[postgis-users] How to fix not valid polygons?

Stephen Woodbridge woodbri at swoodbridge.com
Sun Dec 18 11:50:22 PST 2005

OK, so now I need to take a step backwards since some of the polygons I 
have are not valid. I am wondering how I might go about deconstructing a 
(multi?)polygon into a set of line segments that can be passed to 
polygonize() along the lines of:

insert into newtable select a, b, ..., polygonize(deconstruct(the_geom)) 
from oldtable;

This is not quite what I want because polygonize() returns a collection 
and what I really want is a row for each polygon with the attributes of 
the original.

So the deconstruct() function could probably be implemented by using 
dump() to simplify collections and multipolygons into polygons, then 
extracting the polygon or polygon rings, getting the points from each of 
these and then generating linestrings based on adjacent pairs and then 
returning the lot of them as a set to be consumed by polygonize().

The only problem is I do not see any way to do this short of mangling 
the text of the astext() representation of these.

npoints() will tell me how many points are in the polygon, but 
num_points() and pointn() only work on linestrings.

It would be nice to have some documentation on object deconstruction.

Anyway, is this the best approach? Do you see problems with this 
approach? What are my other options? getting good polygons to start with 
does not appear to be an option at this point.


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