IGRF at a Point

Use the IGRF > IGRF at a Point menu option (geogxnet.dll(Geosoft.GX.Igrf.Igrfpt;Run)*) to calculate IGRF (International Geomagnetic Reference Field) or DGRF (Definitive Geomagnetic Reference Field) model values for a single point at a specified date.

The option is available with the following extensions and menus:

Geophysics: IGRF
Geophysics Leveling: Tie Line Levelling > Levelling Corrections

Compute Single Point GRF Values dialog options

IGRF / DGRF	Select "IGRF" or "DGRF" from the drop-down list. If the survey is older than the last year that is a multiple of 5, select "DGRF". See the Application Notes below for further details. Script Parameter: IGRFPT.MODEL
Date required (yyyy/mm/dd)	Specify the required date using the format yyyy/mm/dd. Script Parameter: IGRFPT.DATE
IGRF model year (ignored for DGRF)	Select the model year. The default is "auto", which uses the most recent model year before current date. See the Application Notes below for further details. Script Parameter: IGRFPT.MODEL_YEAR
Longitude (-180 to 180)	Specify the longitude (-180 to 180 degrees). Script Parameter: IGRFPT.LONG
Latitude (-90 to 90)	Specify the latitude (-90 to 90 degrees). Script Parameter: IGRFPT.LAT
Elevation (meters)	Specify the elevation (in metres). Script Parameter: IGRFPT.ELEV
Field strength (nT)	Enter the calculated field strength (in nT). Script Parameter: IGRFPT.FIELD
Inclination (degrees)	Specify the calculated field inclination (degrees from horizon). Script Parameter: IGRFPT.INCL
Declination (degrees)	Enter calculated field declination (degrees from azimuth). Script Parameter: IGRFPT.DECL

Application Notes

Specify the location and date for the required model. Click on the Calculate button to calculate the GRF. The field strength, inclination, and declination will be calculated and displayed in the bottom three fields of the dialog and written to the script parameters. You may change the input parameters and recalculate the results. Click Close when you are done.

If the IGRF model year is left as the default value ("auto"), the most recent model year before the current date is used in the computations. You may also specify a model year. If a survey crosses the year end of an IGRF model year (every five years), there will be a discontinuity in the calculated values across the year end. In this case, specify the model year as the start of the first 5-year period. This will force the use of the specified model for the entire survey.

If the DGRF is requested for a date outside of known DGRF 5-year epochs, Oasis montaj will automatically default to IGRF and compute the IGRF value for the input location.

The models are defined in ASCII files named IGRF.DAT and DGRF.DAT, located in the Oasis montaj \etc folder. For the IGRF models, the model is interpolated to the survey date using the secular variation coefficients in the IGRF.DAT file. For the DGRF models, the secular coefficients are calculated by interpolation between model years, and the required model date values are determined using these coefficients.

The new coefficients are appended to the DGRF table every five years. For backward compatibility, they are retained in the IGRF table.

The Technical Note "Out-of-Cycle Update of the US/UK World Magnetic Model for 2015-2020" contained information about the development and source data for the World Magnetic Model 2015-2020 Out of Cycle update. The coefficients were needed for surveys conducted between years 2015-2020. When the 2020 DGRF coefficients were released, the Out-of-Cycle coefficients were no longer needed; however, for backward compatibility they are available in the IGRF table.
The latest WMM (World Magnetic Model) coefficients are available for download from ngdc.noaa.gov.