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=00B0F...) 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