[mkgmap-dev] natural=coastline not supported?

Sun Jul 26 03:11:33 BST 2009

Christian Gawron wrote:
> <snip>
> By trying to add sea polygons by hand, I learned that the polygons must
> not be too big - otherwise island are messed up (they are "overdrawn" by
> the sea). 
> Does someone know the reason for this and can specify what the maximum
> size of a polygon (with holes) can be?
>
> BTW: This seems to imply that multipolygons also can not exceed a
> certain size - or is there already some code to divide resulting
> polygons that are too large?  
>
> Best wishes
> Christian
>   
I made some sea polygons by hand also, and found the same  as you are 
experiencing. I first of all thought the sea polygons must be small, but 
this turned out to be incorrect, there is no limit to size as such, - 
the disappearing islands appear to depend on how the large polygon is 
subdivided by mkgmap for the different zoom levels. You can observe 
where it goes wrong is you look at the resulting img file in gpsmapedit, 
the observed errors depend on the position of the islands, but I do not 
know how you might correct this.

Dividing the ploygons up into a small grid worked for me, but there are 
visual defects when I zoom out with gaps between the polygons.

After some experimentation, I got the best results using large poylgons 
for the sea, and rendering the islands separately as a land type. I 
control the background color and draw order of the islands using a 
custom TYP file. I am using mp format input files so I can take 
advantage of the editing controls in gpsmapedit and can change how the 
islands are defined easily. I guess you can do the same by hand editing 
the OSM file.

You might also try some patches that were submitted on 10-7-2009 by 
Rudi. His Problem #3 sounds like the same issue. I did not try these 
patches yet.

Garvan