sph2grdgmt - Online in the Cloud

PROGRAM:

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.

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