grdredpol(1) GMT grdredpol(1)

## NAME

grdredpol - Compute the Continuous Reduction To the Pole, AKA differen- tial RTP.

## SYNOPSIS

grdredpolanom_grd-Grtp_grd[-Cdec/dip] [-Eiinc_grd] [-Eddec_grd] [-F<m/n>] [-Mm|r] [-N] [-Wwin_width] [-V[level] ] [-Tyear] [-Zfiltergrd] [-V[level] ] [-nflags ]Note:No space is allowed between the option flag and the associated arguments.

## DESCRIPTION

grdredpolwill take a.ncfile with a magnetic anomaly and compute the reduction to the pole (RTP) anomaly. This anomaly is the one that would have been produce if the bodies were magnetized vertically and the anomalies were observed at the geomagnetic pole. Standard RTP procedure assumes the direction of magnetization to be uniform throughout the causative body, and the geomagnetic field to be uniform in direction throughout the study region. Although these assumptions are reasonable for small areas, they do not hold for large areas. In the method used here computations are carried out in both the fre- quency and the space domains. The idea is that a large area may be decomposed in small size windows where both the ambient field and the magnetization vector change by a very small amount. Inside each of those windows, or bins, a set of filter coefficients are calculate and reconstruct for each individual point the component filter using a first order Taylor series expansion.

## REQUIRED ARGUMENTS

anom_grdThe anomaly grid to be converted.-Grtp_grdis the filename for output grdfile with the RTP solution

## OPTIONAL ARGUMENTS

-Cdec/dipUse this (constant) declination and inclination angles for both field and magnetization. This option consists in the classical RTP procedure.-Eiinc_grd-Eddec_grdGet magnetizationINCLINATIONandDECLINATIONfrom these grids [default: use IGRF for each of the above parameters not provided via grid]. Note that these two grids do not need to have the same resolution as the anomaly grid. They can be coarser.-Fm/nThe filter window size in terms of row/columns. The default value is 25x25.-Mm|rSet boundary conditions. m|r stands for mirror or replicate edges (Default is zero padding).-NDo NOT use Taylor expansion.-Rwest/east/south/northdefines the Region of the output points. [Default: Same as input.]-TyearDecimal year used by the IGRF routine to compute the declination and inclination at each point [default: 2000]-WwidthThe size of the moving window in degrees [5].-Zfilter_grdWrite the filter file to disk.-V[level] (morea|) Select verbosity level [c].-n[b|c|l|n][+a][+bBC][+c][+tthreshold] (morea|) Select interpolation mode for grids.

## CONSEQUENCES OF GRID RESAMPLING

Resample or sampling of grids will use various algorithms (see-n) that may lead to possible distortions or unexpected results in the resampled values. One expected effect of resampling with splines is the tendency for the new resampled values to slightly exceed the global min/max lim- its of the original grid. If this is unacceptable, you can impose clipping of the resampled values values so they do not exceed the input min/max values by adding+cto your-noption.

## EXAMPLES

Suppose thatanom.grdis a file with the magnetic anomaly reduced to the 2010 epoch and that thedec.grdanddip.grdcontain the magnetiza- tion declination and inclination respectively for an area that encloses that of theanom.grd, compute theRTPusing bins of 2 degrees and a filter of 45 coefficients. gmt grdredpol anom.grd -Grtp.grd -W2 -F45/45 -T2010 -Edec.grd/dip.grd -V To compute the sameRTPbut now with the field and magnetization vec- tors collinear and computed from IGRF : gmt grdredpol anom.grd -Grtp.grd -W2 -F45/45 -T2010 -V

## REFERENCE

Luis, J.L. and Miranda, J.M. (2008), Reevaluation of magnetic chrons in the North Atlantic between 35N and 47N: Implications for the formation of the Azores Triple Junction and associated plateau.JGR, VOL.113, B10105, doi:10.1029/2007JB005573

## COPYRIGHT

2017, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe 5.4.2 Jun 24, 2017 grdredpol(1)

gmt5 5.4.2 - Generated Thu Jun 29 08:07:28 CDT 2017