Finding North with a lensatic compass

Finding North with a lensatic compass

by tonytran2015 (Melbourne, Australia).

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(Blog No.76).

#find North, #finding North, #compass, #lensatic, #magnetic, #true North.

Finding North with a lensatic compass.

A magnetic compass uses the local magnetic field generated by the rotating core of the earth to give the rotational axis of the earth. The two directions are reasonably close.

The magnetic compass is a secondary directional instrument to people who can observe celestial bodies for navigation but is a primary practical directional instrument to people who work under the forrest canopy or underground and cannot observe any Celestial body. (Gyro-compasses are too expensive for most people).

This blog shows how to use lensatic compasses to their full capability.

1. Basic operation (working with Magnetic and True Norths).

Figure: True North, Magnetic North and Magnetic Deviation.

Obtain the magnetic deviation for your area. This deviation is commonly between (-45) degrees and +45 degrees and it slowly changes to a new value every year. The deviation is obtainable from the internet or by the method given in reference [2]. Positive magnetic declination means the Magnetic North is on the East of True North.

If you are using a compass with a rotating compass rose which always points to magnetic North, just read the graduation on the card and ADD the signed magnetic declination to it.

2. Using a lensatic compass

A lensatic compass provides a small lens to read the heading of the compass from a rotating rose.

Figure 1: An old ex-US Army lockable dry compass (made around 1965, initially known as M-1950 compass) from disposal stores remains robust and can keep on working although the clear glass has become partially clouded. The lid is open by 70 degrees..

Figure 2: A lensatic compass with the lid open to vertical postion (90 degrees).

Figure 3: A lensatic compass with the lid open by 135 degrees.

Figure 4: A lensatic compass with the lid open by 180 degrees.

The steps are:

1. Open the lid by 90 degrees to its vertical position. The lid has an aiming slit with a sighting wire.

2. Open the small sighting tab with the lenses by 90 degrees to vertical posotion. This also unlock the compass rose of the M-1950 compass.

3. Place your thumb through the ring to keep the compass vertically on top of your hand.

4. Place your sighting eye behind the lens to see the scale on the rose and the target point simultaneously. The sighting tab may have to move closer or away from the scale to produce a clear reading.

5, When the slit on the sighting tab, the wire on the lid and the target all line up, the figure on the rose gives the target direction relative to magnetic North on the rose.

6. Add the SIGNED magnetic declination to it to get the direction relative to TRUE North.

3. Reconnect to true North.

When using a magnetic compass, form the habit of reconnecting the true North given by the compass to the true North given by the Sun, the Moon, terrain, landmarks, etc… This prevents many possible disasters.

Figure: Cross checking directions with those from a Celestial method whenever possible is a desirable habit. This illustration is from reference [7].

Figure: Cross checking directions with those from a Celestial method whenever possible is a desirable habit. This illustration is from reference [4].

4. Precaution against magnetic anomalies .

Besides magnetic deviation caused by the core of the earth, there are also deviations near ground surface caused by man-made objects and ore bodies. These are called “Magnetic Anomalies”. An anomaly produces non-parallel magnetic field lines and non-constant magnetic field strength.

The following steps help to detect any magnetic non-uniformity in order to detect such anomalies and to avoid disorientation when using a magnetic compass (A magneto-meter is better for this detection as it supplies the extra accurate value of field strength but it requires batteries.).

1/- Count the yaw and pitch oscillation frequencies of the needle of any new compass. Write them on a piece of paper for future reference.

2/- Check the polarity and the yaw and pitch frequencies before packing the compass for any trip. If the frequencies have decreased the needle may have lost part of its magnetism and re-magnetization may be needed.

3/- Have a habit of cross checking directions with those from Celestial methods (given in references [4,5,6,7]) whenever the Sun or the Moon or bright stars can be seen. The cross checking with the direction determined by the method given in [7] also verifies that your watch is still working.

