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grdredpol(1)                          GMT                         grdredpol(1)




NAME

       grdredpol - Compute the Continuous Reduction To the Pole, AKA differen-
       tial 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  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_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 a|)
              Select verbosity level [c].

       -n[b|c|l|n][+a][+bBC][+c][+tthreshold] (more a|)
              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 +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  magnetiza-
       tion 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 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
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