The lensatic compass has a lens on the rear sight to aid in accurate readings. It is used by the military for laying in artillery or sighting on a target. It can be used as an important aid in determining your current position on a map and for land navigation. The compass has a protective cover containing a sighting wire with luminous sighting dots above and below the wire, a lanyard ring attached to a lens with a sighting slot above it, a thumb loop and a bezel ring. The face of the compass has a floating dial with fixed index line, a luminous magnetic arrow, a luminous bezel line and a luminous heading. The face also features an inner and an outer scale, where the outer scale denotes the mil system of angular measurement and the inner scale is marked in degrees.
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Things you need
- Topographic map
Form an L using your pointer finger and the thumb of your right hand. Slip your thumb through the thumb loop and curl your index finger around the base of the compass. This helps to stabilise the compass.
Push the lens up to a 45-degree angle and attach it to the cover to form a triangle. The lens enables an accurate reading of the compass dial, and the notch above the lens is used in conjunction with the sighting wire.
Bring the compass up so that the back of your thumb rests against your cheek. Keep the compass steady and take a reading. This method of holding the compass is called "compass to cheek," and is accurate to within 3 degrees.
Peer through the lens and change the compass dial or bezel seen through the magnifying lens to the desired azimuth or angular direction. Aim the sighting wire so that it lies in the centre of the target object to determine the angle. An azimuth of 0° is north; an azimuth of 90° is east, 180° is due south and 270° due west.
Read the scale on the compass to take your initial reading. The bearing can be read in degrees or mils, whichever you prefer. To set the azimuth of this bearing on your compass, move the marking on the bezel so that it lines up with the north-pointing arrow, then aim the sighting wire at the target and take the reading.
Place your map on a flat surface. Open your lensatic compass all the way so the front cover and the back cover form a flat line.
Place the compass on the map with the long side parallel with the north/south grid line on the map and orient the map so that the north-pointing magnetic arrow lines up with north on the map. This eliminates any error due to the declination angle.
Move your compass without moving your map, so that it lines up on the map from your current point to your desired destination.
Take a reading from your compass. This is the heading you must follow to reach your destination. Mark the readings on your map if you are tracking your path.
Look for two prominent points in the distance such as a hilltop, a man-made structure, or an intersection. They should be separated by some distance. Locate these features on your map.
Take an azimuth reading for the first object. Subtract this measurement by 180° or 1600 mls. This angle is called the back azimuth. Then take an azimuth reading for the second object. Take this as amount and subtract it by 180° or 1600 mls.
Draw lines at the back azimuth angle you calculated on the map through the features that you are using as reference points. If you have calculated accurately, the point where the lines intersect on the map is your location.
Verify your location by finding more features and repeating the same steps. The lines from these measurements each should run through the same point where the first two lines crossed. The more back azimuths you can draw, the more accurate your location calculation will be.
Set your initial azimuth while there is still light, or rotate the bezel ring until the luminous line is over the fixed black index line.
Locate the desired azimuth using your compass and divide the reading by three. The result is the number of clicks that you have to rotate the bezel ring. The bezel turns 120 clicks for one full rotation, and each individual click represents 3°.
Turn the bezel the desired number of clicks. If the desired azimuth is smaller than 180°, count in a counter-clockwise direction; if the desired azimuth is larger than 180°, subtract the reading from 360° and divide by 3 to obtain the number of clicks, then turn the bezel in a clockwise direction.
Follow the direction you calculated to your desired location.
Tips and warnings
- Perform a detailed inspection before using your compass. Check the floating dial to ensure functionality, make sure the sighting wire is straight, the glass and the crystal parts are not broken, the numbers on the dial are legible and that the dial does not stick.
- If the luminous chemical tritium in your compass has become dim, you can charge it up with a flashlight.
- Most lensatic compasses are not moisture-proof or waterproof.
- The needle of magnetic compasses can be affected by nearby metallic objects or by electric circuits.
- If the compass is tilted only a few degrees off level, significant azimuth errors are introduced.
- The readings by two users can differ significantly, and it's easy to make errors in transcribing the reading.
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