Everything about Compass Error

What is Compass Error

Compass Error is the angular difference between true north and compass north. It is named east or west to indicate the side of true north on which the compass north lies. The Compass Error is a combination of two separate and distinct components, namely variation and deviation.

Why we take Compass Error

The compass error should be checked each watch and on every new course to measure the error of the gyrocompass, which is the angle the gyro north makes with the True north to establish trends in deviation on different courses at varying latitudes.This is important because should the gyro compass fail, this information will be needed to set courses using the magnetic compass.  It is worth noting that some modern coasters only have a magnetic compass (they do not have a gyrocompass).

This is important because should the gyro compass fail, this information will be needed to set courses using the magnetic compass.  It is worth noting that some modern coasters only have a magnetic compass (they do not have a gyrocompass).

Different methods of finding compass error

  • By Azimuth of a Celestial Body: This has been discussed in detail in a separate module. Sun, selected stars, moon and planets can be used. 
  • By Amplitude of a Celestial Body: Usually, the sun or moon is used for taking amplitude which is explained in detail in a separate module. The Amplitude is taken when the body is on the true horizon. Stars and planets being small and dim when are on the horizon are not suitable.
  • When at Berth: The difference between the observed direction of the jetty and the charted direction when the ship is fully alongside will give the compass error.  Caution: This will only be accurate if the vessel is close to the jetty throughout its length and the jetty extends to the full length of the vessel.  This may not be accurate if either the bow or stern is away from the jetty or the jetty is not complete. In some ports, the ship may never be alongside the berth due to various reasons and hence this method of taking an error should be double checked as soon as possible after departure by either taking an azimuth of a celestial body or by taking a transit bearing.
  • Transit bearings: This is a line on the chart where an observer would see two identifiable objects in line. A bearing taken when both the objects are in line can be used to determine the gyro and compass errors by comparing charted and observed bearings. 
  • Leading Lights: This is similar to transit bearings where two identifiable objects are used to draw a line of bearing on the chart. These bearing lines are used to indicate track to be followed when approaching a port or a channel.  The difference between the charted bearing and the heading of the ship when on the leads can be used to determine the error on both gyro and magnetic compass.  BEWARE OF PARALLAX ERROR WITH THIS METHOD.
  • Horizontal sextant angles: When used to plot ship’s position, the observed bearing of one of the objects used for plotting the position when compared to the bearing line drawn on the chart from the observed ship’s position to the same object would give the compass error.

What are the different celestial methods of finding compass error?

Ans. The fastest method of finding compass error is by Polaris observation but this method is usable only in Northern hemisphere. Simply the LHA Aries at the time of observation is adequate data for finding the azimuth of pole star.

Compass error can also be found out by Amplitude method using sun or moon at their theoretical rising or setting. This method is also equally fast.’’ Azimuth calculations of a heavenly body can also give compass error. Compass error can also be found for a certain heading, in a given position, if the sunrise as well as sunset bearings are available on that heading. The 3 digital bearings at sunrise and sunset are added. Half of the difference of this sum with 3600 gives the error.

Why do you take the sun’s amplitude when its Lower limb is about the semi diameter above the visible horizon?

Ans. The main reason is the refraction at this moment (34.5’). Actually, at this moment the centre of the sun lies on the observer’s rational horizon. This is approximately equal to twice the semi-diametre of sun.

Which compass is more reliable? Magnetic or Gyro?

Ans. In navigable latitudes, mostly, gyro compass only is used for navigation, including navigation in pilotage waters. The course input from the gyro compass is taken to various navigational equipments. In most of the navigational operations, a high degree of reliability is placed on gyro. The only drawback, which the gyro has is that it works on power and has moving parts. The functioning of gyro may fail or the power itself may fail, though the alarms and backup power arrangements are provided.

Magnetic compass shows magnetic north in a position. The north magnetic pole is north of Bathurst Island in Canadian Arctic, whereas the south pole is 150 km offshore off Adelie coast of Antarctica. These poles are quite remote from the geographic poles. The angle between the geographic meridian and the magnetic meridian, is called variation at that location & is available from the compass rose printed on the chart.

The magnetic compass does not need electrical power for showing the direction and practically does not have mechanical parts. Even if everything else fails the magnetic compass will continue to work. The reliability about its functioning is very high, though the magnitude of error may be high and may not be known to the watch-keeper accurately. Thus, the accuracy to which a magnetic compass error may be known to the observer, is generally lower than the accuracy to which a gyro error is known. This means that if the two compasses are functional, the reliability of gyro compass is more in terms of providing accurate heading.

It is more difficult to manually steer on magnetic compass than on gyro compass. One of the reasons being, the magnetic compass cannot be kept duly corrected for heeling error, caused due different sources. The heeling error comes from various soft iron causes and the permanent vertical component. The correction however, is by permanent vertical magnets. Thus, ‘like for like’ is not done for heeling error correction. Then, the heeling error is different for port and starboard roll. On a given heading, as the ship rolls, the gyro heading will be steady but the reading of magnetic compass heading is likely to vary considerably.

Magnetic compass reliability will go down close to the magnetic poles and secondary poles. The directional ability goes down as the horizontal component, ‘H’ of the magnetic force, becomes very weak and the variation change is rapid due to closeness of ‘isogonic’ lines. Gyro compass, on the other hand, becomes sluggish in high latitudes. Most gyros become useless beyond 80oNorth.

What is directional gyro?

Ans. The Arma Brown gyro has a provision about modifying it into a direction seeking instrument from the North seeking one. The North axle is made horizontal and set in a certain azimuth. The forces equal and opposite to ‘Drifting’ and ‘Tilting’ are applied to it. In the absence of the frictional forces, axle will continue to show the direction. This application is used in very high latitudes.

Different terrestrial methods of finding out the compass error?

Ans. One of the fastest ways of determining the compass error is by taking a transit bearing when passing a set of lights. The lights should be appropriate for this purpose and must be charted too. This method gives compass error and also a terrestrial position line for plotting. The opposite bearings (1800 apart), taken on same heading will also give compass error and a position line.

Leading lights can also be used for finding compass error. The main purpose of the leading lights though, is to maintain the ship in approach channel, counteracting the cross drifts. Another, quick method to find the compass error is by comparing the ship’s head with the direction of wharf on a large scale plan chart. Compass error can also be found using the horizontal angle principle. By horizontal angle method, one is also able to find the ship’s position.

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