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Magnetic Declination – Definition, Conclusion, FAQs

Aug 13, 2022

Magnetic Declination

 The north can be classified into three categories: True north, magnetic north, and grid north. American practical navigator Nathaniel Bowditch defines magnetic declination as the angle between the geographical and magnetic meridian at any location and is stated in degrees and minutes west or east to show the direction or position of magnetic north from true north.

Discovery of Magnetic Declination

The Chinese produced the earliest documented calculation of magnetic declination in 720 AD. Although the idea of declination was well-known in Europe by the early 1400s, it wasn’t until 1510 that Georg Hartman precisely measured the declination in Rome. It was clear that the calculation of magnetic declination was crucial for navigation. Mariners immediately developed systems for calculating it and started gathering declination values from different locations worldwide. Edmund Halley created the first magnetic declination map or chart of the Atlantic Ocean in 1700, using the unique notion of depicting declination as contouring lines on a map. Since then, declination maps have been created often.


What Is Magnetic Declination?

Magnetic declination can be elaborated as the angle between true north and magnetic north on a horizontal plane. The declination varies with location and time on the surface of the earth. Magnetic declination is also commonly known as magnetic variation. It is represented by the Greek letter “δ”.

The magnetic declination is positive (favorable) when the magnetic north is east of true north and negative (unfavorable) when the magnetic north is west of true north. Other phrases include agonic lines (where the lines along the declination are the same) and isogonic lines (where the lines along the declination are zero).



Magnetic declination and variation are interchangeably used to define the angle between true north and magnetic north and true north. Also, the magnetic deviation can be occasionally used to denote the term. However, there are slight distinctions between the three terminologies described below.

  • Declination

Declination is the most preferred word by scientists who study and research the magnetic field. The term is also frequently employed by land navigators to navigate the direction.

  • Variation

Sailors and pilots prefer this term over “declination” as the latter also has an astronomical meaning, which refers to the angle of a planet or star above the celestial equatorial plane. Furthermore, the term variation is employed by geo-magneticians to refer to variations in the magnetic field over time.

  • Deviation

In vehicles such as aircraft or ships, the compass is affected by the magnetization of the iron used in the building of a vehicle as well as by the magnetic field of the earth. As a result, causing the compass needle to point in the opposite direction. This type of directional mistake is called “Deviation”. People frequently use declination interchangeably with deviation.


What Is Magnetic Dip?

The magnetic dip can be elaborated as the angle formed by the earth’s electromagnetic field lines concerning the horizontal plane. It was discovered in 1544 by Georg Hartmann and is known as the dip angle or magnetic inclination. The inclination indicates the direction of the earth’s magnetic lines. A positive inclination means that the lines are heading downward in the Northern Hemisphere, and a negative inclination means that the lines are heading upward in the Southern Hemisphere.

Robert Norman discovered a dip circle, a technique for calculating the dip angle, in 1581. Other terminologies include aclinic lines (where the center of the contour lines is not equal at the earth’s surface) and isoclinic lines (where the contour lines are equal at the earth’s surface).


How to Determine Magnetic Declination?

There are several ways to determine the magnetic declination:

Magnetic Declination Map 2021:

The Canadian topographical map shows a figure in the margin giving the declination for the year the map was issued. It mentions a statement that informs the user about the annual change in declination, highlighted below the figure. The user can determine the current declination by dividing the annual change by the number of years after the chart was released, adding the sum of the changes to the published declination value.


The magnetic declination in 1992 ranges from 16°05′ to 14°03′ easterly at the west edge’s center. The average annual change declines by 11.5.

Here’s an illustration:


Magnetic declination (in 1998): 13° 15′ W

Annual change: the decline of 5.’

Consider it negative because the annual change is declining. Declination in 2003 is thus:

=13° 15′ W − 5 x 5′

=13° 15′ W − 25′

=12° 50′ W

It’s crucial to remember that the annual change changes with time. As a result, utilizing the annual change of magnetic declination map to update the declination on an outdated map is likely to produce inaccurate results.

