This document, billed as a US government secret history of a surveillance tunnel dug in Berlin during the cold war (“Clandestine Services History: The Berlin Tunnel Operation 1952-1956“, CS document #150), tells of a difficult moment for the surveyors:
The lack of an adequate base line made the surveying problem especially difficult. The engineers decided at one point that an object of known size in the East Zone would be useful as a reference point, so a baseball game was organized with the objective of knocking a baseball as far into the East Zone as possible. This scheme was frustrated by the friendliness of the East German guards who kept returning the baseball. Nonetheless, the engineers expressed confidence that they knew their position when the tunnel was completed to a point which could be contained in a six-inch cube. They were correct.