Magnetic Declination
by Ray Hart
Magnetic declination is the angular difference between True North (TN) and Magnetic North (MN). The left and right edges of a USGS topographical map—inside the margins—align with the geographical north and south poles, and the poles are positioned at the end-points of the Earth’s rotational axis. A compass needle points to the magnetic north pole which is currently located in the western part of Hudson Bay (Churchill, Manitoba). Since True North and Magnetic North do not point to the same location, you cannot take a compass azimuth and plot it directly on a map, nor can you set a map bearing directly on a compass. Some compasses have an adjustment to automatically compensate for magnetic declination. After the adjustment is made, the compass is oriented to True North making it easier to use with a map. If your compass does not have the optional adjustment, then you will need to calculate the difference when setting a compass azimuth based on a map bearing, or when plotting a map bearing based on a compass azimuth. This document provides an easy way to remember how make the manual conversions.
The mnemonic “empty sea, add water” provides an easy way to remember how to make the conversions. The words “empty sea” sound like the letters ‘M’, ‘T’’, ‘C’ which stand for Map-To-Compass; this is the conversion direction. The “add water” part means to “add west” declination (‘w’ in water for west). From this, all the derivations can be generated.
Map to Compass add West declination
Map to Compass subtract East declination
Compass to Map add East declination
Compass to Map subtract West declination
In Oregon, we’re only concerned with the middle two derivations because the magnetic declination is to the East. The mnemonic “Declination East Compass Least, Declination West Compass Best” is used by many wilderness travelers, but the derivations may not be as obvious for some. Knowing how to calculate the compass azimuth or map bearing is important when an adjustable compass is not available.
For example,
The magnetic declination in the Portland, OR area is currently 18 degrees east. A heading of 18 degrees on a map needs to be converted to 0 degree on a compass. Since we’re taking a map bearing and converting it to a compass azimuth, the 18 degrees of east declination is subtracted from the map bearing (map-to-compass subtract east declination). So, a true 18 degrees bearing minus 18 degrees of declination calculates to be a 0 degree compass azimuth. So, by setting your compass azimuth to zero on the bezel, you’re actually moving on a true heading of 18 degrees. Conversely, a compass azimuth of 0 degree magnetic needs to be converted to 18 degrees true for the map bearing (compass-to-map add east declination).
The position of the magnetic north pole changes annually, so make sure you look at the date when your map was printed before making the conversions. If the map is very old, you will need to discover the current magnetic declination for the area you are in. |
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