I frequently drives outside mapped areas, because I update the OSM map. It is convenient to have navit show what roads are on the map already.
But navit always snaps to the closest road, even if it is several hundred meters away. Snapping to the closest road within the combined error margins of the map and the GPS is of course reasonable. The current GPS accuracy is (usually) available from the GPS readout. Map accuracy is usually unknown - but a map made from GPS traces (like the osm map) is usually accurate to within typical gps accuracy. The map accuracy should ideally be user configurable. Those who want to cling to the roads can then set a large error margin, those who want to go off-road can set a low error margin.
