[postgis-users] return boundary

[Tim Bowden]

On Tue, 2008-11-25 at 10:48 +0900, Tim Bowden wrote:
> On Mon, 2008-11-24 at 17:15 -0800, Martin Davis wrote:
> > They're similar, but I'm not sure they're 100% the same in all cases.
> > 
> > In the "mainland plus islands" case it seems there is a requirement to 
> > ensure that all the input polygons are contained in the result.  The 
> > concave hull concept as presented is based purely on a point set, and 
> > thus might not necessarily cover all of the input polygons.
> 
> Very true, the concave hull may well result in a boundary inside the
> original polygon boundary, but then doing a union of the concave hull
> and the original polygon would solve this I think (at least for the
> bounding polygon.

Wrong.  The concave hull from points could also be outside the original
polygon boundary, in which case this solution wouldn't work.  Bit messy
really.  You would need to retain the original polygons and identify
those vertices that are between points in different original polygons
and use them in some manner to splice them into a combined polygon.

Tim
-- 
Experience is that marvelous thing that enables you recognize a mistake
when you make it again.