NAME¶
sphdistance - Calculate nearest distances from Voronoi construction of spherical
data
SYNOPSIS¶
sphdistance infiles -Ggrdfile [
-C ] [
-D ] [
-E ] [
-F ] [
-H[
i][
nrec] ] [
-Ixinc[
unit][
=|
+][/
yinc[
unit][
=|
+]]
] [
-Lunit ] [
-Qvoronoi.d ] [
-Rwest/
east/
south/
north[
r] ] [
-V ] [
-:[
i|
o] ] [
-b[
i|
o][
s|
S|
d|
D[
ncol]|
c[
var1/...]]
] [
-m[
i|
o][
flag] ]
DESCRIPTION¶
sphdistance reads one or more ASCII [or binary] files (or standard input)
containing lon, lat and performs the construction of Voronoi polygons. These
polygons are then processed to calculate the nearest distance to each node of
the lattice and written to the specified grid. The Voronoi algorithm used is
STRIPACK. As an option, you may provide pre-calculated Voronoi polygon file in
the format written by
sphtriangulate, thus bypassing the memory- and
time-consuming triangularization.
- infiles
- Data files with the point 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 computed distances.
OPTIONS¶
- -C
- For large data set you can save some memory (at the expense of more
processing) by only storing one form of location coordinates (geographic
or Cartesian 3-D vectors) at any given time, translating from one form to
the other when necessary [Default keeps both arrays in memory]. Not
applicable with -Q.
- -D
- Used with -m to skip the last (repeated) input vertex at the end of
a closed segment if it equals the first point in the segment. Requires
-m [Default uses all points].
- -E
- Instead of computing distances, return the ID numbers of the Voronoi
polygons that each grid node is inside [Default computes distances].
- -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.
- -L
- Specify the unit used for distance calculations. Choose among e
(m), k (km), m (mile), n (nautical mile), or d
(spherical degree). A spherical approximation is used unless
ELLIPSOID is set to an actual ellipsoid. -N Read the
information pertaining to each Voronoi polygon (the unique node lon, lat
and polygon area) from a separate file [Default acquires this information
from the ASCII segment headers of the output file]. Required if binary
input via -Q is used.
- -Q
- Append the name of a file with pre-calculated Voronoi polygons [Default
performs the Voronoi construction on input data]. For binary data
-bi you must specify the node information separately (via
-N).
- -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.
- -V
- Selects verbose mode, which will send progress reports to stderr [Default
runs "silently"].
- -:
- 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 2 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 construct Voronoi polygons from the points in the file testdata.txt and then
calculate distances from the data to a global 1x1 degree grid, use
sphdistance testdata.txt
-Rg -I1
-Gglobedist.grd
To generate the same grid in two steps using
sphtriangulate separately,
try
sphtriangulate testdata.txt
-Qv > voronoi.d
sphdistance -Qvoronoi.d
-Rg -I1
-Gglobedist.grd
SEE ALSO¶
GMT(1),
sphinterpolate(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.