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