Updated: 13 March 2013
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 2010 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 (ie 1990.0, 1995.0, 2000.0, 2005.0) 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 ionsphere. 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 2010 revision of the AGRF at 2010.0
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-11) at 2010.0.
World Declination [jpg_108k] (main field only) from the 11th generation International Geomagnetic Reference Field (IGRF-11) at 2010.0.
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 example 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 [jpg_24k] of these seven components of the magnetic vector.
Converting Compass Bearings to True North Bearing
Let us say the magnetic declination for Perth (31 57'S, 115 51'E) at 1 July 2002 is -1.6 degrees. A compass bearing of 72 degrees in Perth converts to a true bearing of 70.4 degrees [72 + (-1.6)].
Map and compass users often require the angle between grid north and magnetic north. Grid north differs from true north by the "grid convergence". The MGA94 grid convergence for the Perth location above is -0.6 degrees. A true bearing of 70.4 degrees in Perth converts to a grid bearing of 69.8 degrees [70.4 + (-0.6)].
Grid convergence and magnetic declination are shown in diagrammatic form on some topographic maps. The signs of these values can be deduced from the diagram.
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