# grdtrend (1) - Linux Man Pages

## grdtrend: Fit trend surface to grids and compute residuals

## NAME

grdtrend - Fit trend surface to grids and compute residuals## SYNOPSIS

**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.

## 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***3* 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.

## REQUIRED ARGUMENTS

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

## OPTIONAL ARGUMENTS

**-D***diff.nc*-
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. **-T***trend.nc*-
Write the fitted trend to the file
*trend.nc*. **-V**[*level*]*(more ...)*- Select verbosity level [c].
**-W***weight.nc*-
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.
**-****-version**- Print GMT version and exit.
**-****-show-datadir**- Print full path to GMT share directory and exit.

## 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 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.

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

## COPYRIGHT

2015, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe