Reduce to Magnetic Pole (REDP)

Use the Reduce to Magnetic Pole option to apply the reduction to magnetic pole filter.

Reduce to Magnetic Pole dialog options

Survey date (YYYY/MM/DD)	Enter the date of the survey. By default this field is populated with the grid creation date. If you modify the date, you must click on the Calculate button located to the right of this entry to recalculate the IGRF field and refresh the Inclination and Declination entries. When clicking OK, only the Inclination and Declination values are saved to the control file; the date is not of essence and it is not saved. As a result, during subsequent invocations of the same filter, the latest inclination and declination are loaded from the control file, and the date reverts back to the grid creation date. This should not cause an issue, because once the inclination and declination are defined, the date becomes inconsequential.
Inclination	Geomagnetic inclination I in degrees from the horizon. This parameter is automatically calculated and can be overridden.
Declination	Geomagnetic declination D in degrees azimuth. This parameter is automatically calculated and can be overridden.
Amplitude correction inclination	Amplitude correction inclination Ia in degrees (see Application Notes below). This parameter is automatically calculated as a function of the Inclination. If you change the Inclination, you must click on the Calculate button to the right of this entry to recalculate the Amplitude correction inclination.

Application Notes

The IGRF parameters of an input magnetic grid are a function of its geolocation and survey date. If the grid does not have a coordinate system assigned, you will be prompted to define it. Once calculated, these parameters are added to the workspace and called by some MAGMAP filters. Specifically, the IGRF field strength is required for calculating the apparent susceptibility and the IGRF field direction is used in Reduction to the Pole. The field strength, inclination, and declination parameters are calculated together to be synchronized.

Reduction to Pole has an amplitude component (sin(I)) and a phase component (i cos(I) cos(D-θ)). When reducing to pole from near-equatorial latitudes, in the vicinity of the declination azimuth, the filter becomes unstable, producing strongly dominating trends that would obstruct real features. The problem is attenuated by specifying a modulating inclination angle I_a. This is at the expense of under-correcting the amplitudes of features along the declination direction.

In near-equatorial latitudes, to minimize the amplitude distortion, the amplitude correction inclination should be set relative to the inclination. It should be set large enough to counteract the instability and small enough to minimize the amplitude distortion.

Reduce to Magnetic Pole equation

if (| Ia | < | I |), Ia = I

Where:

I	Geomagnetic inclination in °
D	Geomagnetic declination in ° azimuth from north
Ia	Inclination for amplitude correction (never less than I) ->