[postgis-users] Topology: Simplifying Coastlines with Islands

[Sandro Santilli]

On Mon, May 28, 2012 at 04:19:01PM -0400, Stephen Woodbridge wrote:
> On 5/28/2012 3:58 PM, Sandro Santilli wrote:
> >On Mon, May 28, 2012 at 02:50:43PM +0200, Michiel J. van Heek wrote:

> >>If we want to handle these kind of
> >>cases automatically, the easiest way would be to removed the
> >>islands, but I don't like that idea, because some islands may be
> >>very big, like the arctic islands of Canada. It is like removing
> >>Great Britain from the map. :-)
> >
> >One way would be to split the edge introducing nodes making it
> >harder for each sub-edge to collide to anything. I wonder how
> >finding such new nodes could be automated. But we're running too
> >much. Zoom on the Brazil coast first, and extrapolate the actual
> >error message. Oh, note that running a second time might give you
> >more successes, so worth reaching a "stable state" before going
> >on.
> 
> If you can detect that an edge collides with another polygon, then
> you could take the the points where the edge intersects the polygon
> an project them back onto the original edge and split the edge at
> those projected points.
> 
> But I'm not sure that helps anything because making it harder does
> not insure success.

The ST_ChangeEdgeGeom _always_ checks for collision, and the
SimplifyEdgeGeom is already based on a probability of success so
not a big problem to add some more heuristic steps (IMHO).

--strk;

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