The World Magnetic Model - Accuracy, limitations, magnetic poles and error model
Accuracy of the WMM2015
Changes of the fluid flow in the Earth's outer core lead to unpredictable changes
in the Earth's magnetic field. Fortunately, the system has large inertia, so that these
changes take place over time scales of many years. By surveying the field for a few years,
one can precisely map the present field and its rate of change and then linearly extrapolate
it out into the future. Provided that suitable satellite magnetic observations are available,
the prediction of the WMM is highly accurate on its release date (Fig. 1) and then
subsequently deteriorates towards the end of the 5 year epoch (Fig. 2), when it has to be
updated with revised values of the model coefficients.
Limitations of the WMM2015
It is important to recognize that the WMM and the charts produced from this model characterize only
the long-wavelength portion of the Earth's internal magnetic field, which is primarily generated in
the Earth's fluid outer core. The portions of the geomagnetic field generated by the Earth's crust
and upper mantle, and by the ionosphere and magnetosphere, are largely unrepresented in the WMM.
Consequently, a magnetic sensor such as a compass or magnetometer may observe spatial and temporal
magnetic anomalies when referenced to the WMM. In particular, certain local, regional, and temporal
magnetic declination anomalies can exceed 10 degrees. Anomalies of this magnitude are not common but
they do exist. Declination anomalies of the order of 3 or 4 degrees are not uncommon but are usually
of small spatial extent. See EMM home page for a model which includes
crustal fields down to 50 km wavelength.
Fig. 1. Estimated WMM2015 declination inaccuracy without considering crustal and disturbance field contributions. Click for high-resolution image.
Error model for the WMM2015
The WMM2015 software and online calculator include an error model providing uncertainty estimates for every geomagnetic element (X, Y, Z, H, F, I and D) and every location at the Earth's surface. This model is built upon the results of the error analysis (subsection 3.3 of the WMM2015 report), while taking into account the geometrical relationships between the various components. It includes both the commission error (due to inaccuracies in model coefficients) and the omission error (due to missing contributions to the total geomagnetic field such as crustal and external fields). The error values (Table 1) may be interpreted as one standard deviation difference between a hypothetical measurement and the calculator result for a location.
|X||138 nT |
|Y||89 nT |
|Z||165 nT |
|H||133 nT |
|F||152 nT |
|D|| √0.242+(5432/H)2 ° |
Table 1. Uncertainty estimates provided by the WMM2015 error model for the various field components. H is expressed in nT in the formula providing the error in D.
A global map of the magnetic declination error is provided in Fig. 2. As can be seen on this map, the error is lower at mid- to low-latitudes, while it is larger near the magnetic poles and in an area close to South Africa where the horizontal field is very low.
Fig. 2. Global distribution of the declination error provided by the WMM2015 error model Click for high-resolution image.
Magnetic Poles and Magnetic Center
Based on the WMM2015 coefficients for 2015.0 the geomagnetic north pole is at 72.62°W longitude and 80.37°N latitude, and the geomagnetic south pole is at 107.38°E longitude and 80.37°S latitude. The axis of the dipole is currently inclined at 9.69° to the Earth's rotation axis.The WMM can also be used to calculate dip pole positions. These model dip poles are computed from all the Gauss coefficients using an iterative method. In 2015.0 the north dip pole computed from WMM2015 is located at longitude 159.18°W and latitude 86.27°N and the south dip pole at longitude 136.59°E and latitude 64.26°S. More information about geomagetic poles here.
Details of the model derivation are described in the NOAA Technical Report: The US/UK World Magnetic Model
for 2015-2020 (pdf).