SPHDISTANCE(1) Generic Mapping Tools SPHDISTANCE(1)NAMEsphdistance - Calculate nearest distances from Voronoi construction of
spherical data
SYNOPSISsphdistance 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] ]
DESCRIPTIONsphdistance 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 sphtrian‐
gulate, thus bypassing the memory- and time-consuming triangulariza‐
tion.
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 coordi‐
nates (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 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.
-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.
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 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 ALSOGMT(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.
GMT 4.5.14 1 Nov 2015 SPHDISTANCE(1)
