IGRF Channel

Use the IGRF Channel option (IGRF GX) to compute the IGRF (International Geomagnetic Reference Field) or DGRF (Definitive Geomagnetic Reference Field) strength, inclination and declination from given longitude, latitude and optional elevation channels.

The option is available with the following extensions and menus:

• Geophysics: IGRF > IGRF Channel
• Geophysics Leveling: Tie Line Levelling > Levelling Corrections > IGRF Channel
• UAV Merge Sorties: UAV Merge Sorties > Pre-processing > Remove IGRF

Compute IGRF Channels dialog options

Application Notes

In source-free regions at the Earth's surface and above, the main field, with sources internal to the Earth, is the negative gradient of a scalar potential V which is represented by a 13^th degree truncated series expansion (see International Geomagnetic Reference Field & International Geomagnetic Reference Field: the 12th generation):

Where:

A: Earth’s conventional mean reference spherical radius  6371.2 Km

N=13

r: radial distance at the designated longitude/latitude

θ: geocentric colatitude

φ: East longitude

 t: date

Pnm: Schmidt quasi-normalized associated Legendre function of degree n & order m

gnm & hnm: Gauss coefficients as function of time in units of nanotesla published by NOAA on 5 year increments

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 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.

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.

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.

If a date channel is selected, only the first date of each line in the database is the date used for the IGRF calculation. If the date changes within one database line, it is either due to using UTS time that occasionally may straddle over midnight, or because two line segments flown at different times may have been merged together. In the former case, the effect of straddling over midnight has virtually no effect on the IGRF calculation, as the IGRF field changes very slowly with time. In the latter case, you should break down the line into two lines using the date as the line break indicator.
In essence, two lines surveyed on different dates should not be merged into a single line even if they do align.

If you want to use an elevation channel, specify the channel name. Otherwise, the system assumes that the string in this parameter is a single real number value, and it uses the elevation as a constant in computing the IGRF channels. If the string cannot be converted into a valid number, the GX will not continue.