1GRDREDPOL(1)                          GMT                         GRDREDPOL(1)
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NAME

6       grdredpol - Compute the Continuous Reduction To the Pole, AKA differen‐
7       tial RTP.
8

SYNOPSIS

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

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

36       anom_grd
37              The anomaly grid to be converted.
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39       -Grtp_grd
40              is the filename for output grdfile with the RTP solution
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OPTIONAL ARGUMENTS

43       -Cdec/dip
44              Use this (constant) declination and inclination angles for  both
45              field  and  magnetization. This option consists in the classical
46              RTP procedure.
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48       -Eiinc_grd -Eddec_grd
49              Get magnetization INCLINATION and DECLINATION from  these  grids
50              [default: use IGRF for each of the above parameters not provided
51              via grid].  Note that these two grids do not need  to  have  the
52              same resolution as the anomaly grid. They can be coarser.
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54       -Fm/n  The  filter  window  size  in  terms of row/columns. The default
55              value is 25x25.
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57       -Mm|r  Set boundary conditions. m|r  stands  for  mirror  or  replicate
58              edges (Default is zero padding).
59
60       -N     Do NOT use Taylor expansion.
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62       -Rwest/east/south/north
63              defines  the  Region  of  the  output  points. [Default: Same as
64              input.]
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66       -Tyear Decimal year used by the IGRF routine to compute the declination
67              and inclination at each point [default: 2000]
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69       -Wwidth
70              The size of the moving window in degrees [5].
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72       -Zfilter_grd
73              Write the filter file to disk.
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75       -V[level] (more ...)
76              Select verbosity level [c].
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78       -n[b|c|l|n][+a][+bBC][+c][+tthreshold] (more ...)
79              Select interpolation mode for grids.
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CONSEQUENCES OF GRID RESAMPLING

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

91       Suppose  that  anom.grd  is a file with the magnetic anomaly reduced to
92       the 2010 epoch and that the dec.grd and dip.grd contain the  magnetiza‐
93       tion declination and inclination respectively for an area that encloses
94       that of the anom.grd, compute the RTP using bins of  2  degrees  and  a
95       filter of 45 coefficients.
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97              gmt grdredpol anom.grd -Grtp.grd -W2 -F45/45 -T2010 -Edec.grd/dip.grd -V
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99       To  compute  the same RTP but now with the field and magnetization vec‐
100       tors collinear and computed from IGRF :
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102              gmt grdredpol anom.grd -Grtp.grd -W2 -F45/45 -T2010 -V
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REFERENCE

105       Luis, J.L. and Miranda, J.M. (2008), Reevaluation of magnetic chrons in
106       the  North Atlantic between 35N and 47N: Implications for the formation
107       of the Azores Triple Junction and associated plateau. JGR,  VOL.   113,
108       B10105, doi:10.1029/2007JB005573
109

111       2019, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe
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1165.4.5                            Feb 24, 2019                     GRDREDPOL(1)
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