As mkgmap should have all the code needed for that - could there be a new function of mkgmap to exchange / write .typfile to an gmapsupp.img?

Why? 

Well gmaptool is not running under newer OSx versions anymore (because it is 32 bit only, not 64 bit version existing) - and I do not know any other tool to do this. Alternatively - anyone knows another tool for this?

Intended usage:

mgkmap.jar --gmapsupp --family-id=XXXX --product-id=1 gmapsupp.img 123.typ

I actually never tried if this is possible - but I assume not. (changing FID/PID is optional - I would not need it but maybe for consistency allow it too).

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Felix Hartman - Openmtbmap.org & VeloMap.org