The World Magnetic Model - Accuracy and Limitations

The WMM geomagnetic model and the charts produced from this model characterize only that portion of the Earth’s magnetic field that is generated in the Earth’s fluid outer core (the main magnetic field). The portions of the geomagnetic field generated by the Earth’s crust and upper mantle, and by the ionosphere and magnetosphere, are not represented 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 of small spatial extent and are relatively isolated.

Difference in declination for the WMM as compared to independent satellite models for 2005.0 Click image for larger version.

Accuracy

On land, spatial anomalies are produced by mountain ranges, ore deposits, ground struck by lightning, geological faults, and cultural features such as trains, planes, tanks, railroad tracks, power lines, etc. The corresponding deviations are usually smaller at sea, and decrease with increasing altitude of an aircraft or spacecraft. In ocean areas, these anomalies occur most frequently along continental margins, near seamounts, and near ocean ridges, trenches, and fault zones, particularly those of volcanic origin. Ships and submarines are also sources of magnetic anomalies in the ocean.

However, from a global main field perspective, the declination (D), inclination (I), and grid variation (GV) RMS errors of WMM2005 are estimated to be less than 1.0° at the Earth’s surface over the entire 5-year life span of the model. Also, the RMS errors at the Earth’s surface horizontal intensity (H), the vertical component (Z), and the total intensity (F) of WMM2005 are estimated to be well below 200 nT over the entire 5-year life of the model. Thus, the WMM2005 meets and exceeds the accuracy requirements detailed in MIL-W-89500 (Defense Mapping Agency, 1993) for the entire life span of the model.

Difference in total field for the WMM as compared to independent satellite models for 2005.0 Click image for larger version.

The crustal contributions could be included in an extended model, expanded to high degrees, as is common for modern gravity field models, such as EGM96 (Lemoine et al., 1998). Since the crustal field is almost constant in time, it can be inferred from all available marine, aeromagnetic, and high resolution CHAMP and future SWARM satellite data, measured at all times. However, this extended model would differ significantly in format from the current WMM, requiring changes in supporting software.

Geomagnetic Poles

The geomagnetic poles, otherwise known as the dipole poles, can be computed from the first three Gauss coefficients. From the WMM2005 coefficients for 2005.0 the geomagnetic north pole is at longitude 71.78°W and geodetic latitude 79.74°N and the geomagnetic south pole is at longitude 108.22°E and geodetic latitude 79.74°S. The magnetic poles, otherwise known as the dip poles, are computed from all the Gauss coefficients using an iterative method. At 2005.0 the north magnetic pole is located at longitude 118.32°W and geodetic latitude 83.21°N and the south magnetic pole is at longitude 137.86°E and geodetic latitude 64.53°S. In practice the geomagnetic field is not exactly vertical at these dip poles but is vertical on oval-shaped loci traced on a daily basis, with considerable variation from one day to another, and approximately centered on the dip pole positions.

Details of the model derivation are described in the NOAA Technical Report: The US/UK World Magnetic Model for 2005-2010 (zip ~8.3 Mb or pdf format ~8.7 Mb).