GRDTREND
Section: Misc. Reference Manual Pages (l)
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NAME
grdtrend - Fit and/or remove a polynomial trend in a grd file
SYNOPSIS
grdtrend grdfile -Nn_model[r]
[ -Ddiff.grd ] [ -Ttrend.grd ] [ -V ]
[ -Wweight.grd ]
DESCRIPTION
grdtrend reads a 2-D gridded file and fits a low-order polynomial
trend to these data by [optionally weighted] least-squares. The trend surface is
defined by:
m1 + m2*x + m3*y + m4*x*y + m5*x*x + m6*y*y + m7*x*x*x + m8*x*x*y + m9*x*y*y + m10*y*y*y.
The user must specify -Nn_model, the number of model parameters to use;
thus, -N4 fits a bilinear trend, -N6 a quadratic surface,
and so on. Optionally, append r to the -N option to perform a
robust fit. In this case, the program will iteratively reweight the data based on
a robust scale estimate, in order to converge to a solution insensitive to outliers.
This may be handy when separating a "regional" field from a "residual" which should
have non-zero mean, such as a local mountain on a regional surface.
If data file has values set to NaN, these will be ignored during fitting; if output
files are written, these will also have NaN in the same locations.
No space between the option flag and the associated arguments.
- grdfile
-
The name of a 2-D binary grd file.
- -N
-
n_model[r] sets the number of model parameters to fit. Append r for robust fit.
OPTIONS
No space between the option flag and the associated arguments.
- -D
-
Write the difference (input data - trend) to the file diff.grd.
- -T
-
Write the fitted trend to the file trend.grd.
- -V
-
Selects verbose mode, which will send progress reports to stderr [Default runs "silently"].
- -W
-
If weight.grd exists, it will be read and used to solve a weighted
least-squares problem. [Default: Ordinary least-squares fit.] If the robust
option has been selected, the weights used in the robust fit will be written to
weight.grd.
REMARKS
The domain of x and y will be shifted and scaled to [-1, 1] and the basis functions are built from
Legendre polynomials. These have a numerical advantage in the form of the matrix which must be
inverted and allow more accurate solutions. NOTE: The model parameters listed with -V are
Legendre polynomial coefficients; they are not numerically equivalent to the m#s in the equation
described above. The description above is to allow the user to match -N with the order of the
polynomial surface.
EXAMPLES
To remove a planar trend from hawaii_topo.grd and write result in hawaii_residual.grd, try
grdtrend hawaii_topo.grd -N3 -Dhawaii_residual.grd
To do a robust fit of a bicubic surface to hawaii_topo.grd, writing the result in
hawaii_trend.grd and the weights used in hawaii_weight.grd, and reporting the progress, try
grdtrend hawaii_topo.grd -N10r -Thawaii_trend.grd
-Whawaii_weight.grd -V
SEE ALSO
gmt, grdfft, grdfilter
#include "refs.i"
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- OPTIONS
-
- REMARKS
-
- EXAMPLES
-
- SEE ALSO
-
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