A magnetic reference supplies declination to convert magnetic MWD azimuth to true azimuth. Magnetic dip and total field also support survey checks and some correction methods. Reference uncertainty contributes to the survey error budget alongside sensor errors, magnetic interference, and other sources.
The choice depends on the survey date, location, available magnetic data, and required position uncertainty. A model's update frequency or maximum spatial resolution alone does not establish its suitability.
Model comparison
The table identifies the versions available in October 2026. The maximum spherical harmonic degree describes the spatial expansion; it is separate from the model's generation or release year.
| Model version | Developer / provider | Update interval | Field represented | Main application |
|---|---|---|---|---|
| IGRF-14 | IAGA collaborative model | Every 5 years | Core-dominated main field, degree 13 | Scientific reference |
| WMM2025 | NOAA NCEI / BGS | Every 5 years | Core-dominated main field, degree 12 | Global navigation |
| WMMHR2025 | NOAA NCEI / BGS | Every 5 years | Core + crustal field, degree 133 | Navigation with finer spatial detail |
| BGGM2026 | British Geological Survey | Annual | Core + crustal field, degree 1440 | Magnetic MWD surveying |
| HDGM2026 | NOAA NCEI / University of Colorado CIRES | Annual | Core + crustal field, degree 1040 | Magnetic surveying and positioning |
See the provider specifications for IGRF, WMM, WMMHR, BGGM, and HDGM. The table summarizes spatial content; the products also differ in their treatment of external fields and uncertainty.
HDGM provider update, October 2026: NOAA announced that the 2026 models are its final HDGM and HDGM-RT releases. Existing users remain supported through the end of 2026. Monarch Magnetics will provide subsequent models under a University of Colorado license; HDGM2027 is its planned first annual release. See the NOAA transition notice and Monarch's provider information.
Update cadence and validity
The core field changes over time. Five-year models use secular variation to calculate the field for the requested survey date; they do not return unchanged values from their release epoch. Forecast uncertainty can increase as the prediction interval grows.
WMM2025 is valid from January 2025 through December 2029. Annual BGGM and HDGM updates can incorporate new observations and shorten the core-field forecast interval. HDGM2026's main field and secular variation are valid through December 2026. Use the selected product's documented validity period rather than its update interval alone.
Spatial resolution and data coverage
Standard WMM and IGRF omit most short-wavelength crustal detail. WMMHR2025 is a freely available alternative that includes crustal coefficients through degree 133, with approximately 300 km full-wavelength resolution at the equator.
BGGM's degree-1440 expansion has a theoretical full-wavelength resolution of approximately 28 km. HDGM2026's degree-1040 expansion corresponds to approximately 38 km full wavelength at the equator. NOAA reports the latter as approximately 19 km half wavelength; these are two conventions for the same scale.
These values describe spatial resolution, not guaranteed local accuracy. Near-surface magnetic data coverage affects the resolved crustal field. HDGM supplies standard error estimates where magnetic survey data are unavailable. Check the coverage and applicable uncertainty at the wellsite.
Uncertainty and the ISCWSA model
BGGM supplies scalable 1σ estimates for a location and date. HDGM, WMM, and WMMHR also provide uncertainty estimates, using different methods. Compare their definitions, confidence level, and applicability before using them in a survey uncertainty calculation.
The reference estimates must map to the selected ISCWSA tool model. BGS's public global/random outputs are not a direct replacement for every Rev5 component. Its DECG and DECR outputs already include the BH-dependent contributions; adding those contributions again would count them twice. Use the supplied parameter mapping.
For the reference terms and correlation assumptions, see how your main field model shapes the ISCWSA error model. For WMM, also check the published height and depth limitations before applying its uncertainty estimates to a downhole survey.
Local crustal and temporal corrections
Global models can leave local crustal contributions unresolved. In-Field Referencing (IFR1) uses local vector observations or transformed aeromagnetic or marine data to estimate those contributions. It combines observations and modeling; it is not a direct measurement of the complete field at every survey station.
IFR2 adds an estimate of temporal variation at the survey time. Its quality depends on observatory coverage, data quality, and how the measured disturbance is transferred to the wellsite. Both IFR1 and IFR2 retain uncertainty.
These distinctions follow the ISCWSA reference categories and error model definition.
Selecting a reference for the survey
- IGRF and standard WMM: ISCWSA includes both in its low-resolution category. They can support a survey where their limitations and applicable uncertainty meet the survey requirements. Their scientific or navigation origins do not establish a universal prohibition on use.
- WMMHR: evaluate its additional crustal detail and supplied uncertainty separately from standard WMM. Its degree-133 expansion does not imply the same coverage or performance as BGGM or HDGM.
- BGGM or HDGM: evaluate their finer spatial resolution, local data coverage, validity, and uncertainty against the requirements for the definitive survey. A product name alone does not establish an acceptable survey.
- IFR1 and IFR2: evaluate IFR1 where unresolved crustal contributions matter, and IFR2 where temporal contributions matter. Tight geological targets or close collision spacing justify examining the error budget; they do not automatically require both services.
For collision avoidance, evaluate the reference errors and modeled correlations between the compared wells. A smaller individual-well ellipse does not by itself establish acceptable clearance.
Record the model and version, survey date, coordinates, height or depth reference, north reference, applied corrections, uncertainty model, and QC results. Those records explain how the survey was processed and allow the reference choice to be reviewed.
References
- NOAA NCEI: IGRF specifications.
- NOAA NCEI / BGS: WMM2025 and WMM accuracy and limitations.
- NOAA NCEI / BGS: WMMHR2025.
- British Geological Survey: BGGM specifications, errors, and IFR description.
- NOAA NCEI: HDGM2026 specifications and error model.
- NOAA NESDIS (25 August 2026): HDGM provider transition. Monarch Magnetics: HDGM2027 continuity information.
- ISCWSA: Geomagnetic reference categories and Definition of the ISCWSA Error Model, Rev5.13.