Ground Zero Indicator The Ground Zero Indicator , known by the acronym GZI was a specially designed shadowgraph instrument used by the British Royal Observer Corps during the Cold War to locate the Ground Zero of any nuclear explosion . Overview . The GZI consisted of four horizontally mounted cardinal compass point pinhole camera s within a white enamelled metal drum , each ' camera ' contained a sheet of photosensitive paper mounted within a clear plastic cassette on which were printed horizontal and vertical calibration lines delineating compass bearing and elevation above the horizon . The bright flash from a nuclear explosion would burn a mark on one or two of the papers within the drum . The position of the burn spot enabled the bearing and height of the burst to be estimated . With triangulation between neighbouring posts these readings would give an accurate height and position . The altitude of the explosion was important because a ground or near ground burst would produce radioactive fallout , whereas an air burst would produce only short distance and short lived initial radiations ( but no fallout ) . Once combined with the peak - overpressure readings from post Bomb Power Indicator readings the power of the burst in megatons could also be calculated by the Triangulation Team in the group control building , using a hand held plastic calculator device . Operations . Normally stored below ground the drum was mounted at the start of exercises or at Transition To War on a special above ground mount set in the concrete of the entrance hatch . Concentric lugs on the mount ensured that the drum was mounted correctly even in the dark . The light sensitive photographic paper was unfixed chemically and had to be protected from daylight by being carried in a protective pouch strapped to the observer ' s chest . The cassettes were always lodged in a specific order so they could be changed in the pitch black with practice . Differing notched cutouts at the bottom of the cassettes ensured they could not be mounted at the incorrect cardinal point . Daylight inevitably darkened the unfixed papers so routine changes were made at mid day during winter operations and twice daily , at mid day and sunset , during summer months . The sun burned a distinctive suntrail across the papers when there was no cloud cover . Sixty seconds after any reading on the Bomb Power Indicator an observer exited the post and changed the GZI cassettes . The exposed papers were returned underground for assessment . The pre - exposed papers had a graticule grid exposed on the papers that showed degrees of bearing . The fireball from any nuclear burst within range would have burned a mark on the paper . The " spot size " and bearing would be reported to the group control together with an indication that the spot is touching or clear of the horizon , essential for indicating an air or ground burst . Codeword . Reports following readings on the GZI were prefixed with the words " Nuclear Burst " and the monitoring post ' s operational identifier e . g . " Nuclear Burst Shrewsbury 56 post - ( time ) oh six hundred - Bearing zero seven zero - spotsize zero six - touching " . Training . For realistic exercise training photographic papers with simulated burst spots were prepared by the " Exercise Division " of the United Kingdom Warning and Monitoring Organisation at Cowley . Delivered sealed , the time of opening was written on the outside of the sealed envelope . Observers trained blindfolded so that they could exit the post up the vertical steel ladder and complete a GZI cassette change quickly and even in the pitch dark of a winter ' s night .