Why are variation and deviation applied when using a compass?

• A. Variation corrects true to magnetic north; deviation accounts for vessel-generated magnetic fields; both are applied to obtain true headings.
• B. They are used to adjust the vessel's draft.
• C. They are used to correct solar interference.
• D. They are optional extras.

Answer: (A)

Converting compass readings to true headings requires applying two corrections: variation and deviation. The compass shows a magnetic heading, which is measured relative to magnetic north. Variation is the angular difference between true north and magnetic north at your location; it can be found on charts and is the general correction you use to translate magnetic headings into true headings. Deviation is the error caused by the ship’s own magnetism and electrical equipment; each vessel has its own deviation value determined by swinging the compass and recording how the ship’s magnetic field skews the reading.

You apply deviation to the observed compass reading first to get a corrected magnetic heading, then apply variation to convert that to a true heading. For example, if the compass reads 90 degrees, deviation is 3 degrees west (so correct by subtracting 3 to get 87 degrees), and variation is 5 degrees east (add 5 to get a true heading of 92 degrees). This process is essential so your heading aligns with the charted course and other navigational data.