xenial (1) grdfilter.1gmt.gz

Provided by: gmt-common_5.2.1+dfsg-3build1_all bug

NAME

       grdfilter - Filter a grid in the space (or time) domain

SYNOPSIS

       grdfilter ingrid distance_flag xwidth[/width2][modifiers] outgrid [ increment ] [ i|p|r ] [ region ] [  ]
       [ [level] ] [ -f<flags> ]

       Note: No space is allowed between the option flag and the associated arguments.

DESCRIPTION

       grdfilter will filter a grid  file  in  the  time  domain  using  one  of  the  selected  convolution  or
       non-convolution  isotropic  or  rectangular  filters  and  compute distances using Cartesian or Spherical
       geometries.  The output grid file can optionally be generated as a  sub-region  of  the  input  (via  -R)
       and/or  with  new increment (via -I) or registration (via -T). In this way, one may have "extra space" in
       the input data so that the edges will not be used and the output can be  within  one  half-width  of  the
       input edges. If the filter is low-pass, then the output may be less frequently sampled than the input.

REQUIRED ARGUMENTS

       ingrid The grid file of points to be filtered. (See GRID FILE FORMATS below).

       -Ddistance_flag
              Distance flag tells how grid (x,y) relates to filter width as follows:

              flag = p: grid (px,py) with width an odd number of pixels; Cartesian distances.

              flag = 0: grid (x,y) same units as width, Cartesian distances.

              flag = 1: grid (x,y) in degrees, width in kilometers, Cartesian distances.

              flag = 2: grid (x,y) in degrees, width in km, dx scaled by cos(middle y), Cartesian distances.

              The  above options are fastest because they allow weight matrix to be computed only once. The next
              three options are slower because they recompute weights for each latitude.

              flag = 3: grid (x,y) in  degrees,  width  in  km,  dx  scaled  by  cosine(y),  Cartesian  distance
              calculation.

              flag = 4: grid (x,y) in degrees, width in km, Spherical distance calculation.

              flag = 5: grid (x,y) in Mercator -Jm1 img units, width in km, Spherical distance calculation.

       -Fxwidth[/width2][modifiers]
              Sets  the filter type. Choose among convolution and non-convolution filters. Use any filter code x
              (listed below) followed by the full diameter width. This gives an isotropic filter; append /width2
              for  a  rectangular  filter  (requires -Dp or -D0).  Some filters allow for optional arguments and
              modifiers.

              Convolution filters (and their codes) are:

              (b) Boxcar: All weights are equal.

              (c) Cosine Arch: Weights follow a cosine arch curve.

              (g) Gaussian: Weights are given by the Gaussian function, where width is 6 times the  conventional
              Gaussian sigma.

              (f)  Custom:  Weights  are  given by the precomputed values in the filter weight grid file weight,
              which must have odd dimensions; also requires -D0 and output spacing must match input  spacing  or
              be integer multiples.

              (o)  Operator:  Weights are given by the precomputed values in the filter weight grid file weight,
              which must have odd dimensions; also requires -D0 and output spacing must match input  spacing  or
              be  integer  multiples.  Weights  are assumed to sum to zero so no accumulation of weight sums and
              normalization will be done.

              Non-convolution filters (and their codes) are:

              (m) Median: Returns median value. To select another quantile append +qquantile in  the  0-1  range
              [Default is 0.5, i.e., median].

              (p)  Maximum  likelihood probability (a mode estimator): Return modal value. If more than one mode
              is found we return their average value. Append +l or +u if you rather want to return the lowermost
              or uppermost of the modal values.

              (h)  Histogram mode (another mode estimator): Return the modal value as the center of the dominant
              peak in a histogram. Append /binwidth to specify the binning interval.  Use modifier +c to  center
              the bins on multiples of binwidth [Default has bin edges that are multiples of binwidth].  If more
              than one mode is found we return their average value. Append +l or +u if you rather want to return
              the lowermost or uppermost of the modal values.

              (l) Lower: Return the minimum of all values.

              (L) Lower: Return minimum of all positive values only.

