U.S. military Lensatic Compass in use. Corporal Brian Askew, a Corporal's School student from Marine Wing Support Squadron 273, shoots an azimuth using a compass during two days of field training, 21 July 2005.
Today in WW II: 25 Oct 1943 American and New Zealand troops land at Mono and Stirling, Treasury Islands, south of Bougainville [25-27 Oct]. More↓
One page of a Lensatic Compass Instruction Sheet, World War II.
Compass and map skills are fundamental to warfighter readiness. The compass has been a basic instrument for unit leaders, artillerymen, and scouts since colonial times. A trained soldier with map and compass is never lost and will reach his planned objective. Magnetic compasses are suitable for military units for reconnoitering, determining direction, orienting maps, fire control and other uses. During World War I and through the early days of World War II, U.S. military compasses were in the form of a pocket watch. The modern "lensatic compass" developed from earlier "prismatic compass" designs, a line that ended with the M-1938 model. The most common modern compass for land navigation is the improved lensatic compass, carried by U.S. soldiers and Marines since World War II.
The liquid filled "Silva" or "Suunto" compasses with their clear plastic base are smaller and lighter than the military Lensatic Compass. The military compass design has survived due to its ruggedness and good fit with military training and usage for more than a half century. The commercial style compasses are procured and have NSNs but they are not regular issue for field use.
This section of Olive-Drab has these pages about U.S. military compasses:
There are other compasses in use with the U.S. military, such as mounted compasses in aircraft, ships, and gun platforms or the unusual sun compass. The compasses covered in these pages are the most common, used by large numbers of personnel and available to militaria collectors.
U.S. Military Compass & Direction Finding Terminology
Here are the definitions of some terms used for military land navigation by compass:
Compass: A device that provides a constant, reliable orientation of direction. The magnetized needle of the magnetic compass will point north and south as it lines up with the earth's magnetic field.
Map: A representation of a portion of the earth's surface, drawn to a standardized scale, as seen from above. Maps use colors, symbols, and lettering to identify features on the surface.
Azimuth: The direction of an object, measured clockwise around the observer's horizon from north. In the degree system, due north has an azimuth of 0° (zero degrees), due east 90°, south 180° and west 270°. There are 360 degrees in the full compass circle. See also the mil system defined below.
Altitude: the distance an object appears to be above the horizon. The angle is measured up from the closest point on the horizon. Azimuth and altitude can be used together to give the location of an elevated object.
Mil: In the mil system, the compass circle is divided into 6400 units. The mil unit of angular measurement is defined so that 1 mil equates to 1 meter separation at 1,000 meters distance. One mil also equates to one yard at 1000 yards, the unit doesn't matter. If two objects at 10,000m distance (say, hilltops) are 100 mils apart in azimuth on your compass (or scope) then you instantly know they are 1000m apart. This is a much easier calculation than using degrees, therefore modern military compasses have scales in both degrees and mils.
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