“Knowledge is power,” wrote Francis Bacon. (Being a man of letters and fluent in Latin, he really said “Nam et ipsa scientia potestas est.”) When it comes to powerful information, maps provide a particularly potent form. Before the advent of aerial photography and radar, accurate geographic information could provide a winning advantage, whether in business or war. George Washington, a surveyor and mapmaker himself, complained during the American Revolution that “The want of accurate maps has been a grave disadvantage to me.”

       Because of their immense strategic value, navigational maps have historically been kept secret, lest they fall into the hands of enemies or competitors. No mariners’ charts earlier than the 14th century have survived -- not because there were none, but because the knowledge such charts contained was so valuable that the maps would have been destroyed when they became obsolete or were no longer needed. During the exploration of the New World, Spanish merchants’ logs and charts were weighted with lead so that if a hostile force attacked, these important documents could easily be tossed overboard rather than surrendered. One captain was said to have deliberately wrecked his ship rather than give up his maps.

       A more modern method of keeping map secrets is simply to omit key features. During the Cold War even maps that were themselves secret concealed key information: In Russia the Chelyabinsk-70 nuclear complex was never shown on any official Soviet maps. The United States is not exempt; the infamous “Area 51” is designated for mapping purposes as “Nellis Air Force Bombing and Gunnery Range,” though hundreds of people work there and the facility is widely rumored to be used for secret weapons research. (The Central Intelligence Agency, however, is clearly marked on maps of Fairfax County.) Spy planes and satellites have made it difficult to conceal major geographic features such as military installations and factories, though it may be difficult for a bomber to determine whether the factory produces nerve gas or baby formula.

       The tradition of guarding important cartographic information to achieve a business advantage persists today in the real estate industry. When they are showing houses to customers, real estate agents use special maps which clearly identify schools, hospitals, and playgrounds, but leave garbage dumps and prisons unmarked.