Magnetic Declination

A compass needle is attracted to magnetic north, while most maps are oriented to a different point on the earth, the geographic north pole ("true north"). This difference between the direction to true north and the direction to magnetic north, measured in degrees, is called magnetic declination. A simple compass adjustment or modification is necessary to correct for magnetic declination.

In areas west of the line of zero declination, the magnetic needle points somewhere to the east (to the right) of true north (fig. 4-9), so these areas are said to have east declination. It works just the opposite on the other side of the line of zero declination. Here, the magnetic needle points somewhere to the west (left) of true north, so these areas have west declination.

Consider a mountain traveler in Colorado, where the declination is 14 degrees cast. The true bearing is a measurement of the angle between the line to true north and the direction-of-travel line. The magnetic needle, however, is pulled toward magnetic north, not true north. So instead it measures the angle between the line to magnetic north and the direetion-of-travel line. This "magnetic bearing" is 14 degrees less than the true bearing. To get the true bearing, you must add 14 degrees to the magnetic bearing.

Like those in Colorado, climbers in all areas west of zero declination must add the declination to the magnetic bearing. In central California, for example, about 18 degrees must be added. In Washington State, it is about 20 degrees.

East of the zero-declination line, the declination is subtracted from the magnetic bearing. In Maine, for example, the magnetic bearing is 20 degrees greater than the true bearing. Subtracting the declination of 20 degrees gives a wilderness traveler in Maine the true bearing.

This is all very simple in theory but can be confusing in practice, and the wilderness is no place to practice mental arithmetic that can have life-and-death consequences. A more practical way to handle the minor complication of declination is to pay somewhat more for your compass and get one with an adjustable declination arrow instead of a fixed orienting arrow. The declination arrow can be easily set for any declination. Then the bearing you read at the index line will automatically be the true bearing, and concern about a declination error is one worry you can leave at home.

On compasses without adjustable declination ar

Fig. 4-9. Relationship between true and magnetic north in the United States

rows, you can get the same effect by sticking a thin strip of tape to the top or bottom of the rotating housing to serve as a customized declination arrow. Trim the tape to a point, with the point aimed directly at the specific declination for the area where you will be climbing.

In Colorado, your taped declination arrow must point at 14 degrees east (clockwise) from the 0 point (marked N for north) on the rotating compass dial (fig. 4-10a). In Maine, the declination arrow must point at 20 degrees west (counterclockwise) from the 0 point on the dial (fig. 4-10b). In Wash

DECLINATION ARROW

DECLINATION ARROW

DECLINATION ARROW

Fig. 4-10. Compass declination corrections: a, for the area west of the zero-declination line; b, for the area east of the zero-declination line.

ington State, the declination arrow must point at 20 degrees east (clockwise) from the 0 point.

To take or to follow a bearing in the field, follow exactly the same procedure used in the earlier examples from Ohio, where the declination is zero. The only difference is that from now on, you will align the magnetic needle with the declination arrow instead of with the orienting arrow.

From here on in this chapter, it is assumed you are using a compass with a declination arrow-either an adjustable arrow or a taped arrow that you have added. For all bearings in the field, you will align the needle with this declination arrow. Unless otherwise stated, all bearings referred to are true bearings, not magnetic.

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Responses

• estella
What is the true north declination for colorado?
8 years ago
• uwe
What is declination washington state?
8 years ago
• Marco
Where is the magnetic declination zero.?
8 years ago
• Kimberly
What is the Difference between Magnetic bearings and True bearings.?
8 years ago
• maurizio
Is california's magnetic bearing east or west?
7 years ago
• Marvin
7 years ago
• natalino
What is the declination at midvale utah?
2 years ago