Lesson 10: Magnetic Compass and Compass Error

On most NOAA general and harbor charts, a smaller compass rose is printed within the outer True degrees compass rose. This is referred to as a Magnetic compass rose. Instead of being aligned with a meridian of longitude, the magnetic compass rose always points in another direction, towards the earth’s magnetic North Pole (some 200 miles north of the Boathia Peninsula in Canada…the South Magnetic Pole is located in South Victoria Land in Antarctica). You can see this difference between both North poles on NOAA Chart 12221 TR as the two (2) north poles (True and Magnetic) vary in alignment with each other. This offset or angular difference is referred to as Variation and changes as a vessel’s position changes. In this instance, the stated variation is 10 minutes West (for the year 1990). Variation is the angular difference between the true north and the magnetic north pole. If the northerly part of the magnetic meridian lies to the right of the true north-pole, the variation is Easterly. If the northern part of the magnetic meridian lies to the left of the true north-pole, the variation is Westerly. As one travels on the Earth’s surface, the amount of this Variation changes as the angular difference in alignment between these two poles changes or varies and in some cases, this difference becomes zero (0) degrees. This only occurs when both poles are in direct alignment with one another.

General and harbor charts usually indicate the degree(s) of magnetic variation for its area of coverage. Before we can use a magnetic compass aboard any vessel, we first have to deal with all the magnetic influences that make it deviate from true geographical north-pole. Compass Error is the effect of two factors - variation and deviation.

When correcting from True degrees to Per Magnetic Compass (or PMC) - Westerly Variation and Deviation is always added and Easterly Variation and Deviation is always subtracted. Conversely, when correcting for a heading on the ship’s steering magnetic Compass or PMC back to a True direction, always subtract Westerly Variation and Deviation and always add Easterly Variation and Deviation.

Deviation is the magnetic influence on the compass card caused by external iron masses, structural components, electro-mechanical devices, and other electrical items or articles located onboard the vessel and will never change with ship’s position as variation does. Deviation only varies with changes in the vessel’s heading. Go to the Compass Deviation Table with either “C” or “M” to find Deviation values. Never use “T”!

Need to Know

The compass is a simple navigational instrument which uses magnetic attraction to determine circular direction. Since opposite magnetic poles attract, the fixed magnet on the compass card is attracted to the earth’s magnetic north pole. The compass card is aligned with the magnet(s) and always points towards magnetic north (excluding deviation). Remember – as the compass heading changes – the compass card does not move – it remains aligned with magnetic north.
Magnetic Variation and never, ever Deviation, changes with a vessel location at sea and will significantly vary from location to location and therefore, potentially from chart to chart. Although the learning exercises and exam questions will use the same “frozen 009º W”, correction factor for variation, out there, in the real world, the amount of magnetic variation changes from place to place on the earth’s surface. In addition, the actual variation changes on a yearly basis. Almost all NOAA general and harbor charts will have the annual increase or decrease in variation (including direction) clearly printed within the compass rose.
There are 32 cardinal points on a compass (each point equals 11¼ °).

To convert True direction to a Compass heading - use the following formula -T V M D C. This is where T means True degrees, V means magnetic Variation, M means Magnetic degrees, D means compass Deviation, and C means ship’s Compass.

When converting True to Magnetic, add westerly variation and subtract easterly variation. If correcting from Magnetic to True, subtract west variation and add east variation.

To arrive at Compass – correct Magnetic for Deviation caused by onboard magnetic sources. Deviation is always either East or West. Deviation is found on the Compass Deviation Table by using either Compass or Magnetic. Determine a Deviation value which most closely agrees with either Compass or Magnetic (interpolate).

Deviation changes as the ship’s heading changes.

By swinging the ship’s steering compass, the compass card’s Deviation from absolute magnetic truth can be defined in degrees (in a east or west direction) in the Compass Deviation Table. Therefore, Compass is a heading corrected for both Variation and Deviation or Compass Error.
To correct Compass heading to True direction, reverse the formula: C D M V T .
Always write the course in degrees clearly indicated by a T for True or M for Magnetic above the projected course line.
PMC indicates Per (steering) Magnetic Compass. PGC is Per (steering) Gyro-Compass.


Here’s How To Do It:

One of the easier ways to do compass calculations is to begin by writing down the first letters of the phrase “True Virtue Makes Dull Company” or just T – V – M – D – C in either a vertical or horizontal line with arrows indicating which direction you add east (subtract west) and add west (subtract east) and then do the math. Never get Deviation values from True, only from Compass or Magnetic.

Using 009 W Variation and 002 E Deviation.

True to Compass Compass to True

When Correcting “T” to “C” When Correcting “C” to “T”


Do this: + W à - E - W ß + E

True 115º 115º
Variation + 009º W - 009º W
Magnetic 124º ß (use this number to find D) 124º
Deviation - 001º E + 001º E
Compass 123º 123º (use this number to find D)

SHIP’S COMPASS DEVIATION TABLE

Ship’s Head Deviation (in degrees)
000 2 East
030 3 East
060 4 East
120 1 East
150 1 West
180 2 West
210 3.5 West
240 3 West
270 1.5 West
300 0
330 1 East
Remember – When going from True to Compass add Westerly Variation and or Deviation and subtract Easterly Variation and or Deviation.

When going from Compass to True subtract Westerly Variation and or Deviation and add Easterly Variation and or Deviation.