[mkgmap-dev] Multipolygons and basic question on polygons with holes

Sat Jan 2 21:32:14 GMT 2010

I'm not involved in the multipolygon code at all, but some comments:
> I realise that it would be easier if I made a drawing so look at
> http://www.mkgmap.org.uk/tmp/islands.png
>
> In nicely working maps A and B are coincident, as are C and D. In our
> maps C and D are not, our original code also made them coincident, but
> this failed when the polygon was big enough to need splitting.  This
> is why I suggest attempting to split large polygons first.
>
>   
Reading this, it means for me, that the splitting code ist the source of 
the problems. Maybe we need an more complex code for splitting, which 
handles concave polygons well. I imagine something like cut it with a 
pair of scissors. But I have no algorithm at hand.
> Point 3 is illustrated by the second diagram, where a sea polygon is
> split into X and Y and avoids the need for a DABC cut as in the first.
> It may not be easy to take advantage of this with an algorithm, but
> I'd probably make use of that a lot if doing it by hand.
>
>   
Sorry, but I can't see the difference. The picture on the right side 
shows a splitted polygon, but there are two places with parallel lines. 
Both polygons have concave segments too. So there is the same need for a 
DABC cut. Where is my misunderstanding?

Regards,
Johann