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sph2grd(1)                            GMT                           sph2grd(1)




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, nau-
              tical mile or US survey foot, respectively,  and  will  be  con-
              verted  to  the equivalent degrees longitude at the middle lati-
              tude 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  coordi-
              nates:  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  recalcu-
              lated  from  the  number  of nodes and the domain. The resulting
              increment value depends on whether you  have  selected  a  grid-
              line-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 a|)
              Specify the region of interest.


OPTIONAL ARGUMENTS

       table  One or more ASCII [or binary, see -bi] files holding the spheri-
              cal 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: Mathemati-
              cal 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 geomag-
              netism.

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

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

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

       -icols[+l][+sscale][+ooffset][,^<i>a|] (more a|)
              Select input columns and transformations (0 is first column).

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

       -x[[-]n] (more a|)
              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 explana-
              tion 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  preci-
       sion  once  GMT  operates on the grid or writes out new grids. To limit
       loss of precision when processing data you should always consider  nor-
       malizing 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 apackinga of grids, writing  out  floating  point
       data as 1- or 2-byte integers. To specify the precision, scale and off-
       set, 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 aza. 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
       alongitudea, alatitudea, or atimea 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(1), grdfft(1), grdmath(1)


COPYRIGHT

       2017, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe



5.4.2                            Jun 24, 2017                       sph2grd(1)

gmt5 5.4.2 - Generated Thu Jun 29 16:20:06 CDT 2017
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