Hi, the attached patch improves the overview map, but so far only when precomp-sea is NOT used. I tested it with --generate-sea=multipolygon,floodblocker so that mkgmap really has a multipolygon with the natual=sea data. For each level in the overview map it uses the original multipolygon data to compute the rings which will might visible at the given resolution. This requires more time compared to the current code but the result is much better and some fine tuning is possible. To be able to use this also with --precomp-sea we need some changes in the code which generates sea.zip so that one multipolygon relation for each tile is stored. I've uploaded the results here: https://files.mkgmap.org.uk/download/511/compare.7z What do you think? Gerd