Hi all, Frank Stinner found more about the meaning of the byte in DEM header at offset 0x12 of the zoom level header. This value gives a shrink factor that is used to minimize the deltas. All height delta values are divided by this factor, so that e.g. with a factor of 5 the values 12,19,20,10 become 2,4,4,2 before they are encoded. This reduces the number of bytes but also reduces precision. Garmin uses it in many maps, of with increasing value for each map level. I've started to implement this in DemDisplay but it seems that there is something else to change. Gerd