Magnetic declination

Over much of the Globe'due south surface, compass needles bespeak roughly north. Nevertheless, because of the complex shape of the Earth's magnetic field there are few places where a compass needle will signal exactly n. A compass lines upwardly with the horizontal component of the magnetic field in a direction called magnetic north. Truthful north, on the other paw is the direction from a given location to the north geographic pole. The angle between magnetic north and true north is called magnetic declination. Many people believe that a compass needle points at the North Magnetic Pole. This is not true; if you follow your compass needle you volition eventually arrive at the North Magnetic Pole, but non by the most directly route.

Terminology

Both declination and variation are used to describe the angle between magnetic north and true north. The term departure is also used from fourth dimension to time. Here is an explanation of the differences between the three terms.

Declination
This is the term preferred past those who written report the magnetic field; information technology is also the term most normally used by land navigators. Sometimes the term "magnetic declination" is used.
Variation
This term is preferred by mariners and pilots considering the word "declination" likewise has an astronomical usage - the angle of a star or planet above the angelic equator. However, the give-and-take "variation" is used past geomagneticians to refer to fourth dimension changes in the magnetic field.
Deviation
In a vehicle such as a ship or shipping, a compass is influenced past the magnetism of the iron used in the construction of the vehicle equally well as the Earth's magnetic field. This causes the compass needle to point in the wrong direction. This directional fault is chosen "departure". Many people incorrectly use divergence when they mean declination.

The first known determination of magnetic declination was made past the Chinese in about 720 AD. In Europe, the concept of declination was known in the early on 1400s, but the beginning precise measurement of declination was non fabricated until 1510, when Georg Hartman adamant the declination in Rome. The importance of declination for navigation was obvious. Mariners rapidly devised methods for determining it and began compiling declination values from locations around the earth. In 1700 Edmund Halley came up with the idea of showing declination equally contour lines on a map ; he used this novel concept to produced the first declination chart of the Atlantic Ocean. Declination charts have been produced on a regular basis ever since.

Halley map of atlantic ocean with magnetic declination contour lines

Halley map

The secular variation of the magnetic field causes declination to modify with time. Changes in declination can be quite large. At Yellowknife, NWT, for example, the declination is changing by more than 1 degree every three years. On the other hand, at Ottawa, the yearly change in declination is almost cipher. The diagram shows the change in declination at several locations in Canada.

Secular variation of declination for Vancouver, Churchill, Ottawa, and Halifa

Secular variation

Magnetic declination besides undergoes changes that are much more rapid than secular variation and are a upshot of magnetic activity. These variations can be polish and cyclic, with amplitudes of several minutes of arc in southern Canada, or, during magnetic storms, large and erratic. Changes in declination become increasingly irregular in both amplitude and frequency as one approaches the Northward Magnetic Pole, a result of the weak horizontal component of the magnetic field. The number of times per year that a compass user will be affected by changes in declination acquired by magnetic storms will depend both on the user's application and location. The diagram shows the percent of days in a typical yr during which magnetic declination will fluctuate by more than a given amount from its normal value.

Probability of deviations

Probability of deviations

Most standard orienteering compasses accept a precision of about 2 degrees. It can be seen that in Southern Canada users of such compasses will seldom feel fluctuations larger than 2 degrees. However, someone on a canoe trip in the NWT would detect his or her compass in error past more than ii degrees on more than ane twenty-four hours in iv.

How to determine declination

At that place are several ways for compass users to make up one's mind declination:

From a map

Canadian topographic charts contain a diagram in the margin which gives the declination for the year in which the chart was published. Beneath the diagram is a statement informing the user about the annual modify of declination. Past multiplying the almanac change by the number of years that have elapsed since the nautical chart was published and adding the full change to the published declination value, the user obtains the present mean solar day declination.

Image depicting an example of the declination information which is displayed on topographic charts as desribed in text

Topographic declination diagram

Argument of magnetic declaration taken from Canadian topographic map:

Magnetic declination 1992 varies from 16°05′ easterly at middle of west edge to 14°03′ easterly at eye of due east edge. Mean annual change decreasing 11.v.

Hither is an example:

  • Declination in 1998: thirteen° 15′ W
  • Annual change: decreasing five′

Since the annual change is decreasing, treat information technology as negative. Therefore Declination in 2003 is:

= xiii° 15' W - 5 ten 5′ = thirteen° xv' West - 25' = 12° fifty' W

It is important to retrieve that the annual change does not remain constant with time. Therefore, using the annual alter to update the declination on an old map is likely to result in an fault in the updated declination.

It is too important to know that the declination shown in the diagram on the topographic map is not the true declination. Declination is defined as the bending betwixt magnetic north and the true north. However, the diagram on the topographic map sheet gives the value of the angle between magnetic northward and grid north, which is referenced to the grid lines shown on the map. This angle is properly called grid declination. The bending between grid n and true north is called the convergence angle. To obtain the true declination information technology is necessary to add or decrease the convergence angle to the Grid Declination. The diagram illustrates four possible combinations.

Illustration of the four possible cases for the orientation of magnetic north, true north, and grid north as described in text

Grid (square), true (star) and magnetic n (arrow)

In the diagram,

  • the star indicates true north;
  • the square indicates grid northward;
  • the arrow indicates magnetic n;
  • G refers to filigree declination;
  • C is the convergence angle;
  • D refers to the declination.

All quantities are considered positive. 4 cases are illustrated:

  1. Magnetic northward west of true north; filigree n due west of true north;
  2. Magnetic north westward of true due north; filigree north east of true north;
  3. Magnetic due north east of true north; grid north west of true north;
  4. Magnetic north east of truthful north; grid north east of true due north.

Declination is besides shown on aeronautical charts every bit contour lines and on hydrographic (marine) charts either equally contour lines or in the form of a compass rose, depending on the scale of the chart. In all cases, true declination is given.

From a magnetic declination chart

Maps of the Earth's magnetic field are available from the National Geophysical Information Centre of the National Oceanic and Atmospheric Administration : https://www.ngdc.noaa.gov/geomag/magfield-wist/