Hi Frank Your superb program works a real treat - What an amazing feat to have more or less deciphered the DEM sub file . I can now give the whole of the UK a proper DEM look! Thanks Gerd for providing us with a script as I would have been completely lost despite the help file. Unfortunately I too get this odd comment about mapset approx (42mb) but only 27.8 gig available ! At the moment , I'n not too fussed as DEM files tend to darken the GPS display , in my opinion. I tried setting different flags in the tdb but to no avail ! I suspect there is a length block issue in the DEM file itself which is ignored by Basecamp & Mapsource and only used when creating a gmapsupp. thanks again Nick -- Sent from: http://gis.19327.n8.nabble.com/Mkgmap-Development-f5324443.html