Hi I've been using --generate-sea rather than --precomp-sea for a few weeks and there doesn't seem to be an obvious performance penalty and the img size is slightly better (but I have been changing other things along the way). However - It doesn't seem to know about the rule that land must be on the left of the coastline and sea on the right. Taking the geofabrik download of UK/Hampshire, the Isle of Wight is (correctly) mostly excluded - just 3 bits of coastline that don't meet each other or the bottom edge of the tile. With --generate-sea=multipolygon, I sometimes get the top half, which is correctly defined land, ie has coastline to the tile edge, as sea and the sea as nothing. The bits of disconnected coastline sometimes have a triangle of sea where it should be, sometimes where the land is! With extend-sea-sectors,gap=350 it is much better behaved. Maybe the default behaviour should be to ignore these disconnected bits of coastline (with a warning), hence both sides will show as sea. The coast of the requested area should behave correctly. Ticker