Dear Saaduk. The procedure is quite simple. I think you should read this thing about filenames and btw which maps you are using for Pakistan?
Adding multiple maps in Garmin Nuvi GPS
Theoretically, the Garmin Nuvi GPS navigator can be loaded with
hundreds of map. The limit is on the maximum file size (about 2GB per
file) and available storage memory.
The maps are stored in specific mapset files with *.img extension in
the flash memory of the GPS unit. Each mapset file contains one or
many maps in it. Beside that, the Garmin GPS navigator can also
recognise and load up one additional mapset file from SD card residing in the "Garmin" folder and named as "gmapsupp.img".
Here are the mapset files supported by Garmin Nuvi 255w GPS navigator:
* gmapbmap.img - the default base map
* gmapprom.img - the primary mapset
* gmapprom1.img - the secondary mapset
* gmapoem.img - the OEM mapset that cannot be unselect by user (if it exist)
* gmapsupp.img - the supplemental mapset which you can have one in
the GPS flash memory and another one in the attached SD card.
Therefore, the Garmin Nuvi GPS supports up to 6 mapset files,
including 1 in SD card. Normally new maps installed by the user will
be stored in gmapsupp.img.
You will lose all the maps in the previous mapset file if it is overwritten by a new file with same filename. If you want to keep the existing maps, ensure that you store the mapset file with an unused filename (from the 5 filenames above). Alternatively, you can make use of special tools to merge new maps into existing mapset file.
You can view the name of your base map in Tools > Settings > System >
About > Basemap.
You can view the name of maps in the other mapset files in Tools >
Settings > Map > Map Info. There, you can tell the GPS to load the map
by checking on its name, or not to use the map by unchecking it.
In case there are 2 or more non-overlaping maps available for the same
location, the map with the highest priority number (defined in the map
itself) will be used by the GPS.
Note that certain map which are locked by vendor need a 25 bytes
unlock key stored in a *.unl file in the same folder and with the same
name as the mapset file where the locked map is stored in.
