons to lines, typology related

[Peter]

> Its a pity that st_intersection cant work on sets then it could be used to
>  return linestrings where adjacent polygons intersect.

Well upon reflection, it can, kind of (to answer my own question):

CREATE TABLE sharedlines AS
SELECT
   ST_intersection(a.the_geom, b.the_geom) AS the_geom
FROM polys a, polys b
where ST_touches(a.the_geom, b.the_geom) and a.gid!=b.gid;

> Probably slow though.

Surpisingly not. Much, much, quicker than st_union. st_union has given me quite a bit of grief lately, eg see
http://www.webspaces.net.nz/page.php?view=polygon-dissolve-and-generalise

Ill add the above to it later.

peter


>
> Am i wasting time working on this or should i wait for postgis 2.0?
> If the latter is their a topology primer somewhere?
>
> Peter
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