grdtrend(1) GMT grdtrend(1)
NAME
grdtrend - Fit trend surface to grids and compute residuals
SYNOPSIS
grdtrend grdfile -Nn_model[+r] [ -Ddiff.nc ] [ -Rregion ] [
-Ttrend.nc ] [ -Wweight.nc ]
Note: No space is allowed between the option flag and the associated
arguments.
DESCRIPTION
grdtrend reads a 2-D grid 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, -N3 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 out-
liers. This may be handy when separating a aregionala field from a
aresiduala 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 fit-
ting; if output files are written, these will also have NaN in the same
locations.
REQUIRED ARGUMENTS
grdfile
The name of a 2-D binary grid file.
-Nn_model[+r]
n_model sets the number of model parameters to fit. Append +r
for robust fit.
OPTIONAL ARGUMENTS
-Ddiff.nc
Write the difference (input data - trend) to the file diff.nc.
-Rxmin/xmax/ymin/ymax[+r][+uunit] (more a|)
Specify the region of interest. Using the -R option will select
a subsection of the input grid. If this subsection exceeds the
boundaries of the grid, only the common region will be
extracted.
-Ttrend.nc
Write the fitted trend to the file trend.nc.
-V[level] (more a|)
Select verbosity level [c].
-Wweight.nc
If weight.nc 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.nc.
-^ or just -
Print a short message about the syntax of the command, then
exits (NOTE: on Windows just use -).
-+ or just +
Print an extensive usage (help) message, including the explana-
tion of any module-specific option (but not the GMT common
options), then exits.
-? or no arguments
Print a complete usage (help) message, including the explanation
of all options, then exits.
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. See grdmath if you need to evaluate the trend using the
reported coefficients.
GRID FILE FORMATS
By default GMT writes out grid as single precision floats in a
COARDS-complaint netCDF file format. However, GMT is able to produce
grid files in many other commonly used grid file formats and also
facilitates so called apackinga of grids, writing out floating point
data as 1- or 2-byte integers. (more a|)
EXAMPLES
To remove a planar trend from hawaii_topo.nc and write result in
hawaii_residual.nc:
gmt grdtrend hawaii_topo.nc -N3 -Dhawaii_residual.nc
To do a robust fit of a bicubic surface to hawaii_topo.nc, writing the
result in hawaii_trend.nc and the weights used in hawaii_weight.nc, and
reporting the progress:
gmt grdtrend hawaii_topo.nc -N10+r -Thawaii_trend.nc -Whawaii_weight.nc -V
SEE ALSO
gmt(1), grdfft(1), grdfilter(1), grdmath(1)
COPYRIGHT
2017, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe
5.4.2 Jun 24, 2017 grdtrend(1)
gmt5 5.4.2 - Generated Thu Jun 29 08:31:07 CDT 2017