sentences:  A base map usually had an edition number followed by a letter, so a map marked 6C had an edition 6 base map and the trench over print of edition C. This system was not strictly adhered to, since sometimes, two maps with the same number and letter can be found but with different detail and different dates.On many of the maps, the words "Trenches corrected to" are followed by either a single date or a separate date for British and German trenches.Trench maps were updated from aerial photographs and intelligence reports although sometimes captured German maps were used as a source of detail.