Section 4

Correcting the Compass

4.1

Deviation

Ferrous metals and electrical devices create magnetic fields. Most cruising boats contain enough metal in their engine blocks, keels, electronics, electrical appliances and wiring to affect the magnets in a boat's steering compass. This influence deflects the compass card of a boat's compass and creates a compass error called deviation.

The amount and direction of deviation change as a boat's heading changes. This is because the location of the influencing objects changes relative to magnetic north and to each other as a boat turns. Deviation also varies from boat to boat because the mix of magnetic influences differs from boat to boat.

A boat's compass may have compensating magnets underneath the compass card that can be adjusted to minimize errors caused by magnetic influences. However, some degree of deviation usually persists.

While variation depends on geographic location and equally affects all boats in the same locality; deviation varies from boat to boat and changes as the heading of the boat changes.

This example shows how deviation varies with the heading of the boat. On the left, the magnetic influence of the engine is in line with magnetic north and no deviation is caused. As the boat turns, the compass should continue to line up with magnetic north, however it's pulled to the east by the influence of the engine. This produces an eastward compass deviation.

Correcting for Deviation

Deviation, like variation, is measured in degrees east or west and must be converted to get a true heading, bearing or course. A reading for which deviation has not been converted is a compass reading or ship's compass reading. In the process of converting, it's changed to a magnetic reading and then to a true reading. The method of converting to and from compass and magnetic is similar to converting to and from magnetic and true. A similar mnemonic to that used as a guide to convert for variation can be used to convert for deviation:

Deviation West, Compass Best; Deviation East, Compass Least

With deviation west, a compass reading affected by deviation is "best" or greater than the magnetic. With deviation east, the compass reading is "least" or less than magnetic.

The Deviation Table

Deviation varies from boat to boat. A boat with a steering compass affected by magnetic influences should have a deviation table. This table shows the amount and direction of deviation for different headings. Lesson 4.2 examines deviation tables and shows how to create one.

A practice deviation table is provided for use in the tests and plotting exercises. It shouldn't be used for real-world navigation.

Deviation and the Hand-held Compass

A hand-held compass can be "walked away" from the boat's magnetic influences. To take deviation-free readings stand at the lifelines or gunwales, or just forward of the mast on a sailboat. You can then compile a deviation table by comparing the headings from the hand-held compass -- the magnetic readings -- with those from the steering compass. (See lesson 4.2.)

"C" for Compass

When noting or plotting a compass heading, bearing or course, the letter "C" should follow the degrees, as in 218°C. This prevents confusion with true or magnetic readings.

Get Help