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. Not sure what would happen if there are tin's involved). > > 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. I suspect it would not be hard to find cases where the optimum approach (for the current algorithm) would be manual intervention in the form of breaking the problem (or solutions- easier?) into multiple pieces and then pick and choosing to stitch them together again to get a final result. Awful way to go though. Regards, Tim > 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 > > > > > -- Experience is that marvelous thing that enables you recognize a mistake when you make it again. _______________________________________________ postgis-users mailing list [email protected] http://postgis.refractions.net/mailman/listinfo/postgis-users
