A reliable source for that specific edit should be provided.
A well-known source that has a reputation for neutral (not biased) territorial control coverage, can be used (is deemed reliable) for all edits.
A well-known source that does not have a reputation for neutral (not biased) territorial control coverage, can be used (is deemed reliable) only for edits that are unfavorable to the side it prefers (favorable to the side it opposes).
A source that is not well-known (or that has proven inaccurate for all edits) cannot be used (is deemed unreliable) for any edit. This includes all maps (see item 2. next).
Maps from mainstream media are approximate and therefore unreliable for any edit. Maps from amateur sources are below the standards of Wikipedia for any edit. They violate WP:SPS and WP:CIRCULAR.
WP:SPS: "Anyone can create a personal web page or publish their own book, and also claim to be an expert in a certain field. For that reason, self-published media, such as books, patents, newsletters, personal websites, open wikis, personal or group blogs, Internet forum postings, and tweets, are largely not acceptable as sources."
WP:CIRCULAR: "Do not use websites that mirror Wikipedia content or publications that rely on material from Wikipedia as sources."
