NAME

       sph2grd - Compute grid from spherical harmonic coefficients

SYNOPSIS

       sph2grd [ table ]  -Ggrdfile
        -Iincrement
        -Rregion  [   -D[g|n]  ]  [  -E ] [  -F[k]filter ] [  -N[norm] ] [  -Q ] [  -V[level] ] [
       -bibinary ] [ -hheaders ] [ -iflags ] [ -r ] [ -x[[-]n] ]

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

DESCRIPTION

       sph2grd reads a spherical harmonics coefficient table with records of L, M, C[L,M], S[L,M]
       and evaluates the spherical harmonic model on the specified grid.

REQUIRED ARGUMENTS

       -Ggrdfile
              grdfile is the name of the binary output grid file. (See GRID FILE FORMAT below.)

       -Ixinc[unit][+e|n][/yinc[unit][+e|n]]
              x_inc  [and  optionally  y_inc]  is  the  grid spacing. Optionally, append a suffix
              modifier. Geographical (degrees) coordinates: Append m to indicate arc minutes or s
              to  indicate  arc  seconds.  If  one  of  the  units e, f, k, M, n or u is appended
              instead, the increment is assumed to be given in meter, foot,  km,  Mile,  nautical
              mile  or  US  survey  foot,  respectively,  and will be converted to the equivalent
              degrees longitude at the middle latitude of the region (the conversion  depends  on
              PROJ_ELLIPSOID).  If  y_inc  is given but set to 0 it will be reset equal to x_inc;
              otherwise it will be converted to degrees  latitude.  All  coordinates:  If  +e  is
              appended  then the corresponding max x (east) or y (north) may be slightly adjusted
              to fit exactly the given increment  [by  default  the  increment  may  be  adjusted
              slightly  to fit the given domain]. Finally, instead of giving an increment you may
              specify the number of nodes  desired  by  appending  +n  to  the  supplied  integer
              argument;  the  increment  is  then  recalculated  from the number of nodes and the
              domain. The resulting increment value  depends  on  whether  you  have  selected  a
              gridline-registered  or  pixel-registered  grid;  see App-file-formats for details.
              Note: if -Rgrdfile is used then the grid spacing has already been initialized;  use
              -I to override the values.

       -Rxmin/xmax/ymin/ymax[+r][+uunit] (more ...)
              Specify the region of interest.

OPTIONAL ARGUMENTS

       table  One  or  more  ASCII  [or  binary,  see  -bi]  files holding the spherical harmonic
              coefficients. We expect the first four columns to hold the degree L, the  order  M,
              followed by the cosine and sine coefficients.

       -D[g|n]
              Will  evaluate  a derived field from a geopotential model.  Choose between Dg which
              will compute the gravitational field or  Dn  to  compute  the  geoid  [Add  -E  for
              anomalies on the ellipsoid].

       -E     Evaluate expansion on the current ellipsoid [Default is sphere].

       -F[d]filter
              Filter  coefficients  according  to  one  of  two  kinds of filter specifications:.
              Select -Fk if values are given in km [Default is coefficient harmonic degree L]. a)
              Cosine  band-pass: Append four wavelengths lc/lp/hp/hc.  Coefficients outside lc/hc
              are cut; those inside lp/hp are  passed,  while  the  rest  are  tapered.   Replace
              wavelength  by  -  to  skip,  e.g.,  -F-/-/50/75 is a low-pass filter.  b) Gaussian
              band-pass: Append two wavelengths lo/hi where filter  amplitudes  =  0.5.   Replace
              wavelength by - to skip, e.g., -F70/- is a high-pass Gaussian filter.

       -N[norm]
              Normalization  used for coefficients.  Choose among m: Mathematical normalization -
              inner products summed over surface equal 1 [Default].  g  Geodesy  normalization  -
              inner products summed over surface equal 4pi. s: Schmidt normalization - as used in
              geomagnetism.

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

       -bi[ncols][t] (more ...)
              Select native binary input. [Default is 4 input columns].

       -h[i|o][n][+c][+d][+rremark][+rtitle] (more ...)
              Skip or produce header record(s). Not used with binary data.

       -icols[+l][+sscale][+ooffset][,...] (more ...)
              Select input columns and transformations (0 is first column).

       -r (more ...)
              Set pixel node registration [gridline].

       -x[[-]n] (more ...)
              Limit number of cores used in multi-threaded algorithms (OpenMP required).

       -^ or just -
              Print a short message about the syntax of the command, then exits (NOTE: on Windows
              just use -).

       -+ 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  all  options,
              then exits.

GRID VALUES PRECISION

       Regardless  of  the  precision of the input data, GMT programs that create grid files will
       internally hold the grids in 4-byte floating point arrays. This is done to conserve memory
       and  furthermore  most  if  not  all  real  data can be stored using 4-byte floating point
       values. Data with  higher  precision  (i.e.,  double  precision  values)  will  lose  that
       precision  once  GMT  operates  on  the  grid  or  writes  out new grids. To limit loss of
       precision when processing data you should always consider normalizing the  data  prior  to
       processing.

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[+sscale][+ooffset][+ninvalid], 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 invalid is the value used to indicate missing data. See
       grdconvert and Section grid-file-format of the GMT Technical Reference  and  Cookbook  for
       more information.

       When  writing  a netCDF file, the grid is stored by default with the variable name "z". To
       specify another variable name varname, append ?varname to the file name. 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.

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

       To create a 1 x 1 degree global grid file from the ASCII coefficients in EGM96_to_360.txt,
       use

              gmt sph2grd EGM96_to_360.txt -GEGM96_to_360.nc -Rg -I1 -V

REFERENCE

       Holmes, S. A., and Featherstone, W. E., 2002, A unified approach to the Clenshaw summation
       and the recursive computation of very high degree and order normalized associated Legendre
       functions: J. Geodesy, v. 76, p. 279-299.

SEE ALSO

       gmt, grdfft, grdmath

COPYRIGHT

       2019, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe