No it's not a Linux thing it's a Roadmap thing...

Roadmap recognizes several map formats.-
- The state by state roadmaps available at the sourceforge page which are based on U.S. Census Bureau Tiger Data.
- Maps downloaded from openstreetmaps.org. These maps are downloaded and compiled using the getmaps_osm program. 'getmaps' in your start menu.

Roadmap will display both maps simultaneously if you have both. So it's best to use one or the ther for a particular area. For example use the tiger maps for one's home state, then download maps on the fly when traveling using getmaps.

If you're using the n810 all you need to do is-
1. create a dir 'roadmaps' on the root directory of either MMC.
2. fire up getmaps point the map location at your 'roadmaps' directory. (I've got 'roadmaps' directories on both MMCs. Roadmap is designed to look on the Internal card for Tiger maps, and on the external card for OSM maps. This allows a way to gps keep things organized.
3. Wait for a GPS fix, press the 'GPS Position' button and then the 'Get OSM Maps' button.
4. Start Roadmap. If you still have a fix from step 2 you can press 'g' and go to your current location.

Hope this helps,
kernelpanic

p.s.- To use the state maps download usdir.rdm.tar.gz, bndSB.tar.gz and whatever states you want. Then create a directory on your internal mmc and untar. In xterm this would be-

$ mkdir /media/mmc2/roadmaps
$ cd /media/mmc2/roadmaps
$ tar -xvzf <location of tar.gz file>