1GRDTREND(1) GMT GRDTREND(1)
2
6 grdtrend - Fit trend surface to grids and compute residuals
7
9 grdtrend grdfile -Nn_model[+r] [ -Ddiff.nc ] [ -Rregion ] [
10 -Ttrend.nc ] [ -Wweight.nc ]
11 Note: No space is allowed between the option flag and the associated
13 arguments.
14
16 grdtrend reads a 2-D grid file and fits a low-order polynomial trend to
17 these data by [optionally weighted] least-squares. The trend surface is
18 defined by:
19 m1 + m2*x + m3*y + m4*x*y + m5*x*x + m6*y*y + m7*x*x*x + m8*x*x*y +
20 m9*x*y*y + m10*y*y*y.
21 The user must specify -Nn_model, the number of model parameters to use;
23 thus, -N3 fits a bilinear trend, -N6 a quadratic surface, and so on.
24 Optionally, append +r to the -N option to perform a robust fit. In this
25 case, the program will iteratively reweight the data based on a robust
26 scale estimate, in order to converge to a solution insensitive to out‐
27 liers. This may be handy when separating a "regional" field from a
28 "residual" which should have non-zero mean, such as a local mountain on
29 a regional surface.
30 If data file has values set to NaN, these will be ignored during fit‐
32 ting; if output files are written, these will also have NaN in the same
33 locations.
34
36 grdfile
37 The name of a 2-D binary grid file.
38 -Nn_model[+r]
40 n_model sets the number of model parameters to fit. Append +r
41 for robust fit.
42
44 -Ddiff.nc
45 Write the difference (input data - trend) to the file diff.nc.
46 -Rxmin/xmax/ymin/ymax[+r][+uunit] (more ...)
48 Specify the region of interest. Using the -R option will select
49 a subsection of the input grid. If this subsection exceeds the
50 boundaries of the grid, only the common region will be
51 extracted.
52 -Ttrend.nc
54 Write the fitted trend to the file trend.nc.
55 -V[level] (more ...)
57 Select verbosity level [c].
58 -Wweight.nc
60 If weight.nc exists, it will be read and used to solve a
61 weighted least-squares problem. [Default: Ordinary least-squares
62 fit.] If the robust option has been selected, the weights used
63 in the robust fit will be written to weight.nc.
64 -^ or just -
66 Print a short message about the syntax of the command, then
67 exits (NOTE: on Windows just use -).
68 -+ or just +
70 Print an extensive usage (help) message, including the explana‐
71 tion of any module-specific option (but not the GMT common
72 options), then exits.
73 -? or no arguments
75 Print a complete usage (help) message, including the explanation
76 of all options, then exits.
77
79 The domain of x and y will be shifted and scaled to [-1, 1] and the
80 basis functions are built from Legendre polynomials. These have a
81 numerical advantage in the form of the matrix which must be inverted
82 and allow more accurate solutions. NOTE: The model parameters listed
83 with -V are Legendre polynomial coefficients; they are not numerically
84 equivalent to the m#s in the equation described above. The description
85 above is to allow the user to match -N with the order of the polynomial
86 surface. See grdmath if you need to evaluate the trend using the
87 reported coefficients.
88
90 By default GMT writes out grid as single precision floats in a
91 COARDS-complaint netCDF file format. However, GMT is able to produce
92 grid files in many other commonly used grid file formats and also
93 facilitates so called "packing" of grids, writing out floating point
94 data as 1- or 2-byte integers. (more ...)
95
97 To remove a planar trend from hawaii_topo.nc and write result in
98 hawaii_residual.nc:
99 gmt grdtrend hawaii_topo.nc -N3 -Dhawaii_residual.nc
101 To do a robust fit of a bicubic surface to hawaii_topo.nc, writing the
103 result in hawaii_trend.nc and the weights used in hawaii_weight.nc, and
104 reporting the progress:
105 gmt grdtrend hawaii_topo.nc -N10+r -Thawaii_trend.nc -Whawaii_weight.nc -V
107
109 gmt, grdfft, grdfilter, grdmath
110
112 2019, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe
1135.4.5 Feb 24, 2019 GRDTREND(1)