How do you plan to ensure anonymity when judging to avoid bias? It might be worth submitting the maps to someone not on the judging panel first, who can then remove the mappers' names from each set. The judging panel would just see "Set 1, Set 2, etc.", or something, and wouldn't know whose set was which until after judging is complete.
I know everyone says they try to maintain impartiality and be fair, but the cynic in me says that removing friendship/rivalry/grudge bias completely is impossible, and this is a straightforward solution that wouldn't take much effort to implement.