A Cruising Boat (Magnetic) Compass should be capable of accuracies to within about 1 degree of error (I cannot steer that accurately)
. For a variety of reasons, including the resolution of my compass card, I’ve always assumed a compass accuracy of 2 degrees. This permits a cross-track error of 2 miles over a 60 mile run, which I’ve never experienced.
The Magnetic Compass, used on a large commercial
ship, should have an Accuracy of 0.5 degrees, whereas an electronic compass to should have 0.2 degree RMS Accuracy, and 0.1 deg. Resolution .
However, there are a variety of reasons that a compass may give an incorrect bearing. The most commonly encountered problems are summarized below.
1. The Compass must be aligned with the keel
, or it will suffer from a constant Alignment Error (on all headings).
2. The initial deviation must be less than 20 degrees on all Cardinal Headings. The integral compensation rod cannot correct more than 20 deg error.
3. It’;s impossible to adjust a compass that’s subjected to variable magnetic fields, such as caused by motors (autopilot), etc.
It is generally agreed that the IGRF (International Geomagnetic Reference Field) achieves an overall accuracy of better than 1degree in declination; the accuracy is better than this in densely surveyed areas such as Europe
and North America, and worse in oceanic areas such as the south Pacific
. The accuracy of all models decreases in the Arctic near the North Magnetic Pole. Global model calculations are produced every five years.
The horizontal force of the magnetic field, responsible for the direction in which a compass needle is oriented, decreases in strength as it approaches the North Magnetic Pole, where it is zero. Close to the pole, an area is reached where the frictional forces in the pivot are comparable to the horizontal forces of the magnetic field.
Ie: In Ketchikan the free compass needle points 17 degrees from the vertical: in the Nome and Anchorage areas, about 15 degrees; at Whitehorse, 14 degrees; at Fairbanks, 13 degrees; and at Barrow, only 11 degrees from vertical. Hence in Alaska
and northern Canada
, the magnetic field exerts little horizontal force on a compass needle. The force that is exerted causes the compass to point horizontally well east of north, by amounts ranging from less than ten degrees in the Aleutians to as much as 40 degrees near Barter Island.
In various parts
of the world, magnetic ores on or just below the seabed may give rise to local magnetic anomalies resulting in the temporary deflection of the magnetic compass needle when a ship passes over them. The areas of disturbance are usually small unless there are many anomalies close together. The amount of the deflection will depend on the depth
and the strength of the magnetic force generated by the magnetic ores. However, the magnetic force will seldom be strong enough to deflect the compass needle in depths greater than about 5,000 Feet. Similarly, a ship would have to be within a mile of a nearby land mass containing magnetic ores for a deflection of the needle to occur. Local magnetic anomalies are depicted by a special symbol on most charts
, and are mentioned in Sailing Directions. The amount and direction of the deflection of the compass needle is also given, if known.
Deflections may also be due to wrecks lying on the bottom in moderate depths, but investigations have proved that, while deflections of unpredictable amount may be expected when very close to such wrecks, it is unlikely that deflections in excess of 7 degrees will be experienced, nor should the disturbance be felt beyond a distance of 300 Yards.
Greater deflections may be experienced when in close quarters with a ship carrying a large cargo such as iron ore, which readily reacts to induced magnetism.
carrying direct current
can cause deflection of the compass needle. The amount of the deflection depends on the magnitude of the electric current
and the angle the cable makes with the magnetic meridian. Small vessels with an auto-pilot dependent upon a magnetic sensor may experience steering
difficulties if crossing such a cable.
The effects of magnetic and ionospheric storms, which may persist with varying intensity for several days, are usually greatest in higher latitudes, causing radio
'black-outs' and simultaneous deviations of the magnetic compass needle by several degrees (in and around auroral zones).
Accuracy and durability are the most important points to consider when buying
a compass. Some key specifications to check are:
1. Compass accuracy
2. Bezel resolution (5 degree, 2 degree, 1 degree, 1/2 degree - lower is better)
3. Dampening time (how long it takes for the needle to settle)
GPS HEADING ACCURACY:
GPS collects data about our very recent track or course over the ground, providing a “Historical” track. That track includes all of the other forces working on the boat in addition to where we’re pointing the vessel. These other forces include tides, currents, leeway, and any other anomalies. GPS generates a HAS BEEN FUNCTION (ie the course & speed average HAS BEEN 162 degrees @ 6.2 knots). It does not generate real time information.