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SPHINTERPOLATE(1) Generic Mapping Tools SPHINTERPOLATE(1)
NAME
sphinterpolate - Gridding in tension of spherical data
SYNOPSIS
sphinterpolate infiles -Ggrdfile [ -F ] [ -H[i][nrec] ] [
-Ixinc[unit][=|*][/yinc[unit][=|+]] ] [ -Qmode[/options] ] [
-Rwest/east/south/north[r] ] [ -V ] [ -Z ] [ -:[i|o] ] [
-b[i|o][s|S|d|D[ncol]|c[var1/...]] ][ -m[i|o][flag] ]
DESCRIPTION
sphinterpolate reads one or more ASCII [or binary] files (or standard
input) containing lon, lat, f and performs a Delaunay triangulation to
set up a spherical interpolation in tension. The final grid is saved
to the specified file. Several options may be used to affect the
outcome, such as choosing local versus global gradient estimation or
optimize the tension selection to satisfy one of four criteria.
infiles
Data files with the (lon, lat, f) coordinates in ASCII (or
binary; see -b). If no files are given the standard input is
read.
-G Name of the output grid to hold the interpolation.
OPTIONS
-F Force pixel node registration [Default is gridline
registration]. (Node registrations are defined in GMT Cookbook
Appendix B on grid file formats.)
-H Input file(s) has header record(s). If used, the default number
of header records is N_HEADER_RECS. Use -Hi if only input data
should have header records [Default will write out header
records if the input data have them]. Blank lines and lines
starting with # are always skipped.
-I 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 c to indicate arc seconds.
If one of the units e, k, i, or n is appended instead, the
increment is assumed to be given in meter, km, miles, or
nautical miles, respectively, and will be converted to the
equivalent degrees longitude at the middle latitude of the
region (the conversion depends on 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 Appendix B for details. Note: if -Rgrdfile
is used then grid spacing has already been initialized; use -I
to override the values.
-Q Specify one of four ways to calculate tension factors to
preserve local shape properties or satisfy arc constraints
[Default is no tension].
-Q 0 Piecewise linear interpolation; no tension is applied.
-Q 1 Smooth interpolation with local gradient estimates.
-Q 2 Smooth interpolation with global gradient estimates. You may
optionally append /N/M/U, where N is the number of iterations
used to converge at solutions for gradients when variable
tensions are selected (e.g., -T only) [3], M is the number of
Gauss-Seidel iterations used when determining the global
gradients [10], and U is the maximum change in a gradient at the
last iteration [0.01].
-Q 3 Smoothing. Optionally append /E/U [/0/0], where E is Expected
squared error in a typical (scaled) data value, and U is Upper
bound on weighted sum of squares of deviations from data.
-R west, east, south, and north specify the Region of interest, and
you may specify them in decimal degrees or in
[+-]dd:mm[:ss.xxx][W|E|S|N] format. Append r if lower left and
upper right map coordinates are given instead of w/e/s/n. The
two shorthands -Rg and -Rd stand for global domain (0/360 and
-180/+180 in longitude respectively, with -90/+90 in latitude).
Alternatively, specify the name of an existing grid file and the
-R settings (and grid spacing, if applicable) are copied from
the grid.
-T Use variable tension (ignored with -Q 0 [constant]
-V Selects verbose mode, which will send progress reports to stderr
[Default runs "silently"].
-Z Before interpolation, scale data by the maximum data range [no
scaling].
-: Toggles between (longitude,latitude) and (latitude,longitude)
input and/or output. [Default is (longitude,latitude)]. Append
i to select input only or o to select output only. [Default
affects both].
-bi Selects binary input. Append s for single precision [Default is
d (double)]. Uppercase S or D will force byte-swapping.
Optionally, append ncol, the number of columns in your binary
input file if it exceeds the columns needed by the program. Or
append c if the input file is netCDF. Optionally, append
var1/var2/... to specify the variables to be read. [Default is
3 input columns].
-bo Selects binary output. Append s for single precision [Default
is d (double)]. Uppercase S or D will force byte-swapping.
Optionally, append ncol, the number of desired columns in your
binary output file. [Default is same as input].
-m Multiple segment file(s). Segments are separated by a special
record. For ASCII files the first character must be flag
[Default is '>']. For binary files all fields must be NaN and
-b must set the number of output columns explicitly. By default
the -m setting applies to both input and output. Use -mi and
-mo to give separate settings to input and output.
ASCII FORMAT PRECISION
The ASCII output formats of numerical data are controlled by parameters
in your .gmtdefaults4 file. Longitude and latitude are formatted
according to OUTPUT_DEGREE_FORMAT, whereas other values are formatted
according to D_FORMAT. Be aware that the format in effect can lead to
loss of precision in the output, which can lead to various problems
downstream. If you find the output is not written with enough
precision, consider switching to binary output (-bo if available) or
specify more decimals using the D_FORMAT setting.
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.
EXAMPLES
To interpolate the points in the file testdata.txt on a global 1x1
degree grid with no tension, use
sphinterpolate testdata.txt -Rg -I1 -Gsolution.grd
SEE ALSO
GMT(1), greenspline(1) sphdistance(1) sphtriangulate(1) triangulate(1)
REFERENCES
Renka, R, J., 1997, Algorithm 772: STRIPACK: Delaunay Triangulation and
Voronoi Diagram on the Surface of a Sphere, AMC Trans. Math. Software,
23 (3), 416-434.
Renka, R, J,, 1997, Algorithm 773: SSRFPACK: Interpolation of scattered
data on the Surface of a Sphere with a surface under tension, AMC
Trans. Math. Software, 23 (3), 435-442.
GMT 4.5.14 1 Nov 2015 SPHINTERPOLATE(1)