Hi Gerd, I have tested some img, changing block size with GMapTool. These are results, first original file size, then size delta for blocks 1k, 2k and 4k: 14.8k -- +3.6k +9.9k +22.0k 109k -- +2.6k +7.7k +16.9k 1M -- +0.0k +5.2k +17.7k 2M -- -1.6k -0.5k +10.7k 3M -- -3.6k -3.6k +2.6k 4M -- -5.7k -4.7k -0.5k 8M -- -14.3k -18.4k -16.4k Looks like 2MB is a tile size, above which block size 2048 can be better. I would change default. You could think about optimization, if you are interested, but gain won't be big. Side notes: Block size for overview map is probably always 512, can be problematic for very big maps with DEM. There is something wrong with FAT in MDR. FAT shouldn't be bigger than about 140kB. Mkgmap creates MDR with FAT of 3.2MB, almost all empty. -- Best regards, Andrzej