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NAME

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

SYNOPSIS

       grdredpol  anom_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

       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.

       -Eiinc_grd -Eddec_grd
              Get magnetization INCLINATION and DECLINATION from 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/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

       2019, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe