I use normally --reduce-point-density=1 --reduce-point-density-polygon=1 --merge-lines. I have no problems with routing. (I also use --remove-short-arcs=3) Basically --reduce-point* -options simplify the shape by removing some of the points if it does not affect the shape too much. I have no problems with drawing speed but I keep the very low optimization on to take care of any exessive details. (Consider for example a road that has points every 2m and the poor GPS trying to draw that on a moving map) Pro of optimization is faster map drawing and smaller map files. The con is reduced details and perhaps a bit coarser map (depending on how much you reduce point density). A test with my personal style file and different options (using same data for Finland and the latest mkgmap): -options at 1m: size is 110 MB -options at 4m: size is 107 MB (a bit too coarse map for my taste) -no optimization: size is 109 MB You may benifit more or less on your region. In my region I consider the result that the size is not really affected but it simplifies anything overly complicated. I have no idea why the map size is increased (by a tiny bit) with my options at 1m compared to no optimization. -- Harri