Callaghan was a member of the 2nd Field Company Engineers. Before the landing at Gallipoli in 1915, he copied maps of the landing beaches with several other Australian and New Zealand soldiers with surveying experience. These men were kept on board a ship at Mudros Harbour, where they slept and exercised separately from the other troops for three weeks while they made the maps. Callaghan never heard what happened to the maps they made, but he assumed they were used by senior officers during the landing.
While he was sequestered, Callaghan made this small 1:120,000 sketch map. It is an amalgamation of all three sheets of the 1:40,000 map used during the landings at Gallipoli and shows towns and known watercourses. It also includes notes on the terrain, such as “cultivation”, “low scrub”, “steep sandy bluff” and “sandy beach”. The map is squared, based on the squaring of the 1:40,000 maps.