True, an accurate representation of an area isn't necessary for many purposes (including travelling). Knowing where roads or rivers lead to, having at least some villagers (or banditing allies) to talk to,... could just as well bring an army to it's destiny. And then there is of course trade (think about the amber route, or depictions on ceramics found in Belgium that very strongly resembled Greek ceramic patterns). Trade may not imply direct contact (as massed migrations suggest), but at least it offers those who buy the end-goods the (possibly severly disformed) stories of the merchants or other intermediataries.