grdtrend man page

grdtrend — Fit trend surface to grids and compute residuals


grdtrend grdfile n_model[r] [ diff.nc ] [ region ] [ trend.nc ] [ weight.nc ]

Note: No space is allowed between the option flag and the associated arguments.


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 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.

Required Arguments

The name of a 2-D binary grid file.
n_model sets the number of model parameters to fit. Append r for robust fit.

Optional Arguments

Write the difference (input data - trend) to the file diff.nc.
-R[unit]xmin/xmax/ymin/ymax[r] (more ...)
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.
Write the fitted trend to the file trend.nc.
-V[level] (more ...)
Select verbosity level [c].
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 use just -).
-+ or just +
Print an extensive usage (help) message, including the explanation 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 options, then exits.
Print GMT version and exit.
Print full path to GMT share directory and exit.


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 "packing" of grids, writing out floating point data as 1- or 2-byte integers. To specify the precision, scale and offset, the user should add the suffix =id[/scale/offset[/nan]], where id is a two-letter identifier of the grid type and precision, and scale and offset are optional scale factor and offset to be applied to all grid values, and nan is the value used to indicate missing data. In case the two characters id is not provided, as in =/scale than a id=nf is assumed. When reading grids, the format is generally automatically recognized. If not, the same suffix can be added to input grid file names. See grdconvert and Section grid-file-format of the GMT Technical Reference and Cookbook for more information.

When reading a netCDF file that contains multiple grids, GMT will read, by default, the first 2-dimensional grid that can find in that file. To coax GMT into reading another multi-dimensional variable in the grid file, append ?varname to the file name, where varname is the name of the variable. Note that you may need to escape the special meaning of ? in your shell program by putting a backslash in front of it, or by placing the filename and suffix between quotes or double quotes. The ?varname suffix can also be used for output grids to specify a variable name different from the default: "z". See grdconvert and Sections modifiers-for-CF and grid-file-format of the GMT Technical Reference and Cookbook for more information, particularly on how to read splices of 3-, 4-, or 5-dimensional grids.


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 -N10r -Thawaii_trend.nc -Whawaii_weight.nc -V

See Also

gmt, grdfft, grdfilter, grdmath


GMT 5.2.1 November 12, 2015