Updated:  08 April 2015

Australian Geomagnetic Reference Field Values

Map of Australia with coloured areas where the AGRF Values program will work, otherwise global magnetic field model will be used for the calculations.

If the chosen location is outside the coloured area in the adjacent image then a global magnetic field model will be used for the calculations.

If location coordinates are unknown, try the place name search. The WGS84 datum is used for location coordinates.


The AGRF Model

The Australian Geomagnetic Reference Field model (AGRF) is a series of spherical cap harmonics which describe the geomagnetic field in the Australian region. From 1990 to 2015 the AGRF has been updated at five yearly epochs. A main field model is produced for each five yearly epoch, along with a prospective secular variation model to extend the life of the model. There will be small discontinuities between successive models (i.e. 1990.0, 1995.0, 2000.0, 2005.0, 2010.0) as individual models are not retrospectively updated.

The AGRF model represents the Earth's main magnetic field originating from the core and the broad scale crustal field. The AGRF does not model short term variations of the magnetic field with time, such as those caused by solar activity or from electrical currents in the ionosphere. The AGRF is derived from vector magnetic data from ground level, aircraft and satellite surveys as well as the network of geomagnetic observatories and repeat stations run by Geoscience Australia and neighbouring countries.

Images of data from the 2015 revision of the AGRF at 2015.0

In the images the magnitude components (F, H, X, Y and Z) have the main field (red contours) in nanoTesla (nT) and the annual change (blue contours) in nT per year. The angular components (D and I) have the main field (red contours) in degrees and annual change (blue contours) in arc-minutes per year. The circular boundary shows the limit of the AGRF model, contours outside the boundary are from the International Geomagnetic Reference Field model (IGRF-12) at 2015.0.

World Declination (main field only) from the 12th generation International Geomagnetic Reference Field (IGRF-12) at 2015.0.

Components of the Magnetic Field

D, the magnetic declination (sometimes called the magnetic variation), is the angle between the horizontal component of the magnetic field and true north. It is positive when the compass points east of true north, and negative when the compass points west of true north. Declination is given in degrees and its annual change is in degrees per year.

The value of magnetic declination should be added to a magnetic compass bearing to yield the true north bearing. (see the examples below)

F, the total field, is the strength of the magnetic field. F is given in nanoTesla (nT) and its annual change in nT/year.

H, the horizontal field, is the strength of the horizontal part of the magnetic field. H is given in nanoTesla (nT), and its annual change in nT/year.

X, Y, and Z are the magnetic field components in the true north, east, and vertically down directions. This forms a standard right-handed coordinate system. X, Y and Z are given in nanoTelsa (nT) and their annual change in nT/year.

I, the magnetic inclination, is the angle between the magnetic field and the horizontal plane. It is positive when the magnetic field points down, as it does in the northern hemisphere, and negative when the magnetic field points up, as it does in the southern hemisphere. Inclination is given in degrees and its annual change is in degrees per year.

Click on the link to view a diagram of these seven components of the magnetic vector.

Converting Between Magnetic, True and Grid Azimuths

Map and compass users often need to convert between a magnetic azimuth and a true azimuth or a grid azimuth.

Magnetic north differs from true north by the magnetic declination which is defined and discussed above and can be calculated for a particular location and time using regional or global magnetic field models.

Grid north differs from true north by the grid convergence. Grid convergence is defined as "the angular quantity to be added algebraically to an azimuth to obtain a grid bearing" (Geocentric Datum Of Australia Technical Manual, Version 2.3 (1))

In the southern hemisphere grid convergence is positive for points east of the grid zone central meridian (grid north is west of true north) and negative for points west of the grid zone central meridian (grid north is east of true north). Grid convergence can be calculated on-line for a given location. (Geodetic Calculation Methods)

Grid convergence and magnetic declination are shown in diagrammatic form on some topographic maps. The signs of these values can be deduced from these diagrams.

Four numerical examples are presented to illustrate conversions between magnetic, true and grid azimuths. The same relationships between azimuths apply to all examples but each example illustrates the application of the relationships for magnetic declination and grid convergence values with differing sign combinations.

Example 1: Negative magnetic declination, negative grid convergence

A magnetic azimuth of 72.0° measured at a Perth location (-31° 57'S, 115° 51'E), on 01 July 2002 where the magnetic declination is -1.8° and the Map Grid of Australia 1994 (MGA94) zone 50 grid convergence is -0.6°.

Example 2: Positive magnetic declination, positive grid convergence

A magnetic azimuth of 251.0° measured at a Canberra location (-35° 18'S 149° 08'E) on 01 July 2004, where the magnetic declination is +12.3° and the MGA94 zone 55 grid convergence is +1.2°.

Example 3: Positive magnetic declination, negative grid convergence

A magnetic azimuth of 135.0° measured at an Alice Springs location (-23° 42'S, 133° 53'E) on 01 January 2009, where the magnetic declination is +5.1° and the MGA94 zone 53 grid convergence is -0.4°.

