GRDREDPOL(1gmt) | GMT | GRDREDPOL(1gmt) |

# NAME¶

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

# SYNOPSIS¶

**grdredpol** *anom_grd* **-G***rtp_grd* [
**-C***dec/dip*] [ **-E****i***inc_grd*] [
**-E****d***dec_grd*] [ **-F***<m/n>*] [
**-M***m|r*] [ **-N** ] [ **-W***win_width*] [
**-V**[*level*] ] [ **-T***year* ] [
**-Z***filtergrd* ] [ **-V**[*level*] ] [ **-n**flags
]

**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.

**-G***rtp_grd*- is the filename for output grdfile with the RTP solution

# OPTIONAL ARGUMENTS¶

**-C***dec/dip*- Use this (constant) declination and inclination angles for both field and magnetization. This option consists in the classical RTP procedure.

**-Ei***inc_grd***-Ed***dec_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.

**-F***m/n*- The filter window size in terms of row/columns. The default value is 25x25.

**-M***m|r*- Set boundary conditions. m|r stands for mirror or replicate edges (Default is zero padding).

**-N**- Do NOT use Taylor expansion.

**-R***west*/*east*/*south*/*north*- defines the Region of the output points. [Default: Same as input.]

**-T***year*- Decimal year used by the IGRF routine to compute the declination and inclination at each point [default: 2000]

**-W***width*- The size of the moving window in degrees [5].

**-Z***filter_grd*- Write the filter file to disk.

**-V**[*level*] (more ...)- Select verbosity level [c].

**-n**[**b**|**c**|**l**|**n**][**+a**][**+b***BC*][**+c**][**+t***threshold*] (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

May 21, 2019 | 5.4.5 |