              (u) Upper: Return maximum of all values.

              (U) Upper: Return maximum or all negative values only.

              In  the  case  of  L|U  it is possible that no data passes the initial sign test; in that case the
              filter will return NaN.

       -Goutgrid
              outgrid is the output grid file of the filter. (See GRID FILE FORMATS below).

OPTIONAL ARGUMENTS

       -Ixinc[unit][=|+][/yinc[unit][=|+]]
              x_inc [and optionally y_inc] is the output Increment. Append m to indicate arc minutes,  or  s  to
              indicate  arc  seconds.  If the new x_inc, y_inc are NOT integer multiples of the old ones (in the
              input data), filtering will be considerably slower.  [Default: Same as input.]

       -Ni|p|r
              Determine how NaN-values in the input grid affects the filtered output: Append  i  to  ignore  all
              NaNs  in  the calculation of filtered value [Default], r is same as i except if the input node was
              NaN then the output node will be set to NaN (only applies if both grids are co-registered), and  p
              which  will  force the filtered value to be NaN if any grid-nodes with NaN-values are found inside
              the filter circle.

       -R     west, east, south, and north defines the Region of the output points. [Default: Same as input.]

       -T     Toggle the node registration for the output grid so as to become the opposite of  the  input  grid
              [Default gives the same registration as the input grid].

       -V[level] (more ...)
              Select verbosity level [c].

       -f[i|o]colinfo (more ...)
              Specify data types of input and/or output columns.

       -^ 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.

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.

GEOGRAPHICAL AND TIME COORDINATES

       When the output grid type is netCDF, the coordinates will be labeled "longitude", "latitude",  or  "time"
       based  on the attributes of the input data or grid (if any) or on the -f or -R options. For example, both
       -f0x -f1t and -R90w/90e/0t/3t will result in a longitude/time grid. When the x, y,  or  z  coordinate  is
       time, it will be stored in the grid as relative time since epoch as specified by TIME_UNIT and TIME_EPOCH
       in the gmt.conf file or on the command line. In addition, the unit attribute of the  time  variable  will
       indicate both this unit and epoch.

EXAMPLES

       Suppose  that  north_pacific_etopo5.nc  is a file of 5 minute bathymetry from 140E to 260E and 0N to 50N,
       and you want to find the medians of values within a 300km radius (600km full width) of the output points,
       which you choose to be from 150E to 250E and 10N to 40N, and you want the output values every 0.5 degree.
       Using spherical distance calculations, you need:

              gmt grdfilter north_pacific_etopo5.nc -Gfiltered_pacific.nc -Fm600 \
                            -D4 -R150/250/10/40 -I0.5 -V

       If we instead wanted a high-pass result then one can perform the corresponding low-pass  filter  using  a
       coarse grid interval as grdfilter will resample the result to the same resolution as the input grid so we
       can compute the residuals, e.g.,

              gmt grdfilter north_pacific_etopo5.nc -Gresidual_pacific.nc -Fm-600 \
                            -D4 -R150/250/10/40 -I0.5 -V

       Here, the residual_pacific.nc grid will have the same 5 minute resolution as the original.

       To filter the dataset in ripples.nc using a custom  anisotropic  Gaussian  filter  exp  (-0.5*r^2)  whose
       distances  r  from  the center is given by (2x^2 + y^2 -2xy)/6, with major axis at an angle of 63 degrees
       with the horizontal, try

              gmt grdmath -R-10/10/-10/10 -I1 X 2 POW 2 MUL Y 2 POW ADD X Y MUL 2 MUL \
                          SUB 6 DIV NEG 2 DIV EXP DUP SUM DIV = gfilter.nc
              gmt grdfilter ripples.nc -Ffgfilter.nc -D0 -Gsmooth.nc -V

LIMITATIONS

       To use the -D5 option the input Mercator grid must be created by img2mercgrd using the -C option  so  the
       origin of the y-values is the Equator (i.e., x = y = 0 correspond to lon = lat = 0).

SEE ALSO

       gmt, grdfft img2grd

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