Provided by: gmt_4.5.11-1build1_amd64 bug


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


       grdtrend grdfile -Nn_model[r] [ -Ddiff.grd ] [ -Ttrend.grd ] [ -V ] [ -Wweight.grd ]


       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 -Nn_model, the number of model parameters to use; thus, -N4  fits  a
       bilinear trend, -N6 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.

              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.


       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

       -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.


       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.


       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.


       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 10r -T hawaii_trend.grd -W hawaii_weight.grd -V


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