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