Provided by: gmt-common_5.2.1+dfsg-3build1_all 
NAME
sph2grd - Compute grid from spherical harmonic coefficients
SYNOPSIS
sph2grd [ table ] grdfile increment region [ [g|n] ] [ ] [ [k]filter ] [ [norm] ] [ ] [ [level] ] [
-bi<binary> ] [ -h<headers> ] [ -i<flags> ] [ -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][=|+][/yinc[unit][=|+]]
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 = 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 + 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.
-R[unit]xmin/xmax/ymin/ymax[r] (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 (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 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 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[/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.
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
2015, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe