Find Magnetic Declination with Google Maps
Copyright © 1997 - 2015 Wayana Software. All Rights Reserved.

Calculation of Magnetic Declination

The calculation of the magnetic declination is based on the work "The US/UK World Magnetic Model for 2015-2020" published by the National Geophysical Data Center and British Geological Survey Geomagnetism Group. This utility uses a mathematical model to determine the magnetic declination of a place, or the difference between true north and magnetic north. This is the standard method used by the Department of Defense United States, the Ministry of Defence of Britain, NATO and the Word Hydrographic Office in determining the magnetic declination. The magnetic declination value obtained on the map is accurate and I believe that this is the same model as used in most hand held GPS receivers.

How can we calculate Magnetic Declination value at any given place?

Use this map to calculate the magnetic declination value for any place on Earth. It uses Google Maps API, World Magnetic Model 2015-2020 and C code translated into the language javascript. You can always find it in this web address.

Using the Magnetic Declination Value

When using Magnetic Declination values, keep in mind that when the declination is East you must subtract the declination value from the calculated true azimuth. When declination is West you must add the declination value to the calculated true azimuth.

Find Magnetic Declination value by moving around the map

a. Drag and drop the map to each location.
b. Zoom in for better accuracy.
c. Place the cursor on the desired city and read on the top of the map the "Magnetic Declination" value.

Move the mouse over the map to find Magnetic Declination !
Copyright © 1997 - 2015 Wayana Software. All Rights Reserved.

Many people are surprised to learn that a magnetic compass does not normally point to true north. In fact, over most of the Earth it points at some angle east or west of true (geographic) north. The direction in which the compass needle points is referred to as magnetic north, and the angle between magnetic north and the true north direction is called magnetic declination. You will often hear the terms "variation", "magnetic variation", or "compass variation" used in place of magnetic declination, especially by mariners.

The magnetic declination does not remain constant in time. Complex fluid motion in the outer core of the Earth (the molten metallic region that lies from 2800 to 5000 km below the Earth's surface) causes the magnetic field to change slowly with time. This change is known as secular variation. Because of secular variation, declination values shown on old topographic, marine and aeronautical charts need to be updated if they are to be used without large errors. Unfortunately, the annual change corrections given on most of these maps cannot be applied reliably if the maps are more than a few years old since the secular variation also changes with time in an unpredictable manner.

Globally, the magnetic field lines (which make up the magnetic meridian) are similar to the lines of longitude which form the geographic meridian. That is, they encircle the globe and converge at a common point in each hemisphere. This point in the northern hemisphere presently lies in the Northwest Territories, Canada: the true magnetic pole is about 11.6° south of the geographic north pole, and about 104.3° west longitude. On a more detailed level however, the magnetic lines are not straight, but bend and arc depending on local magnetic conditions. This bending is called deviation.

The Magnetic Compass

The magnetic compass has been used for navigation for hundreds of years. At one time, it was the only reliable means of direction-finding on days when the sun and stars were not visible. Nowadays, sophisticated equipment is available that enables users to determine their bearing accurately and to pinpoint locations to within a few metres. However, such equipment has not made the compass obsolete. It is still a very practical tool for navigation for many small craft and for people on foot. Even airplanes and ships equipped with more sophisticated equipment often carry compasses as backups.

Regardless of their intended purpose or the complexity of their construction, most compasses operate on the same basic principle. A small, elongated, permanently magnetized needle is placed on a pivot so that it may rotate freely in the horizontal plane. The Earth's magnetic field which is shaped approximately like the field around a simple bar magnet exerts forces on the compass needle, causing it to rotate until it comes to rest in the same horizontal direction as the magnetic field. Over much of the Earth, this direction is roughly true north, which accounts for the compass's importance for navigation.

Area of Compass Unreliability

The horizontal force of the magnetic field, responsible for the direction in which a compass needle is oriented, decreases in strength as it approaches the North Magnetic Pole, where it is zero. Close to the pole, an area is reached where the frictional forces in the pivot are comparable to the horizontal forces of the magnetic field. The compass starts to behave erratically, and eventually, as the horizontal force decreases even more, the compass becomes unusable.

Magnetic Reference Field Models

Since magnetic observations are neither uniformly nor densely distributed over the Earth, and since the magnetic field is constantly changing in time, it is not possible to obtain up-to-date values of declination directly from a database of past observations. Instead, the data are analyzed to produce a mathematical routine called a magnetic reference field "model", from which magnetic declination can be calculated. Global models are produced every five years. These constitute the series of International Geomagnetic Reference Field (IGRF) models. The latest IGRF was produced in 2009, and is valid until 2020.

Since magnetic field models such as the IGRF are approximations to observed data, a value of declination computed using the model is likely to differ somewhat from the "true" value at that location. It is generally agreed that the IGRF achieves an overall accuracy of better than 1° in declination; the accuracy is better than this in densely surveyed areas such as Europe and North America, and worse in oceanic areas such as the south Pacific. The accuracy of all models decreases in the Arctic near the North Magnetic Pole.

Magnetic Declination Calculation

When using magnetic declination values, keep in mind that when you are located west of the 0° line-of-declination, the declination is east, whereby you must subtract the declination value from the calculated azimuth value. When located east of the 0° line, the declination is west, whereby you must add the declination value to the calculated azimuth value.

Magnetic Declination Description

Magnetic declination is sometimes referred to as the magnetic variation or the magnetic compass correction. It is the angle formed between true north and the projection of the magnetic field vector on the horizontal plane. By convention, declination is measured positive east and negative west (i.e. D -6 means 6 degrees west of north). For surveying practices, magnetic declination is the angle through which a magnetic compass bearing must be rotated in order to point to the true bearing as opposed to the magnetic bearing. Here the true bearing is taken as the angle measured from true North.

Declination is reported in units of degrees and minutes. One degree is made up of 60 minutes.

If west declinations are assumed to be negative while east declination are considered positive then

True bearing = Magnetic bearing + Magnetic declination

An example: The magnetic bearing of a property line has an azimuth of 72 degrees East. What is the true bearing of the property line if the magnetic declination at the place in question is 12 degrees West?
A magnetic declination of 12 degrees West means that magnetic North lies 12 degrees West of true North.

True bearing = 72 degrees + ( -12 degrees declination )
= 72 degrees - 12 degrees declination = 60 degrees East

It should be noted that the magnetic declination becomes undefined at the North and South magnetic poles. These poles are by definition the two places where the magnetic field is vertical. Magnetic compasses become quite unreliable when the magnetic field vector becomes steeply inclined.

Magnetic Field Components

There are seven magnetic field elements. The total field vector ( F ), the X component or northward component, the Y component or eastward component, the Z component or vertical component, and the H or horizontal component. These five elements are often referred to as the force elements while the last two components, the inclination and the declination are referred to as the angular elements.

The seven computed magnetic components displayed are:

H - Horizontal Intensity of the geomagnetic field
X - North Component of the geomagnetic field
Y - East Component of the geomagnetic field
Z - Vertical Component of the geomagnetic field
F - Total Intensity of the geomagnetic field
I - Geomagnetic Inclination
D - Magnetic Declination (Magnetic Variation)

Horizontal Component of the Magnetic Field

This is the magnitude of vector constructed by projecting the total field vector onto the local horizontal plane. In terms of the vector components of the field

North Component of the Magnetic Field

This is the magnitude of vector constructed by projecting the total field vector onto an axis lying in the direction of the Earth's rotational pole or true North. X is measured positive in the north direction.

East Component of the Magnetic Field

This is the magnitude of vector constructed by projecting the total field vector onto an axis in the Eastward direction i.e. perpendicular to the X-axis. Y is measured positive in the east direction.

Vertical Component of the Magnetic Field

This is the magnitude of vector constructed by projecting the total field vector onto an axis in the local vertical direction i.e. perpendicular to the horizontal plane. Z is measured positive down.

Total Intensity (Magnetic Field Vector)

The Earths magnetic field, referred to as the Geomagnetic field is a vector field i.e. at each point in space this field has a strength and a direction. This vector, F is referenced to a local coordinate system as follows: the vector is ecomposed into three mutually perpendicular ( orthogonal ) vector components, which are referred as the X, Y, and Z components of the field, where the X and the Y components lie in the horizontal plane with X lying in the northward direction, Y lying in the eastward direction, while the Z component is taken in the local vert- ical directi/n. The strength of the magnetic field is usually given in units of nanoteslas ( nT ) and is taken in the usual mathematical fashion i.e.

The X, Y, and Z components completely describe the magnetic field vector, F however in the study of the Earth's magnetic field it is often convenient to describe this vector's direction through the use of two so-called "angular components" called the declination and the inclination. In addition the strength of the projection of the vector F onto the horizontal plane or the H component is often studied.

Magnetic Inclination

Also called magnetic dip, this is the angle measured from from the horizontal plane to the magnetic field vector . I is measured positive down. If the vector components of F are X, Y, and Z then

I = arc tangent( Z/square root(X*X + Y*Y )) or I = arc tangent( Z/H )

The north magnetic pole is defined as that position where I=90 degrees i.e. straight down. Similarly, the south magnetic pole is defined as that position where I= -90 degrees i.e. straight up.

Magnetic Declination

D = arc tangent( Y / X ).

Get your local Magnetic Declination

The TrackingSat software also computes the main components of the geomagnetic field and their annual changes. The  module is part of the main program. The input parameters and valid entries are:

1) Latitude -90.00 to +90.00 degrees
2) Longitude -180.00 to +180.00 degrees
3) Altitude sea level to 1,000,000 meters (optional)

Date base epoch of the current model to epoch + 5 years
Note: The altitude is referenced to the World Geodetic System 1984 (WGS 84) ellipsoid.

Annual change in each of these magnetic components is also displayed. The annual change is computed by subtracting the main field values for the desired input date from main field values one year later. The output units are displayed using the abbreviations nT (nanoTesla), deg (degrees) and min (minutes) per year.

As geomagnetic model data is only reliable for five years from the epoch date of the model, computing data for a date that exceeds the life of the model may produce inaccurate results. Therefore, when a date is entered that exceeds five years from the epoch date, a warning is printed on the screen.

Frequently Asked Questions

What is the Earth's magnetic field ?

The Earth acts like a great spherical magnet, in that it is surrounded by a magnetic field. The Earth's magnetic field resembles, in general, the field generated by a dipole magnet (i.e., a straight magnet with a north and south pole) located at the center of the Earth. The axis of the dipole is offset from the axis of the Earth's rotation by approximately 11 degrees. This means that the north and south geographic poles and the north and south magnetic poles are not located in the same place. At any point, the Earth's magnetic field is characterized by a direction and intensity which can be measured. Often the parameters measured are the magnetic declination, D, the horizontal intensity, H, and the vertical intensity, Z. From these these elements, all other parameters of the magnetic field can be calculated.

What are the magnetic elements ?

To measure the Earth's magnetism in any place, we must measure the direction and intensity of the field. The parameters describing the direction of the magnetic field are declination (D), inclination (I). D and I are measured in units of degrees. The intensity of the total field (F) is described by the horizontal component (H), vertical component (Z), and the north (X) and east (Y) components of the horizontal intensity. These components may be measured in units of Oersted (1 oersted=1gauss) but are generally reported in nanoTesla (1nT * 100,000 = 1 0ersted).

The Earth's magnetic field intensity is roughly between 25,000 - 65,000 nT (.25 - .65 oersted).
Magnetic declination is the angle between magnetic north and true north. D is considered positive when the angle measured is east of true north and negative when west. Magnetic inclination is the angle between the horizontal plane and the total field vector. In older liturature, the term 'magnetic elements' often refered to D, I, and H.

What are the magnetic poles ?

The magnetic poles are defined as the area where dip is vertical. There are several definitions of magnetic pole. Two common uses are the "surveyed" magnetic dip pole where the magnetic field is measured to be vertical and the "modeled" magnetic dip pole based on a model of Earth's magnetic field where inclination is calculated to be 90 degrees. In reality, the surveyed magnetic pole is not a single point, but more likely an area where many 'magnetic poles' exist.

The task of locating the principal magnetic pole is difficult for many reasons; the large area over which the dip or inclination (I) is nearly 90 degrees, the pole areas are not fixed points, but move tens to hundreds of kilometers because of daily varations and magnetic storms, and finally, the polar areas are relatively inaccessable to survey crews. Based on recent (~1990) surveys carried out by the Canadian Geological Survey and the U.S. Naval Oceanographic Office (among others), the current location of the surveyed magnetic poles are approximately: 78.5 N and 103.4 W degrees, near Ellef Ringnes Island, Canada 65 S and 139 E degrees, in Commonwealth Bay, Antarctica.

The current model dip pole based on the IGRF 1995, computed for mid-1996 is 79.0 N and 105.1 W degrees 64.7 S and 138.6 E degrees. Another pole position is the geomagnetic dipole (geocentric dipole). This is the pole positions based on the first three terms of the current International Geomagnetic Reference Field, a model of the Earth's main magnetic field. Using the IGRF, and computing the symmetric positions where the dipole would intersect the Earth's surface, the pole positions are 79.3N, 71.5W for the north pole and 79.3S, 108.5E for the south pole.

These positions are frequently used to generate the geomagnetic coordinate system.
Is the magnetic field different in different places of the Earth? Yes, the magnetic field is different in different places. It is so irregular that it must be measured in many places to get a satisfactory picture of its distribution. This is done at the approximately 200 operating magnetic observatories world-wide and at several more temporary sites. However, there are some regular features of the magnetic field.

At the magnetic poles, a dip needle stands vertical (dip=90 degrees), the horizontal intensity is zero, and a compass does not show direction (D is undefined). At the north magnetic pole, the north end of the dip needle is down; at the south magnetic pole, the north end is up. At the magnetic equator the dip or inclination is zero. Unlike the Earth's geographic equator, the magnetic equator is not fixed, but slowly changes.

Does the compass needle point toward the magnetic pole ?

No. The compass points in the directions of the horizontal component of the magnetic field where the compass is located, and not to any single point. Knowing the magnetic declination (angle between true north and the horizontal trace of the magnetic field) for your location allows you to correct your compass for the magnetic field in your area. A mile or two away the magnetic declination may be considerably different, requiring a different correction.

What is the magnetic equator ?

The magnetic equator is where the dip or inclination (I) is zero. There is no vertical (Z) component to the magnetic field. The magnetic equator is not fixed, but slowly changes. North of the magnetic equator, the north end of the dip needle dips below the horizontal, I and Z are positive. South of the magnetic equator, the south end dips below the horizontal, I and Z are measured negative. As you move away from the magnetic equator, I and Z increase.

What are magnetic field models and why do we need them ?

Because the Earth's magentic field is constantly changing, it is impossible to accurately predict what the field will be at any point in the very distant future. By constantly measuring the magnetic field, we can observe how the field is changing over a period of years. Using this information, it is possible to create a mathematical representation of the Earth's main magnetic field and how it is changing. Since the field changes the way it is changing, new observations must contiually be made and models generated to accurately represent the magnetic field as it is.

How accurate are the magnetic field models ?

The accuracy of a model in calculating the magnetic field influencing a compass or other magnetic sensor is affected by many things, including where you are using the compass. The magnetic field acting on a compass at the Earth's surface is made up of interactions between the outer core field (perhaps 90% of the total), the Earth's crust (i.e. the kind of rock you are standing on and the underlying geologic structure), the 'external' field influenced by the ionosphere and magnetosphere, and local effects such as power lines, metalic belt buckles, train tracks etc.

Most magnetic field models represent only the portion of the geomagnetic field generated by the Earth's outer core. In general, the present day field models such as the IGRF and World Magnetic Model (WMM) are accurate to within 30 minutes of arc for D and I and about 200 nanoTesla for the intensity elements. It is important to understand that local anomalies exceeding 10 degrees, although rare, do exist. Local anomalies of 3 to 4 degrees also exist in relatively limited spatial areas. One area in Minnesota has a mapped anomalous area of 16 degrees east declination with anomalies a few miles away of 12 degrees west!

How often are new models adopted ?

A new International Geomagnetic Reference Field (IGRF) is adopted every five years. The IGRF for 2015 through 2020 was adopted in December of 1999 by the International Association for Geomagnetism and Aeronomy (IAGA) at the General Assembly of the International Union of Geodesy and Geophysics (IUGG) in Boulder, Colorado.

The 2015-2020 World Magnetic Model developed by the U.S. Department of Defense and the British Geological Survey, was made available in December of 1999. Models need to be revised at least every five years because of the changing nature of the magnetic field. Existing models forward predict the magnetic field based on the rate of change in the several years preceding the model generation. Since that rate of change itself is changing, to continue to use models beyond 5 years introduces progressively greater errors in the field parameters calculated.

Where can I find out more about geomagnetism ?

NGDC has a brief description of the Earth's magnetic field as well as answers to some commonly asked questions. The following references are just a few of the books on magnetism available from your university library. We also recommend library searches for recent articles published in Science magazine, Scientific American, and other popular scientific magazines. Ask your librarian for help, or try searching the Web !