Locate object does not provide measurements, it provides a direction and whether the object is moving.. Canonical lore states very clearly that maps are inaccurate in the FR. That is a canon statement, pulled from a sourcebook written by the man who created the Realms. Whatever explanation we prefer, that is an in-game reality. We can assume that the "easy" methods of offering accurate maps have failed, which would necessarily include magical means.

That maps are as accurate as they are is likely a reflection of the use of magic to assist with their creation.