NAME

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

SYNOPSIS

       grdredpol anom_grd rtp_grd [dec/dip] [ dec_grd/dipgrd] [ <m/n>] [ m|r] [  ] [ win_width] [
       [level] ] [ year ] [ filtergrd ] [ [level] ] [ -n<flags> ]

       Note: No space is allowed between the option flag and the associated arguments.

DESCRIPTION

       grdredpol will take a .nc file 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 frequency 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_grd
              The anomaly grid to be converted.

       -Grtp_grd
              is the filename for output grdfile with the RTP solution

OPTIONAL ARGUMENTS

       -Cdec/dip
              Use  this  (constant)  declination  and  inclination  angles  for  both  field  and
              magnetization. This option consists in the classical RTP procedure.

       -Edip_grd/dec_grd
              Get  magnetization DIP & DEC from these grids [default: use IGRF].  Note that these
              two grids do not need to have the same resolution as the anomaly grid. They can  be
              coarser.

       -Fm/n  The filter window size in terms of row/columns. The default value is 25x25.

       -Mm|r  Set  boundary conditions. m|r stands for mirror or replicate edges (Default is zero
              padding).

       -N     Do NOT use Taylor expansion.

       -Rwest/east/south/north
              defines the Region of the output points. [Default: Same as input.]

       -Tyear Decimal year used by the IGRF routine to compute the declination and inclination at
              each point [default: 2000]

       -Wwidth
              The size of the moving window in degrees [5].

       -Zfilter_grd
              Write the filter file to disk.

       -V[level] (more ...)
              Select verbosity level [c].

       -n[b|c|l|n][+a][+bBC][+c][+tthreshold] (more ...)
              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 limits 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 +c to your -n option.

EXAMPLES

       Suppose that anom.grd is a file with the magnetic anomaly reduced to the  2010  epoch  and
       that  the  dec.grd  and  dip.grd  contain  the  magnetization  declination and inclination
       respectively for an area that encloses that of the anom.grd, compute the RTP using 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  same  RTP but now with the field and magnetization vectors 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

       2015, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe