SPHINTERPOLATE(1) Generic Mapping Tools SPHINTERPOLATE(1)NAMEsphinterpolate - Gridding in tension of spherical data
SYNOPSISsphinterpolate 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] ]
DESCRIPTIONsphinterpolate 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 out‐
come, such as choosing local versus global gradient estimation or opti‐
mize 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 registra‐
tion]. (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 nauti‐
cal miles, respectively, and will be converted to the equivalent
degrees longitude at the middle latitude of the region (the con‐
version 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 pre‐
serve 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 ten‐
sions are selected (e.g., -T only) [3], M is the number of
Gauss-Seidel iterations used when determining the global gradi‐
ents [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 preci‐
sion, 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 ALSOGMT(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)
