WanMil, Good news - I don't think it's your problem. I disabled the sea generation completely and the lines on the tile edges are still there. Haven't a clue what's causing them (and I don't care because they don't show up when I use generate-sea=polygons). One issue that I don't think you can do much about is that mapsource tends to show horizontal and vertical gaps between polygons when you zoom out. They are a bit weird because they come and go as you pan and zoom. Having polygons for sea (as opposed to having polygons for land, which is what I have been using) means that you get more artifacts visible when using mapsource. That's a shame but may well not be a problem when viewing the map on a real gps. Anyway, your new MP code is producing great results (and it's quick). Mark