[postgis-users] Removing / Polygonising multiple overlapping lines

[Ben Madin]

G'day all,

I have a problem with some administrative boundary data I have failing validity checks.

As an example, I had a polygon which kept reporting self-intersections. When I tried :

select st_exteriorring((st_dump(the_geom)).geom) as the_geom from tmp_admin where gid = 91;

So the one polygon returned 887 linestrings, which in themselves were valid, but :

prices=# select gid, st_geometrytype(the_geom), st_isvalid(the_geom), st_issimple(the_geom), st_isring(the_geom) from rubbish where st_isring(the_geom) is false;
 gid | st_geometrytype | st_isvalid | st_issimple | st_isring 
-----+-----------------+------------+-------------+-----------
 103 | ST_LineString   | t          | f           | f
 197 | ST_LineString   | t          | f           | f
 199 | ST_LineString   | t          | f           | f
 313 | ST_LineString   | t          | f           | f
 485 | ST_LineString   | t          | f           | f
 489 | ST_LineString   | t          | f           | f
 470 | ST_LineString   | t          | f           | f
 474 | ST_LineString   | t          | f           | f
 516 | ST_LineString   | t          | f           | f
 887 | ST_LineString   | t          | f           | f
(10 rows)

so for example 485 is in green and looks like (apologies if the picture is scrubbed). The blue island is OK 

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It appears that the one line goes around the island three times, and this was the basis for the polygon. I'm a little stuck with 1.4.0 at the moment, so I can't use the st_lineCrossingDirection function, but what I really want is to produce a polygon which matches the outermost line of the object.

Any ideas... I'd rather do this in postgis as there are more of them...

cheers

Ben