Thanks Wanmil, Those images explained very well what exactly happens.
So what I can do is to add more coastline to the benelux osm data to prevent that this coastline is intercepting itself. I have to merge osm data (German coastline) to the osm data from the Benelux extract. How does this work with osmosis? Or is it possible with mkgmap to combine the coastline with osm tiles?
You can load a separate coastline file using the option coastlinefile=coastlines1.osm.pbf,coastlines2.osm.pbf When using this option the coastlines of your tiles are ignored and are only taken from the given file(s). The file(s) need not contain only coastlines but I think using the complete original benelux and german files here will not be possible due to memory problems. You can use osmosis (please read the osmosis documentation!) to extract the coastlines. The osmosis call might look like: osmosis.bat -rb europe.osm.pbf -tf accept-ways natural=coastline -tf reject-relations --used-node -wb europe_coasts.osm.pbf WanMil