How to convert compass bearing to true bearing

Compass bearings are affected by the magnetic field of the earth and by magnetic objects in the vicinity. The effect of magnetic objects in the vicinity of called 'Deviation'. The effect of the earth's magnetic field is called 'Variation'. Compass corrected for nearby objects (Deviation) gives us 'Magnetic' direction. Correcting for the Earth's magnetic field (Variation) gives us TRUE direction. NOTE: On land when we talk about map reading, we normally refer to 'Variation' as 'Magnetic Declination' or sometimes even 'Deviation'.

Instructions

Difficulty: Moderately Easy

Things You’ll Need:

• table of local deviations
• pencil
• paper
• chart
• paper
• pencil

How to convert from Compass to Magnetic

1. If deviation is known to be zero or if you are far away from any ferrous objects, skip to the section for converting from Magnetic to True
2. If the deviation is EAST, add it to the compass reading
   If the deviation is WEST, subtract it from the compass reading
   
   For example, the compass reads 265 degrees with a measured deviation of 5 degrees EAST at this location. This means that the magnetic reading is 265 + 5 = 270 degrees.
3. Write down the computed magnetic bearing for reference

How to convert from Magnetic to True

4. Read the variation from the compass rose on the chart, adjusting if necessary to the current year
5. If the variation is EAST, add it to the magnetic reading
   If the variation is WEST, subtract it from the magnetic reading
   
   For example, the compass reads 265 degrees with no deviation at this location. This means that the magnetic reading is 265 degrees. If the variation from the chart is 6 degrees WEST, then the TRUE direction is 265 - 6 = 259 degrees.
6. Write down the final compass direction for reference

Tips & Warnings

• The most frequent mistake is getting mixed up between EAST and WEST deviation and variation, and hence adding or subtracting at the wrong time.