[mkgmap-dev] Merging adjacent multipolygons

Fri Feb 26 17:36:34 GMT 2010

> Hi WanMil,
>
>> So in the end what does it mean? The artificial cut will be handled
>> as one segment of the outer polygon and will therefore be tagged with
>> the multipolygon tags. This is the same as happens with all inner
>> cuts by the mp code.
>
> That is not a big deal with artificially split natural=water
> multipolygons, as the border lines of the lake will be the same colour
> as the lake. (And I do not think that there any other huge
> multipolygons than lakes.)

Ok, this is true for your style. But other renderes might use another 
color for the outlines. E.g. have a look at the cycle map 
(http://www.openstreetmap.org/?lat=47.39361&lon=13.65131&zoom=15&layers=00B0FTF) 
you see a green line through the forest which is a segmented multipolygon.

>
>>    From my point of view as long as it isn't widely accepted by the
>> community mkgmap should not implement workarounds that help other
>> tools like Osmarender to work properly.
>
> It is not just rendering tools, but editing tools and the OSM server. I
> tried to view the history of one of the multipolygons that I created,
> and the server timed out, repeatedly. OSM just does not scale if there
> are thousands of 500-node ways in a relation.
>
> I guess this means that I will be relying on generate-sea in the
> future. If someone wants to create lake multipolygons for the remaining
> huge lakes in Finland (for example if they want to tag islands), that
> is up to them.
>
> I have been considering "glueing" artificial natural=coastlines to
> "seal" the Geofabrik Finland extract, to avoid flooding with
> generate-sea. Do you have any suggestion how to do this efficiently?
> Optimally, splitter would input two files, my coastline-glue.osm and
> the finland.osm.bz2 from Geofabrik.
>
> 	Marko

I don't know very much about the generate-sea option. The code is 95% 
undocumented (and therefore it takes a long time to get familiar with 
it).  I just got in touch with it because it generates a multipolygon if 
you set generate-sea=multipolygon.

If you want to use your own coastline file you could use a special tag 
(e.g. natural=markocoastline) and then change the generate-sea source 
code to use this tag instead of natural=coastline. Before starting 
mkgmap you might append the content of your coastline file to the 
geofabrik file.

WanMil