Compass Work. "I sailed with a free wind day after day, marking the position of my ship on the chart with considerable precision; but this was done by intuition, I think, more than by slavish calculations. For one whole month my vessel held her course true; I had not, the while, so much as a light in the binnacle. The Southern Cross I saw every night abeam; the sun every morning came up astern; every evening it went down ahead. I wished for no other compass to guide me, for these were true. If I doubted my reckoning after a long time at sea I verified it by reading the clock aloft made by the Great Architect, and it was right." Joshua Slocum, Sailing Alone Round the World

In navigation or surveying, the compass is the primary device for direction-finding on the surface of the Earth. A compass may operate on magnetic or gyroscopic principles or by determining the direction of the Sun or a star. This discussion deals with the magnetic compass, which is the type universally used on small craft, to find direction in the horizontal plane.

The magnetic field observed at the surface of the Earth is similar to that exhibited by an imaginary bar magnet approximately aligned with the planet's axis of rotation. The magnetism is a physical property of the Earth’s molten mid core, and is thought to be generated by convection currents. The Earth’s magnetic field is oriented with its magnetic north pole near the south geographic pole, and its magnetic south pole near the north geographic pole. The separation between geographic and magnetic poles is about 17 deg. of latitude, and the magnetic poles describe a roughly circular path about the geographic poles approx. every 960 years. The magnetic flux field, (or magnetic lines of force), therefore emanates geographically from south to north.

A freely suspended magnetic material e.g. a soft iron rod, will be magnetised by and aligned to this flux field and become a bar magnet, its north pole always pointing in a variable northerly direction, in alignment with the magnetic field at any particular location. This is the operating principle of the magnetic compass, with the modern compass needle being a permanent bar magnet to enhance the sensitivity of the instrument. The direction of the magnetic flux at a particular point on the Earth’s surface can be resolved into horizontal and vertical components. The horizontal component, at an angle to any north - south meridian, is called the angle of declination, or in nautical terms, magnetic variation from true north, and is one component of compass error in the horizontal plane. The vertical component is called the angle of dip. To generally compensate for dip, compass cards are factory balanced depending on predetermined zones. These angles have been tabulated for all areas of the Earths surface, and map lines joining places with the same variation are called Isogonals, and with the same angle of dip, Isoclinals. These angles are constantly undergoing gradual change, mainly due to the drifting of the magnetic poles, but can also be affected occasionally by external influences like solar flares or magnetic storms. Depending on one’s position, magnetic variation will be described as either east or west of true north. i.e the compass will be deflected either east or west of true north. In Port Curtis locations, variation is about 9 deg. 45 min.East, but in recent years, has been gradually decreasing. It is the direction of the magnetic flux at a particular location that determines the degree of compass deflection relative to true north or to the meridian, not the actual position of the north magnetic pole.

Being sensitive to magnetic influences, a magnetic compass will also be affected by magnetic fields set up by construction materials of the vessel, and installed equipment. These magnetic fields vary, mainly with the heading of the vessel. The effect these fields may have on a compass is called deviation, and is the other component of compass error. Allowance for deviation must be made in determining an accurate compass course. Deviation is minimal in north/south directions and maximum in east/west directions. Deviation therefore depends on the direction in which the vessel is travelling, and the peculiarities of that vessel. Like variation, deviation is determined to be either east or west of true north. For accurate navigation, a vessels compass deviations must be known. Compass error is the sum of variation and deviation.

It is important that the compass corrections be applied in the proper order. When converting a true heading to a compass course, variation is first added or subtracted to obtain a magnetic heading. Then the deviation of the compass for the magnetic heading is added or subtracted to obtain the compass course. When working from compass course to true heading, the reverse order applies.
Which ever way the problem is worked, the magnetic heading is always the first obtained. Variation is a correction of a true heading to give a magnetic heading or v.v. Deviation is a correction of a magnetic heading to give a compass course or v.v.

The fundamental diagram of navigation is the circle, being a two dimensional representation of the Earth sphere. Originally, the nautical compass card was divided into 32 basic divisions called points of the compass, each of 11.25 deg. In modern practice, the compass scale is subdivided into 360 degrees of angle in a clockwise direction, North, East, South, and West, and compass directions are always denoted by three figure notation east of north. e.g. A direction of south west is written as 225 deg. ; east north east is written as 067.5 deg. The four basic points of a compass in 90 deg. divisions, N, E, S, W are called the Cardinal Points.
The shortest line between a start point and a destination point on a chart is called the true heading, intended track, desired track, or sometimes the rhumb line on a Mercator chart, and is a geographic direction. To steer a correct compass course, magnetic variation as noted on the chart must be applied to the true bearing giving a magnetic bearing. Then the vessel’s deviation for that bearing must be applied to the magnetic bearing to give the compass course.

Compass course = True bearing ± variation ± deviation. ------------ (1)

Variation East - Magnetic least. (subtract)      Variation West - Magnetic best. (add)

Deviation East - Compass least (subtract)      Deviation West - Compass best. (add)

In Eastern Australia, variation will normally be East, but deviation may be either East or West, depending on the vessel heading.

