sph2grd - Compute grid from spherical harmonic coefficients
] ] [ -E
] [ -N
] ] [ -Q
] ] [ -bi
binary ] [ -h
headers ] [
flags ] [ -r
] [ -x
No space is allowed between the option flag and the associated
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
- grdfile is the name of the binary output grid file. (See GRID FILE
- 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
-R grdfile is used then the grid spacing has already been
initialized; use -I to override the values.
- Specify the region of interest.
- 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.
- 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].
- Evaluate expansion on the current ellipsoid [Default is sphere].
- 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.
- 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].
- Skip or produce header record(s). Not used with binary data.
- 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
- -^ 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.
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.
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
is a two-letter identifier of the grid type and precision, and
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
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
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
options. For example, both -f0x -f1t
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.
To create a 1 x 1 degree global grid file from the ASCII coefficients in
gmt sph2grd EGM96_to_360.txt -GEGM96_to_360.nc -Rg -I1 -V
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.
gmt, grdfft, grdmath
2018, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe