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

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

grdtrendreads a 2-D gridded 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 grd file.-Nn_model[r] sets the number of model parameters to fit. Appendrfor robust fit.

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.

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.

To remove a planar trend from hawaii_topo.grd and write result in hawaii_residual.grd, trygrdtrendhawaii_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, trygrdtrendhawaii_topo.grd-N10r-Thawaii_trend.grd-Whawaii_weight.grd-V

gmt, grdfft, grdfilter

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