.. index:: ! grdredpol .. include:: ../module_supplements_purpose.rst_ ********* grdredpol ********* |grdredpol_purpose| Synopsis -------- .. include:: ../../common_SYN_OPTs.rst_ **gmt grdredpol** *ingrid* |-G|\ *rtp_grd* [ |-C|\ *dec/dip* ] [ |-E|\ **i**\ *inc_grd* ] [ |-E|\ **d**\ *dec_grd* ] [ |-F|\ *m/n* ] [ |-M|\ **m**\|\ **r** ] [ |-N| ] [ |-W|\ *win_width* ] [ |SYN_OPT-V| ] [ |-T|\ *year* ] [ |-Z|\ *filtergrd* ] [ |SYN_OPT-V| ] [ |SYN_OPT-n| ] [ |SYN_OPT--| ] |No-spaces| 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 ------------------ .. |Add_ingrid| replace:: The anomaly grid to be converted. .. include:: /explain_grd_inout.rst_ :start-after: ingrid-syntax-begins :end-before: ingrid-syntax-ends .. _-G: **-G**\ *rtp_grd* is the filename for output grdfile with the RTP solution Optional Arguments ------------------ .. _-C: **-C**\ *dec/dip* Use this (constant) declination and inclination angles for both field and magnetization. This option consists in the classical RTP procedure. .. _-E: **-Ei**\ *inc_grd* **-Ed**\ *dec_grd* Get magnetization *INCLINATION* and *DECLINATION* from these grids [Default uses IGRF for each of the above parameters not provided via grid]. **Note**: These two grids do not need to have the same resolution as the anomaly grid. They can be coarser. .. _-F: **-F**\ *m/n* The filter window size in terms of row/columns [Default value is 25/25]. .. _-M: **-M**\ **m**\|\ **r** Set the boundary conditions. Directives **m**\|\ **r** stands for mirror or replicate edges [Default is zero padding]. .. _-N: **-N** Do **not** use Taylor expansion. .. _-R: **-R**\ *west*/*east*/*south*/*north* defines the Region of the output points [Default: Same as input]. .. _-T: **-T**\ *year* Decimal year used by the IGRF routine to compute the declination and inclination at each point [Default is 2000]. .. |Add_-V| replace:: |Add_-V_links| .. include:: /explain_-V.rst_ :start-after: **Syntax** :end-before: **Description** .. _-W: **-W**\ *width* The size of the moving window in degrees [5]. .. _-Z: **-Z**\ *filter_grd* Write the filter file to disk. .. include:: ../../explain_-n.rst_ .. include:: ../../explain_grdresample2.rst_ 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