IGRF at a Point

Use the IGRF > IGRF at a Point 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 

IGRF at a Point dialog options

Date (yyyy/mm/dd)

Specify the required date using the format yyyy/mm/dd.

Script Parameter: IGRFPT.DATE

Position format

The default position format (“Normal”) for the Latitude/Longitude coordinates is decimal degrees. Use the drop-down list to select a geographic format: the Latitude and Longitude parameters below will redisplay the coordinates based on the selected format.

The Latitude/Longitude coordinates are displayed in the user specified format, but they will be converted to decimal degrees for processing.

For a list of supported "Geo" formats, check the topic Data Types and Database Formats.

Script Parameter: IGRFPT.POSITION_FORMAT

Longitude

Specify the longitude using one of the options below:

  • Input a numeric value within ±180°.
  • Click on Locate to interactively select a point location on your active map/grid: the Longitude field will auto-update, and the new coordinate value will be displayed based on the current Position format. (See the Application Notes below for further details.)

Script Parameter: IGRFPT.LONG

Latitude

Specify the latitude using one of the options below:

  • Input a numeric value within ±90°.
  • Click on Locate to interactively select a point location on your active map/grid. The Latitude field will auto-update, and the new coordinate will be displayed based on the Position format selection. (See the Application Notes below for further details.)

Script Parameter: IGRFPT.LAT

Elevation (m)

Specify the elevation in metres (0 for sea level).

Script Parameter: IGRFPT.ELEV

Output

[Calculate]

Click on the Calculate button to calculate and populate the IGRF parameters below. The field strength, inclination, and declination will be calculated based on the input values and will be displayed in the bottom three fields of the dialog. You may change the input parameters and recalculate the results. Click Close when you are done.

Total magnetic internsity (nT)

Displays the calculated field strength (in nT).

Script Parameter: IGRFPT.FIELD

Inclination (°)

Displays the calculated field inclination (degrees from horizon).

Script Parameter: IGRFPT.INCL

Declination (°)

Displays the calculated field declination (degrees from azimuth).

Script Parameter: IGRFPT.DECL

Application Notes

*The GX.NET tools are embedded in the geogxnet.dll file located in the "...\Geosoft\Desktop Applications \bin" folder. If running this GX interactively, bypassing the menu, first change the folder to point to the "bin" directory, then supply the GX.NET tool in the specified format. See the topic Run GX for more details on running a GX.NET interactively.

Locate Point

Using the interactive button (Locate) will move the focus to the current map or grid. Click on the map to pick a point:

  • If the map/grid has a projection attached, the ground coordinates are converted to Lat/Long and the Latitude & Longitude fields are updated with these values. They will be displayed using the format defined in Position format.
  • If the map/grid does not have a projection assigned/it is not georeferenced, a prompt will be displayed. You have two options:
    1. Assume geographic units: the Latitude /Longitude fields will auto-update with the coordinates located on the map/grid.
    2. Return to the main dialog without updating the Latitude /Longitude fields.

IGRF vs DGRF

The International Geomagnetic Reference Field (IGRF) is the empirical representation of Earth’s magnetic field as a function of time and in the absence of any crustal or external sources. The model employs the spherical harmonics expansion of the scalar potential in geocentric coordinates. The IGRF model coefficients are based on all available data sources including geomagnetic measurements from observatories, ships, aircrafts and satellites. For further details see ccmc.gsfc.nasa.gov.

The IGRF model coefficients are reviewed and adjusted over time. The coefficients sets, for which no further changes are anticipated, are known as the Definitive Geomagnetic Reference Field (DGRF), and they should be used when the survey is older than the last year that is a multiple of 5. The IGRF coefficient set covering the period from the last x5 year to now are a best guess projecting forward in time and subject to adjustment.

Spherical Harmonic Coefficients in Oasis montaj

The models are defined in the ASCII files named DGRF.DAT and IGRF.DAT, located in the Oasis montaj \etc folder. 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. For the IGRF models, the model is interpolated to the survey date using the secular variation coefficients in the IGRF.DAT file.

Table

Description

DGRF.dat

Contains all the definitive coefficients for every five years since 1945, up to the latest year that was a multiple of 5. This file is appended with a single set of the most recent IGRF coefficients that are subject to change over time. Every five years, the last set (IGRF) is removed, and the latest Definitive coefficient set is added. Then the new most recent IGRF coefficient, covering the time from the last x5 year to now, is appended.

IGRF.dat

Contains all the published forward projected coefficients since 1945. In practice we should not need any, but the last set of IGRF coefficients. We save them, however, for backward compatibility and to be able to reproduce georeference field values previously produced.

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.