[postgis-users] Fixing invalid polygons created from polygonizing rasters

[William Temperley]

Hi all,

We have some polygonized rasters that create invalid GEOS geometries.
Take polygon A I've attempted to illustrate below. It is one ring,
which touches itself at a single point. In line with the OGC spec it
is invalid. If it becomes two polygons, it is valid (See B I've
illustrated below).

It would be great if anyone knew of an algorithm to make A into B.

Thanks

Will


==============
A
____
|   __|___
|   |__|    |
|_______|

MAP=# select isvalid(geomfromtext('MULTIPOLYGON(((0 0, 0 3, 2 3, 2 2,
1 2, 1 1, 2 1, 2 2, 3 2, 3 0, 0 0)))')
);
NOTICE:  Ring Self-intersection
 isvalid
---------
 f

===============
B
____
|  1__|___
|__|__|    |
|______2|


MAP=# select isvalid(geomfromtext('MULTIPOLYGON(((0 0, 0 1, 2 1, 2 2,
3 2, 3 0, 0 0)),((0 1, 0 3, 2 3, 2 2, 1 2, 0 1)))')
);
 isvalid
---------
 t
(1 row)