Can i add an OSM to a Garmin city navigator sd-card

Short answer: Yes.

Long answer: A google search for "gmapsupp edge 800" gave this helpful result.

hth
malenki

If there is enough free space on the memory card, then yes, this should be possible.

I'm not sure how much free space the City Navigator cards usually have, but if you put the card in the Edge, and plug it into your computer, it should appear as a drive. You should be able to check the properties for the drive, to see how much space is available. Depends on what version of the TalkyToaster maps you want, most of them need about 500MB for all of the UK and Ireland. Note there are other OSM based maps which may need less space, or you could use maps for a smaller area.

Before modifying anything, it would be a good idea to back up everything on the card, just in case any of the City Navigator maps get deleted.

For downloading OSM maps, look for one in a "GMAPSUPP" version, this is designed for copying to a GPS device. So get one of these from Talkytoaster (or elsewhere), then unzip it. It will give you a file named something like "130830-British-Isles+Contours-Routable-GMAPSUPP.IMG". You need to copy this file onto the memory card, in the "Garmin" directory. There will already be a file there for the City Nav maps, probably named "gmapsupp.img", so leave that alone. You can rename the Talkytoaster map file if you want, so long as it has the file extension ".img"

Then switch on the Edge, and it should see the maps. You will have to go to the map setup menu to choose which map to enable. If you have multiple maps enabled, one will probably hide the other.