Hi Steve,
So if the tile height or width is an odd number of these shifted units then the centre point will be half way between a shifted unit boundary. Therefore adding any offset will leave you half a shifted unit out.
Thanks for the thorough investigation - much more than I could have done by applying my sort of random voodoo to see if it fixes things. Anyway, a few observations I made in the mean time, that are probably unrelated: - the Garmin map on my Nuvi seems to have no overlap - at least, I cannot find it. (You probably knew this already). - Also, my Garmin Europe map seems to split exactly at whole degrees (you can see the same endings of ways that the OSM map shows). I don't know why this is (probably "just because"). Also, I cannot reproduce their split with routing included, as the "NET (thingy) too large" assertion spoils my build. As said, this is probably unrelated and maybe you knew all this already. Best regards, Valentijn