A map should do three things ...
1. It should provide some sort of representation of reality (even a mythical reality) which the user can recognise
2. It must give the individual user a clear understanding of where they are in relation to this reality and the key (through symbols and meta-information) to find their way round and interpret this map.
3. It should give the user the tools to plan a route (or compare routes) from a starting point to a destination