Last spring, I made an OSM compatible file available to superimpose the GB National Grid on OSM maps using mkgmap. At that time I used Garmin codes 0x1c to 0x1e. These are normally used for administrative boundaries, but I was unable to find any sensible alternative codes.

I have now found that it is possible to use high numbered codes which are not allocated, or are allocated to marine applications. I have successfully applied codes in the 0x10300 range:

national_grid=100km [0x10302 level 3]
national_grid=10km [0x10301 level 2]
national_grid=1km [0x10300 level 1]

These seem to work correctly in MapSource and on a GPS 60CSx.

I have also made suitable TYP file entries, which are available on my download site http://www.rogercalvert.me.uk/OSM/grid, and are compatible with the TYP compiler in recent versions of mkgmap.

Note that the compiled IMG file in the download package still uses the previous codes.

Regards

Roger

Roger Calvert