Example 4: Negative magnetic declination, positive grid convergence

A magnetic azimuth of 310.0° measured at a Carnarvon location (-24° 53'S 113° 40'E) on 01 January 2007, where the magnetic declination is -0.1° and the MGA94 zone 49 grid convergence is +1.1°.

The azimuth conversions are explained and worked through for each of these four examples.


Convert from Magnetic Azimuth to True Azimuth

true azimuth = magnetic azimuth + magnetic declination (#1)

To convert a magnetic azimuth to a true azimuth apply relationship #1, shown immediately above, taking care to retain the sign of all the angles involved.

Example 1:

true azimuth = 72° + (-1.8°)
= 70.2°

Example 2:

true azimuth = 251° + (+12.3°)
= 263.3°

Example 3:

true azimuth = 135° + (+5.1°)
= 140.1°

Example 4:

true azimuth = 310° + (-0.1°)
= 309.9°


Convert from True Azimuth to Magnetic Azimuth

magnetic azimuth = true azimuth - magnetic declination

To convert a true azimuth to a magnetic azimuth re-arrange relationship #1 as shown immediately above, taking care to retain the sign of all the angles involved.

Example 1:

magnetic azimuth = 70.2° - (-1.8°)
= 72.0°

Example 2:

magnetic azimuth = 263.3° - (+12.3°)
= 251.0°

Example 3:

magnetic azimuth = 140.1° - (+5.1°)
= 135.0°

Example 4:

magnetic azimuth = 309.9° - (-0.1°)
= 310.0°


Convert from True Azimuth to Grid Azimuth

grid azimuth = true azimuth + grid convergence (#2 )

To convert a true azimuth to a grid azimuth apply relationship #2, shown immediately above, taking care to retain the sign of all the angles involved.

Example 1:

grid azimuth = 70.2° + (-0.6°)
= 69.6°

Example 2:

grid azimuth = 263.3° + (+1.2°)
= 264.5°

Example 3:

grid azimuth = 140.1° + (-0.4°)
= 139.7°

Example 4:

grid azimuth = 309.9° + (+1.1°)
= 311.0°


Convert from Grid Azimuth to True Azimuth

true azimuth = grid azimuth - grid convergence

To convert a grid azimuth to a true azimuth re-arrange relationship #2 as shown immediately above, taking care to retain the sign of all angles involved.

Example 1:

true azimuth = 69.6° - (-0.6°)
= 70.2°

Example 2:

true azimuth = 264.5° - (+1.2°)
= 263.3°

Example 3:

true azimuth = 139.7° - (-0.4°)
= 140.1°

Example 4:

true azimuth = 311.0° - (+1.1°)
= 309.9°


Convert from Magnetic Azimuth to Grid Azimuth

grid azimuth = (magnetic azimuth + magnetic declination) + grid convergence (#3)

To convert from a grid azimuth to a magnetic azimuth substitute from relationship #1 into relationship #2 to derive relationship #3 as shown immediately above, taking care to retain the sign of all angles involved.

The term "grid magnetic angle" can be introduced for conversions between magnetic and grid azimuths. Grid magnetic angle is sometimes called "grid variation" or "grivation". The grid magnetic angle is defined:

"the angle on the plane of a chosen grid coordinate system at the observer's location measured clockwise from the direction parallel to the grid's Northings' axis to the horizontal component of the magnetic field" The US/UK World Magnetic Model for 2010-2015

grid magnetic angle = magnetic declination + grid convergence (#4)

Substituting from relationship #4 into relationship #3 to express it as

grid azimuth = magnetic azimuth + grid magnetic angle

Example 1:

grid azimuth = 72° + (-1.8°) + (-0.6°)
= 69.6°

Example 2:

grid azimuth = 251° + (+12.3°) + (+1.2°)
= 264.5°

Example 3:

grid azimuth = 135° + (+5.1°) + (-0.4°)
= 139.7°

Example 4:

grid azimuth = 310° + (-0.1°) + (+1.1°)
= 311.0°


Convert from Grid Azimuth to Magnetic Azimuth

magnetic azimuth = grid azimuth - magnetic declination - grid convergence

To convert a magnetic azimuth to a grid azimuth, re-arrange relationship #3 as shown immediately above, taking care to retain the sign of all angles involved.

Alternatively the relationship can be expressed using the grid-magnetic angles as

magnetic azimuth = grid azimuth - grid magnetic angle

Example 1:

magnetic azimuth = 69.6° - (-1.8°) - (-0.6°)
= 72.0°

Example 2:

magnetic azimuth = 264.5° - (+12.3°) - (+1.2°)
= 251.0°

Example 3:

magnetic azimuth = 139.7° - (+5.1°) - (-0.4°)
= 135.0°

Example 4:

magnetic azimuth = 311.0° - (-0.1°) - (+1.1°)
= 310.0°

For more information contact: geomag@ga.gov.au
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