U.S. Army Soldier's Gear: Compass
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.
U.S. Military Compass
|
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.
|
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.
U.S. Army Soldier's Gear: U.S. Military Lensatic Compass
|
The Lensatic Compass, the most common military instrument for measuring direction, was first during World War II, an evolution of the pre-war Compass, Prismatic M-1938. At that time, the U.S. Army Corps of Engineers was responsible for compass procurement so most models had "Corps of Engineers" stamped on the case. During the war, these compasses were manufactured by Superior Magneto Corp. (Long Island City, NY) and W. & L.E. Gurley (Troy, N.Y), with the "Corps of Engineers/US Army" markings.
The Corps of Engineers Lensatic Compass was the most common style in use during World War II, particularly later in the war as the other models were phased out. The compass was issued to officers, squad leaders and specialist troops only, although most troops received training in compass use for land navigation. The World War II Lensatic Compass remained in use until replaced after 1950 with the modern lensatic compass, called "Compass, Magnetic, M-1950" the design that continues in use.
|
Soldier demonstrating taking an azimuth reading with the M-1950 Lensatic Compass. Photo from Army Field Manual FM 21-26 Map Reading (March 1956).
|
Unfortunately this compass design was not moisture-proof or waterproof, leading to problems in the field in Vietnam. Like all magnetic compasses, the needle aligns itself with the local magnetic field and is attracted by nearby objects containing iron or electric circuits. If it is tilted only a few degrees off level, significant azimuth errors are introduced. The readings by two users can differ significantly and errors in transcribing the reading are easy to make.
U.S. Military Lensatic Compass Since the M-1950
|
Compass, Lensatic M-1950.
|
With the Compass, Magnetic M1950 the Lensatic Compass design changed, although only in details. The cylindrical body was extended to a rectangular base that includes a map scale and straight-edge. Other small improvements were made to the earlier wartime Lensatic Compass. The photo to the left shows one of these compasses, produced in November 1951. Note that it is stamped U.S. but does not have any identification of the model number of the compass itself and that the Corps of Engineers is no longer responsible for procurement.
|
The image immediately below from Army Field Manual FM 21-26 Map Reading (March 1956) illustrates the compass and its parts:
From FM 21-26 Map Reading, dated March 1956.
The Lensatic Compass was always associated with a case or pouch, although the case was sometimes issued with the compass and was sometimes a separate item of issue.
Manufacturing History of the Compass, Magnetic
From the 1950's into the 21st Century, the Lensatic Compass was procured in large numbers for the U.S. military ground forces, and other related uses. During that period a number of manufacturers were contracted to build the units and small variations in design were made. This chart summarizes some of the particulars of that history.
| Year | Manufacturer | Nomenclature/Markings |
|---|
| 1951 | Lee and Stemwedel Inc. | U.S. stamped, no other model id. Date 11-1951 |
| 1953 | Marine Compass Co., Pembridge, MA | U.S. stamped, no other model id. Date 2-1953 |
| 1960 | Waltham Precision Instruments | Dates 3-60 and 5-60. Radioactive warning label. |
| 1964-1968 | Union Instrument Corp., Plainfield, NJ | U.S. Compass, Magnetic FSN 6605-846-7618, DA23-195-AMC-00906(T) 9-1966. Radioisotope H3 A.E.C. License No. 8-5970-9 Contains 75mc. Radioactive H3. Do not open. Dispose of per Busanda Inst. 5101.3. If found return to Military Authority. Other dates 1-64, 1-1967, 1-1968, 9-1968. |
| 1966 | Jay Bee Corp., a subsidiary of Stocker & Yale Inc. | Compass, Magnetic
FSN 6605-846-7618. Other dates 1-1970. |
| 1975 | Stocker & Yale Inc., Beverly, MA | MAGNETIC COMPASS, NSN 6605-00-151-5337, 7 MAR 75, DA AK01-75-0-2147 |
| 1979 | Stocker & Yale Inc., Beverly, MA | U.S. Compass, Magnetic NSN 6605-00-151-5337
DAAJ09-79--0-5176 2 SEP 79. H3 content with AEC warning, DISPOSE AS RAD WASTE |
| 1981 | Stocker & Yale, Inc., Beverly, MA | U.S. Army Compass, Magnetic, NSN 6605-00-151-5337, DAAJ09-82-C-A358, 15 DEC 81. |
| 1989 | Stocker & Yale, Inc., Beverly, MA | US Compass, Magnetic NSN 5505-01-196-6971,, DAAK01-89-C-0125 3 Aug 89. SandY 183, 120mC1 3H. Controlled Disposal Required |
| 1991 | Stocker & Yale, Inc., Beverly, MA | U.S. Compass, Magnetic, NSN: 6605-01-196-6971 3/91 |
| 1992 | Cammenga | Compass, magnetic, unmounted, lensatic, luminous, M2. Tritium Lensatic Compass. NSN: 6605-01-196-6971. Mil Spec: MIL-PRF-10436N. |
Compass NSN 6605-00-151-5337 is deemed "operationally unserviceable" and compass 6605-00-846-7618 was condemned in 1978 for safety reasons and was not authorized for use thereafter. Both of these should have been replaced in miliary inventory with NSN 6605-01-196-6971. Since September 1992, the lensatic compass has been produced with tritium luminous markings for the U.S. Government by Cammenga, under NSN 6605-01-196-6971.
Markings on bottom of Compass, Magnetic made by Union Instrument Corp., Plainfield, NJ, 1968. There are many imitations or copies of this popular compass model so beware of cheap imitations that are similar to but not the same as the units produced for the U.S. Government.
|
