I have investigated further and this is what I have found. - I have discovered a section of motorway that is missing, so it appears that all types of way are affected. - The missing ways that I have spotted, all continue beyond the overlap region. They contain nodes within the overlap region but none end within the overlap region. - The output files from splitter contain the missing ways and they are complete; i.e. the way has all its tags and a complete list of its node ids. What is not there, is what you would not expect to be there: the definitions of the nodes which fall beyond the overlap region - their latitudes and longitudes. The effect of this on JOSM is that it draws the nodes that are defined but it does not draw any part of the way. - Splitter does not introduce nodes at the tile boundaries (or if it does, they are not displayed in JOSM, either on the map or in the inspector/Advanced object info). It would seem that mkgmap discards these defective ways. This bug could be fixed in either of two ways: a) Clip the ways in splitter so they are no longer defective. b) Modify mkgmap to accept defective ways and clip them itself. At this point it needs someone with a detailed knowledge of the software, to say how the interface between splitter and mkgmap should work, i.e. whether a) or b) is the correct approach. Only then could bugfixing efforts begin. I hope that the additional information I have provided, will make it that much easier to trace and fix the bug.