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].
- -Q0
- Piecewise linear interpolation; no tension is applied.
- -QQ1
- Smooth interpolation with local gradient estimates.
- -QQ2
- 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].
- -QQ3
- 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 -Q0
[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.
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.