Furthermore, it’s crucial to understand that the magnetic declination map does not depict the actual declination. So, what is magnetic declination? It is described as the angle formed by the true and magnetic north. The figure on the topographic map sheet, which is about the grid lines visible on the map, provides the value of the angle between the grid north and magnetic north. Grid declination is the proper name for this angle. The convergence angle is the angle formed by true north and grid north. The convergence angle must be added or subtracted to get the true declination from the grid declination. The figure shows four possible pairings.

The diagram shows

Magnetic declination diagram

  • The star represents true north;
  • The square represents the grid north;
  • The arrow represents magnetic north;
  • The convergence angle is represented by C;
  • Declination is represented by D, and finally,
  • G represents grid declination.

All volumes are regarded as positive. There are four examples:

  • Magnetic northwest of true north; grid northeast of true north;
  • Magnetic northwest of true north; grid northwest of true north;
  • Magnetic northeast of true north; grid northeast of true north;
  • Magnetic northeast of true north; grid northwest of true north.

Based on the map’s scale, declination is also displayed on aeronautical (aircraft) maps as contouring lines and on hydrographic (marine) maps either as contouring lines or in the way of a compass elevation. In every situation, true declination is conveyed.

Magnetic Declination Calculator:

The calculator provides the declination based on magnetic reference field models after receiving information such as the year, longitude, and latitude of the specified location.

By entering the name of the city followed by the terms “longitude and latitude” into the preferred web browser, a person can quickly obtain its latitude and longitude. (Now, a person can calculate the magnetic declination by zip code, all one needs to do is enter their zip code in a web browser, determine their latitude and longitude of the region, and calculate their magnetic declination). The height above ellipsoid (HAE), also known as elevation above the WGS84 ellipsoid, is the model currently employed by the magnetic declination calculator. Simply put, this is because variations in the gravitational field caused by the earth’s orbit produce variations in mean sea level.

Informative facts

The best part is that almost every smartphone today has built-in GPS systems, allowing smartphone owners to locate their GPS altitude virtually anywhere.

Magnetic Declination Compass:

There are three types of bearings: True, compass, and magnetic. Declination may be calculated with a compass as follows:

T = M + V

M = C + D

A general equation linking compass and true bearings is T = C + V + D.


  • C = Compass bearing
  • M = Magnetic bearing
  • T = True bearing
  • V = Variation
  • D = Compass deviation

V < 0, D < 0 for westerly deviation and variation

V > 0, D > 0 for easterly deviation and variation

Following is the method to determine compass bearing from true bearing:

True bearing – variation = magnetic bearing

Magnetic bearing – deviation = compass bearing

Following is the method to determine true bearing from compass bearing:

Compass bearing + deviation = magnetic bearing

Magnetic bearing + variation = true bearing

When used to find north, it is clear that the compass needle actually points in the opposite direction of true north, toward the earth’s magnetic north.


Declination is merely a representation of the geomagnetic field’s complexity. The dipole is not entirely consistent with the earth’s rotational axis, and the field is not symmetrical. It also contains non-dipolar “additives.” The north-pointing end of a compass would slide down at the north geomagnetic pole and the south geomagnetic pole if one were to hold it on its side and be at the north geomagnetic pole.

Frequently Asked Questions

1. Define the term ‘True North’.

True north, also known as the geographical North Pole, is the direction that runs parallel to the earth’s surface. It is also referred to as geodetic north and differs from grid north and magnetic north, which are directions along the grid lines pointing in the north direction. A compass shows magnetic north.

2. Define the term ‘Grid North’.

According to definition, a grid north is the direction that is northward all along grid lines of a geographic coordinate system. This phrase is used in navigation, and there is very little difference between the grid north and the true north.

3. Define the term ‘Magnetic North’.

The magnetic north is the direction the compass needle points about the earth’s magnetic field. Since the earth’s magnetic poles are not fixed about its axis, the difference between true and magnetic north varies from location to location.

Magnetic Declination


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