Model background

The BGS Global Geomagnetic Model (BGGM) is a mathematical model of the Earth's magnetic field in its undisturbed state. It is revised every year to allow for the inclusion of new data collected since last revision and any development of the modelling methodology. With annual revisions, it is also possible to minimise errors arising from predicting the field at some date after the time span of input data. In 2019 the maximum spherical harmonic degree increased from 133 to 1440. Degree 133, representing features of wavelength 300 km at Earth's surface, is close to the maximum resolution that can be achieved with satellite data above 300 km altitude. By including information from near-surface total intensity anomaly compilations, the theoretical resolution can be increased to 28 km.

Note: Declination has a discontinuity at the ±180° boundary - contouring algorithms thus perform poorly at the poles and the contour lines should be viewed with caution.

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How can I get it?

The model and associated software are available under licence from BGS. Licence holders receive source code for incorporation into their own software throughout company, DLLs, Excel implementation examples, a Windows program for use throughout company, access to the web service and documentation.

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What is it for?

The BGGM is widely used in the oil industry for directional drilling with measurement-while-drilling (MWD) magnetic survey tools. These tools measure the direction of the well-bore relative to the direction of the local geomagnetic field and are used to navigate wells towards precisely known underground targets.

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Estimating Errors for High-Definition Geomagnetic Models

Industry increasingly requests higher-degree, smaller-scale magnetic field models. We have quantified and delivered these models, along with a combined estimate of the main sources of error:

1. High-degree crustal field uncertainty
2. Spatial limitations of crustal field input data
3. Main field forecasting uncertainty
4. External field influences

Model Resolution and Data Sources

For high-degree models such as BGGM (degree 1440), satellite data can consistently model the magnetic field up to degree 133 (~300 km spatial resolution). To further enhance the model, we incorporated global ground aeromagnetic and marine survey data from the WMDAM compilation, enabling computation up to degree 1440 (~28 km resolution).

Model Degree	Approx. Spatial Resolution	Primary Data Source
133	~300 km	Satellite data
1440	~28 km	Satellite + WMDAM aeromagnetic & marine surveys

Validation Against Observations (1960-2018)

We compared BGGM model outputs against magnetic measurements collected at thousands of repeat stations and observatories worldwide between 1960 and 2018. Residuals (model minus observation) were analysed in the linear magnetic field components: X (north), Y (east), and Z (vertical).

These were then converted into derived magnetic elements: Declination (D), Inclination (I), and Total Field (F).

Confidence Interval Computation

To compute confidence intervals (CI), residuals in each component were:

1. Sorted by magnitude (absolute value).
2. The value below which 68.3% of residuals fall was recorded.
3. The value below which 95.4% of residuals fall was recorded.
4. These values were interpolated and gridded onto a 1° × 1° global dataset.

Non-Gaussian Error Behaviour

Magnetic residuals are not Gaussian-distributed. In a Gaussian distribution:

Gaussian Relationship	Confidence Level
1σ	68.3%
2σ	95.4%
3σ	99.7%

However, magnetic data are typically better described by Laplacian statistics, meaning:

2 × 1σ ≠ 95.4%

In practice:

• CI_68.3% < 1σ (Gaussian equivalent)
• CI_95.4% > 2σ (Gaussian equivalent)

Scalable 1σ Equivalent

To provide a conservative and scalable uncertainty metric, we use:

Scalable 1σ = CI_95.4% ÷ 2

This value is plotted in the colour contours of the BGGM global error maps.