Updated: 23 November 2018
Australian Geomagnetic Reference Field Values
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)
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.
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.
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.
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.
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.
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
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
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