v1 + v1 - OK, here's the matched pair - patches for both the splitter and mkgmap. With these in place, the mapsource mouse works as expected across all tile boundaries right up to the outer boundaries. No weird overlap of tile and overview map. Spot on. Please could some others give this a test and confirm that the rounding is now correct. ----------------- Thinking some more about what I was seeing in mapsource when mousing around the Baltic map, I came to the conclusion that the edges of the overview map bounding boxes where in the wrong place so I took a look at mkgmap where it rounds those coords and came up with the attached fix. No great theory here, I just dicked around until it worked as I expected. For the Baltic map, the mouse now works right up to each boundary in both lat and lon directions whereas before it would only work right on one side of each boundary. Please try the patch, especially for maps that are W of Greenwich and S of the equator and see if the overview boxes are right. Cheers, Mark