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

       2018, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe

5.4.3                                             Jan 03, 2018                                   GRDREDPOL(1gmt)