Provided by: gmt_4.5.11-1build1_amd64

**NAME**

grdtrend - Fit and/or remove a polynomial trend in a grid file

**SYNOPSIS**

grdtrendgrdfile-Nn_model[r] [-Ddiff.grd] [-Ttrend.grd] [-V] [-Wweight.grd]

**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,-N4fits a bilinear trend,-N6a quadratic surface, and so on. Optionally, appendrto 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 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.grdfileThe name of a 2-D binary grid file.-N[r]n_modelsets the number of model parameters to fit. Appendrfor robust fit.

**OPTIONS**

No space between the option flag and the associated arguments.-DWrite the difference (input data - trend) to the filediff.grd.-TWrite the fitted trend to the filetrend.grd.-VSelects verbose mode, which will send progress reports to stderr [Default runs "silently"].-WIfweight.grdexists, 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.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-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. Seegrdmathif you need to evaluate the trend using the reported coefficients.

**GRID** **FILE** **FORMATS**

By defaultGMTwrites out grid as single precision floats in a COARDS-complaint netCDF file format. However,GMTis 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 2- or 4-byte integers. To specify the precision, scale and offset, the user should add the suffix=id[/scale/offset[/nan]], whereidis a two-letter identifier of the grid type and precision, andscaleandoffsetare optional scale factor and offset to be applied to all grid values, andnanis the value used to indicate missing data. When reading grids, the format is generally automatically recognized. If not, the same suffix can be added to input grid file names. Seegrdreformat(1) and Section 4.17 of the GMT Technical Reference and Cookbook for more information. When reading a netCDF file that contains multiple grids,GMTwill read, by default, the first 2-dimensional grid that can find in that file. To coaxGMTinto reading another multi-dimensional variable in the grid file, append?varnameto the file name, wherevarnameis 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?varnamesuffix can also be used for output grids to specify a variable name different from the default: "z". Seegrdreformat(1) and Section 4.18 of the GMT Technical Reference and Cookbook for more information, particularly on how to read splices of 3-, 4-, or 5-dimensional grids.

**EXAMPLES**

To remove a planar trend from hawaii_topo.grd and write result in hawaii_residual.grd:grdtrendhawaii_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:grdtrendhawaii_topo.grd-N10r-Thawaii_trend.grd-Whawaii_weight.grd-V

**SEE** **ALSO**

GMT(1),grdfft(1),grdfilter(1)