Bearings by Compass.| Cross bearing fix. | The small craft fixed magnetic compass is essential for navigation, and while it can be used to get an approximate line of sight bearing on a fixed object, the hand held magnetic compass is more appropriate. Bearing sights on at least 3 fixed objects should be taken over the widest arc and over the shortest period of time to obtain a position fix. This is the most common method of determining position.
Write down the sights taken (compass bearings) and convert to true bearings by applying variation and deviation. Remember that because we are converting from compass to true, the mathematics will be opposite to converting from true to compass. If these are East, then add the corrections. If West, subtract the corrections. Apply the true sightings to the chart as follows. Remember that deviation is for the vessels heading, not for the bearing of the object.

a. For each true bearing obtained, set the parallel rule on a compass rose, step the 'sight' to pass through the observed point, and lay off a line at the approximate vessel position. Repeat for the other 'sights'.
b. Alternatively use the square protractor, centering it on each observed point, to lay off each line of sight.
The vessel is on all lines of sight and therefore must be at the point of intersection of the three lines. Due to unavoidable errors the resulting diagram will be in the form of a triangle or ‘cocked hat’. If the cocked hat is small, its centre may be taken as being the position of the vessel. However, to err on the side of caution, the point of the triangle closest to danger should be taken as the position of the vessel. Often, a consistently large triangle indicates an error in the hand bearing compass, possibly due to unsuspected deviation. Take bearings well away from any magnetic influence. (4m from radios.)

Pelorus. Not normally found on small vessels, but its use is of interest. It consists of a baseplate with lubberline, fixed so that the lubber line is true fore and aft, with a fixed or rotating protractor card and sighting device. If the card is fixed, 000 deg will be on the lubber line. All bearings taken will be relative to the true heading. The relative bearing of an object + true heading = true bearing of the object. If the protractor card is adjustable, it can be set to the true heading of the vessel. The observed bearing is then the true bearing.

Lubber Line. The vertical line or pointer on the forward inner surface of the compass bowl in line with the bow, which indicates the direction in which the vessels head is swinging.

The convex of the waxing (new) moon points West. The convex of the waning moon points East.
The Southern Cross star formation is in a generally southerly direction.

When all else fails, carry a crow on board. British coastal vessels customarily carried a cage of crows. Crows detest large expanses of water and head as straight as a crow flies towards the nearest land if released at sea. The lookout perch on sailing vessels thus became known as the crow's nest.

Determining Deviation.

Before a vessel can be accurately navigated, it is necessary that the deviations of its compass be found and tabulated on a card or plotted on a graph. The process of finding deviations is known as 'swinging the ship', or 'swinging the compass'. As the term implies, the vessel is slowly and gently swung on to successive headings, and the differences between the vessel's compass headings and a known magnetic heading are recorded. These differences are the deviation of the vessel's compass. In practice on larger steel vessels, the effect of deviation is complex, but can be minimised or almost eliminated by the skilful positioning of small magnets and iron plugs in proximity to the compass, a task undertaken by a compass adjustor.

There are various methods used, but all involve the use of some form of known reference. Remember that as we are dealing with a magnetic influence, we must compare the compass with a magnetic bearing, i.e. true bearing + or - variation.

1. By alignment of two objects on shore to give a reference magnetic bearing.
The vessel is moored in line with two conspicuous objects using two anchors. The anchors are placed one on either side of the bow and at right angles to the sight line. The anchors are then adjusted to align the vessel with the two objects. The vessel is then swung about the bow and the known magnetic bearing of the aligned objects is compared with the compass bearing of the objects, as the vessel's head is pointed in the direction of the eight major points of the compass at 45 deg. intervals.

2. By use of another compass set up on the vessel in a position which is free of deviation. This 'standard' compass is carefully aligned with its lubber line fore and aft. The vessel would be anchored as above, placed on the eight magnetic headings by 'standard' compass, and the deviations of the vessel compass recorded.

Typical example.

Cardinal Point	Compass Bearing	Known Bearing	Deviation
N	280	278 M	2 W
NE	282	278 M	4 W
E	281	278 M	3 W
SE	279	278 M	1 W
S	276	278 M	2 E
SW	274	278 M	4 E
W	275	278 M	3 E
NW	278	278 M	0

Note that the deviations tend to be greater in east - west directions, and less in north - south directions.

A deviation graph could be developed from this table, with all easterly deviations to the right and all westerly to the left. The graph or table would then be applied to the vessels normal navigation calculations.

As an approximate check for smaller power boat operators, hold the vessel compass steadily in correct fore and aft alignment near the rear of the boat, away from any magnetic influences (at least 1 metre) and note the heading. Mount the compass in the desired permanent position and note any difference in reading. As with other methods, motor and all equipment should be operating. This procedure could be repeated in N,E,S & W directions.

When performing any of the above procedures, the vessel should be in its normal seagoing condition with all electrical gear switched on. Swing the vessel in calm water and allow the compass to settle at each point.

For the general recreational glass or alloy power boat, deviation may not be significant. However, do not underestimate the importance of allowing for deviation. Unless you check as above, you will never be sure. The drawback for small craft operators is that some compass graduations are in increments of 5 deg.

For further compass information, visit | Maps and Compass. |