Description
Object description
Inter-war training film focusing on the magnetic compass and the concept of bearings used in conjunction with topographic maps.
Content description
Reel one:
Film opens with a Hawker Hind in flight, the pilot navigating by correlating features on the ground with their presentation on his aeronautical map, and noting the heading of his aircraft from a compass adjacent to him. The compass is the most important aid for the pilot: camera cuts to a typical compass – a very elementary explanation is given – the concept of the 360 degree compass card, how bearings are determined; explained. The film is designed to accommodate those men with little or no appreciation of the compass, or the associated features, introduced as the film progresses.
Maps are introduced based on imperial units of measurement, the one inch, six inch and ten inch maps: examples of the Ordnance Survey maps are shown, magnetic north, true north, grid north, lines of meridian, explained. Camera pans out from a map, to the corresponding view of the mapped area, and finally to a globe, to introduce the representation of a curved surface on a flat map. The concept of projections is explained. The Mercator map projection finds wide application on aeronautical maps because the angle of a straight flight line plotted on such a map between two points (e.g. two airfields) corresponds with the angle indicated on the magnetic compass.
The components of the aeronautical compass are explained, and the procedures necessary to determine true north from grid north. The earth may be imagined as a magnet with two poles, north and south. The magnetic poles are far away from the geographic poles; due to variation of the magnetic lines of force with time. This is a continuous process, and due allowance made when reading a bearing off a map.
Content description
Reel two:
The magnetic variation with time is caused by physical activities within the earth, magnetic declination is the difference between true north and magnetic north. The value of the magnetic variation is determined from an almanac and map of the earth's magnetic field; the magnetic declination for any location on the topographic map is derived from the compass rose printed on the side of the map, used in conjunction with the deviation values at a known date posted for the four corners of the map sheet. Because these values are constantly changing at a known rate, the posted values are extrapolated to the date of map reading, from which the required declination may be applied to the magnetic bearing.
Maps used by the three Services are based on Ordnance Survey mapping, overprinted with a grid annotated with a letter or number within each square (or grid cell). The grids applied one, four and ten inch maps correspond with each other. The use of the roamer (a plastic transparent scale) is used to determine the grid bearing of a location on a map.
Content description
Reel three:
Cut to film of open countryside: a military exercise is in progress. Observer detects a target, fixes the target's location with the use of a prismatic compass by taking two or three separate magnetic bearings from different positions. This is known as triangulation.
The military prismatic compass is introduced: explained; the lubber line, seen at 0/3600
on the compass housing and sighting glass; intersects the compass card when the compass body is correctly sighted. The procedure to enable an observer to locate their position on the map is explained: the 'back bearing.' Film ends as the narrator recalls the interrelation of grid, true and magnetic bearings, and how they are utilised when navigating by map and compass.