Compass Error

Q. What are the 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.

Q. What is Azimuth?
Ans. Azimuth of a heavenly body is the smaller angle between observer’s celestial meridian and observer’s vertical circle passing through the body, at observer’s zenith. It is also the relevant arc of observer’s rational horizon.

If a straight line is drawn from centre of earth to a heavenly body, the point where this line cuts the surface of earth is called geographic position (GP) of that body. The azimuth at observer’s position therefore can also be understood as the angle between his meridian and the direction of GP.

Q. 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.

Q. Why do you take the sun’s amplitude when its LL 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.

Q. What is the apparent position of moon for the amplitude observation and why?
Ans. The moon’s upper limb is in line with the visible horizon at the time when its centre is on the observer’s rational horizon. The difference compared to the sun is due to the close vicinity of moon to earth, causing the horizontal parallax of moon to be about 10 more than that of the Sun’s.

Q. How do you determine the deviation for the ship’s compass heading, when the error is taken by gyro repeater?
Ans.  The watch-keeper when taking azimuth shouts, ‘write on’. Upon which the helmsman notes down the heading by gyro as well as magnetic compass. Azimuth must be taken when the vessel is upright and not heeled. The gyro error along with the difference between gyro and magnetic heading can give compass error. From this error variation for the present location is removed to get the deviation on the ship’s head. 

Q. What are the normal adjustments on the panel of master gyro?
Ans.  Normally, a marine gyro has adjustments for latitude and speed to take care of damping error and N-S speed error.

Q. What is an Azimuth Circle or Azimuth Ring?
Ans. An azimuth circle helps in determining both bearings of terrestrial objects and azimuths of celestial body. It can be of different types. Thus, the direct reflection by a dark reflecting surface or reflection by a prism, are common principles used. In one type, a brass frame is used to hold prism. The prism is mounted on a horizontal axis with a magnifying lens provided. Appropriate shade can be used when taking bearing of bright objects. Attached spirit levels are used to level the instrument. Prism reflector can be used in ‘arrow up’ position to take celestial bearings whereby the reflected image of celestial body is brought down to the card level. In ‘arrow down’ position the image of card can be raised to the level of terrestrial object. Another type is fitted with sighting vanes. The forward or far vane normally would have a vertical wire and the after or near vane, has a hole to peep.

Q. 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.

Q. 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 toDrifting’ 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.

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