4/- Frequently take the bearing of your destination in both standing and crouching/sitting positions (with different heights), even if you are confident of your current position. This ensures that your compass is ready when needed and you are prepared against any sudden loss of visibility (due to change in weather, terrain, falling down a crevice! . . .). Any quick change in magnetic direction with height or with horizontal travel distance usually reveals a very strong, meter-scaled magnetic anomaly. The anomalies may be caused by buried metal objects (steel-reinforced bunkers, car bodies, tank bodies, unexploded bomb-shells!, magnetic ores,…) or even induced magnetic field from high voltage power lines.

5/- If anomaly is suspected, count the yawing frequency of the needle. The external field strength acting on the needle is proportional to the square of this frequency. For example, if the frequency goes up by 2, the external magnetic field strength may have gone up by 4.

6/- Distant objects such as mountain peaks, light houses, transmission towers can help detecting the sudden change in magnetic deviation when you travel. The magnetic bearings of these distant objects should only change slowly. If there is any sudden change in their bearings you may have to check the accuracy of your compass or the change in magnetic deviation.

Notes:

Compasses are usually NOT suitable for rail travelers as the carriage bodies and the rails underneath it creates magnetic abnormality.

Compasses mounted in cars and boats have special, additional compensaring devive to eliminate the influence by the car or boat bodies.

5. A special mode of operation to reduce own influence.

Figure 3: Hand posture for holding type M-1950 compass for sighting with a stretched arm. The finger through the brass ring is the middle finger. The sighting is carried out using the slit in the sighting tab and the wire in the lid. The lens has no function in this sighting mode. This is my own novel mode of operation not designed by the makers of type M-1950 compasses.

Magnetic material carried on users’ bodies can affect the readings on their compasses. They have to use the following mode of operarion to reduce that effect.

Using the lockable compasses.

Any tilt tolerant, lockable compass can also be used in this mode. Tilt tolerance ensures that the needle can point North before the lock is engaged to freeze it in place. The ex_US Army compass (model M-1950) in figure 1 of Step 2 can be used in this mode, and is surprisingly accurate, as it is tilt tolerant and the inertia of its rotating compass rose ensures that the needle orientation is retained while the lock is engaging. A lockable M-1950 compass is used in the following way (This is my own novel mode of operation, not designed by the makers of type M-1950 compasses. Do NOT complain to them that their compasses are not comfortable to use in this way!) :

Place the lid and the sighting tab of the M-1950 compass in vertical positions to have the compass in unlocked and operating condition. Turn the holding ring near the sighting tab fully downward. Curl the middle finger of your hand and stick it through the holding ring of the compass. Grip the compass by pressing the end of the index finger against the lid hinge and the joint of the thumb against the sighting hinge. Stretch your arm while holding the lensatic compass horizontally level at the height of our eyes, away from your head, with its lid vertical and its lock still fully disengaged (sighting tab in vertical position).

Take aim of the destination by the aiming guide then hold till for 3 seconds. Gently engage the lock (push the sighting tab into horizontal position) by only raising only the first joint of your thumb to push against the bend of the sighting tab while keeping the rest of the body motionless. After having been locked, the whole compass is bought to your comfortable reading distance to read the locked position of the needle. The magnetic bearing of the aiming line is the value on the compass rose at the point nearest to the hinge. It is hard to read but is definitely readable even with the sighting tab fully down.

Please leave any comments and suggestions here so that the posting can be improved !!

References.

[1]. tonytran2015, Beware of perilous flips by magnetic compasses, survivaltricks.wordpress.com, Beware of perilous flips by magnetic compasses, posted on June 14, 2016.

[2]. tonytran2015, Determining local magnetic declination by a magnetic compass, survivaltricks.wordpress.com , Determining local magnetic declination by a magnetic compass, posted on March 31, 2016.

[4]. tonytran2015, Finding directions and time using the Sun and a divider, survivaltricks.wordpress.com , Finding directions and time using the Sun and a divider., posted on May 6, 2015.

[5]. tonytran2015, Finding North direction and time using the hidden Sun via the Moon, survivaltricks.wordpress.com, Finding North direction and time using the hidden Sun via the Moon . Posted on July 6, 2015.

[6]. tonytran2015, Finding North direction and time by stars, survivaltricks.wordpress.com, Finding North and time by stars. Posted on August 28, 2015

[7]. tonytran2015, Finding accurate directions using a watch, posted on May 19, 2015 .

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