Hi all, the current default block-size is 512. This is used for most *.img files and gives a maximum file size of 65534*512 ~ 32MB. For large DEM files you might need a --block-size=1024 or even 2048. If I got that right you may run into the next problem when using this option as the mdr file will be also created with this value, and if no value is given it uses 16384 (which means the mdr file can grow close to 1GB) This disadvantage of a larger block-size is that your img files are typically a bit larger. @Steve: If I got that right we should be able to calculate the optimal block-size as we do know the number of bytes that we will write to the img-file before anything is actually written to the disk. Wouldn't that be much better? Gerd