[mkgmap-dev] Tile names in IMG files

Folks: Inside official Garmin IMG files there are typically several tiles named "I<hex number>.GMP". Does anyone know how the <hex number> is derived please? I guess that it is a bit-interleave of the latitude and longitude of some reference-point in the tile, but it could be a byte-interleave, or a nybble-interleave, or the interleave is done with the number expressed in decimal or whatever. I guess mkgmap must generate tiles too, so this info is likely well-known to those involved in the code. Thanks in advance Steve BTW: I gather that is possible for subtile files to be given decimal-based filenames too. How does that work? Is there any difference in the actual files depending on the naming system used?