[postgis-users] return boundary

[Martin Davis]

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.

Also, the concave hull is defined by a distance tolerance.  In a 
situation with islands both very close and very far, it might be hard to 
find a distance value which would include all islands but not compromise 
the shape of the main polygon too much.

Of course it all depends on the data - there's probably lots of cases 
where both would produce similar results.

Tim Bowden wrote:
> On Mon, 2008-11-24 at 11:45 -0500, Travis Kirstine wrote:
>   
>> I don't think that the geom union would in combination with the
>> boundary would attach the island to the mainland.  I don't think that
>> there is a simple answer to my problem but I have attached a pic to
>> help illustrate what I'm trying to accomplish.
>>
>> Regards
>>
>>     
>
> Looking at that image seems to me it's just another case of what Regina
> pointed to (http://ubicomp.algoritmi.uminho.pt/local/concavehull.html)
> in the concave hull thread.
>
> Regards,
> Tim
>
>   

-- 
Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022