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# grdtrend

Langue: *en*

Version: *369079 (fedora - 01/12/10)*

Section: *1 (Commandes utilisateur)*

## NAME

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

**grdtrend**

*grdfile*

**-N**

*n_model*[

**r**] [

**-D**

*diff.grd*] [

**-T**

*trend.grd*] [

**-V**] [

**-W**

*weight.grd*]

## DESCRIPTION

**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 **-N***n_model*, the number of model parameters to use; thus, **-N***4* fits a bilinear trend, **-N***6* 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.

No space between the option flag and the associated arguments.

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

## OPTIONS

No space between the option flag and the associated arguments.**-D**- Write the difference (input data - trend) to the file
*diff.grd*. **-T**- Write the fitted trend to the file
*trend.grd*. **-V**- Selects verbose mode, which will send progress reports to stderr [Default runs "silently"].
**-W**- If
*weight.grd*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.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**-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 2- or 4-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. When reading grids, the format is generally automatically recognized. If not, the same suffix can be added to input grid file names. See

**grdreformat**(1) and Section 4.17 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

**grdreformat**(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:**grdtrend** hawaii_topo.grd **-N**3 **-D**hawaii_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:

**grdtrend** hawaii_topo.grd **-N**10**r** **-T**hawaii_trend.grd **-W**hawaii_weight.grd **-V**

## SEE ALSO

*GMT*(1),

*grdfft*(1),

*grdfilter*(1)

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