grdtrend(1) GMT grdtrend(1)

## NAME

grdtrend - Fit trend surface to grids and compute residuals

## SYNOPSIS

grdtrendgrdfile-Nn_model[+r] [-Ddiff.nc] [-Rregion] [-Ttrend.nc] [-Wweight.nc]Note:No space is allowed between the option flag and the associated arguments.

## DESCRIPTION

grdtrendreads 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,-N3fits a bilinear trend,-N6a quadratic surface, and so on. Optionally, append+rto the-Noption 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 aaregionalafield from aaresidualawhich 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

grdfileThe name of a 2-D binary grid file.-Nn_model[+r]n_modelsets the number of model parameters to fit. Append+rfor robust fit.

## OPTIONAL ARGUMENTS

-Ddiff.ncWrite the difference (input data - trend) to the filediff.nc.-Rxmin/xmax/ymin/ymax[+r][+uunit] (morea|) Specify the region of interest. Using the-Roption 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.ncWrite the fitted trend to the filetrend.nc.-V[level] (morea|) Select verbosity level [c].-Wweight.ncIfweight.ncexists, 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 toweight.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-Vare 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-Nwith 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 calledapackingaof grids, writing out floating point data as 1- or 2-byte integers. (morea|)

## 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