sphdistance(1) GMT sphdistance(1)
NAME
sphdistance - Make Voronoi distance, node, or nearest-neighbor grid on
a sphere
SYNOPSIS
sphdistance [ table ] -Ggrdfile [ -C ] [ -Ed|n|z[dist] ] [ -Iincre-
ment ] [ -Lunit ] [ -Nnodetable ] [ -Qvoronoi.txt ] [ -Rregion ] [
-V[level] ] [ -bbinary ] [ -dnodata ] [ -eregexp ] [ -hheaders ] [
-iflags ] [ -r ] [ -:[i|o] ]
Note: No space is allowed between the option flag and the associated
arguments.
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 sphtrian-
gulate, thus bypassing the memory- and time-consuming triangulariza-
tion.
REQUIRED ARGUMENTS
-Ggrdfile
Name of the output grid to hold the computed distances (but see
-E for other node value options).
OPTIONAL ARGUMENTS
table One or more ASCII (or binary, see -bi[ncols][type]) data table
file(s) holding a number of data columns. If no tables are given
then we read from standard input.
-C For large data sets 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.
-Ed|n|z[dist]
Specify the quantity that should be assigned to the grid nodes.
By default we compute distances to the nearest data point [-Ed].
Use -En to assign the ID numbers of the Voronoi polygons that
each grid node is inside, or use -Ez for a natural near-
est-neighbor grid where we assign all nodes inside the polygon
the z-value of the center node. Optionally, append the resam-
pling interval along Voronoi arcs in spherical degrees [1].
-Ixinc[unit][+e|n][/yinc[unit][+e|n]]
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 s to indicate arc seconds.
If one of the units e, f, k, M, n or u is appended instead, the
increment is assumed to be given in meter, foot, km, Mile, nau-
tical mile or US survey foot, respectively, and will be con-
verted to the equivalent degrees longitude at the middle lati-
tude of the region (the conversion depends on PROJ_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 coordi-
nates: If +e 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 +n to
the supplied integer argument; the increment is then recalcu-
lated from the number of nodes and the domain. The resulting
increment value depends on whether you have selected a grid-
line-registered or pixel-registered grid; see App-file-formats
for details. Note: if -Rgrdfile is used then the grid spacing
has already been initialized; use -I to override the values.
-Lunit Specify the unit used for distance calculations. Choose among d
(spherical degree), e (m), f (feet), k (km), M (mile), n (nauti-
cal mile) or u survey foot. A spherical approximation is used
unless PROJ_ELLIPSOID is set to an actual ellipsoid.
-Nnodetable
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 head-
ers of the output file]. Required if binary input via -Q is
used.
-Qvoronoi.txt
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).
-Rwest/east/south/north[/zmin/zmax][+r][+uunit]
west, east, south, and north specify the region of interest, and
you may specify them in decimal degrees or in
[A+-]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 for grid creation, give Rcodelon/lat/nx/ny, where
code is a 2-character combination of L, C, R (for left, center,
or right) and T, M, B for top, middle, or bottom. e.g., BL for
lower left. This indicates which point on a rectangular region
the lon/lat coordinate refers to, and the grid dimensions nx and
ny with grid spacings via -I is used to create the corresponding
region. Alternatively, specify the name of an existing grid
file and the -R settings (and grid spacing, if applicable) are
copied from the grid. Appending +uunit expects projected (Carte-
sian) coordinates compatible with chosen -J and we inversely
project to determine actual rectangular geographic region. For
perspective view (-p), optionally append /zmin/zmax. In case of
perspective view (-p), a z-range (zmin, zmax) can be appended to
indicate the third dimension. This needs to be done only when
using the -Jz option, not when using only the -p option. In the
latter case a perspective view of the plane is plotted, with no
third dimension.
-V[level] (more a|)
Select verbosity level [c].
-bi[ncols][t] (more a|)
Select native binary input. [Default is 2 input columns].
-bo[ncols][type] (more a|)
Select native binary output. [Default is same as input].
-d[i|o]nodata (more a|)
Replace input columns that equal nodata with NaN and do the
reverse on output.
-e[~]^<i>apattern^<i>a | -e[~]/regexp/[i] (more a|)
Only accept data records that match the given pattern.
-h[i|o][n][+c][+d][+rremark][+rtitle] (more a|)
Skip or produce header record(s).
-icols[+l][+sscale][+ooffset][,^<i>a|] (more a|)
Select input columns and transformations (0 is first column).
-r (more a|)
Set pixel node registration [gridline].
-:[i|o] (more a|)
Swap 1st and 2nd column on input and/or output.
-^ or just -
Print a short message about the syntax of the command, then
exits (NOTE: on Windows just use -).
-+ or just +
Print an extensive usage (help) message, including the explana-
tion of any module-specific option (but not the GMT common
options), then exits.
-? or no arguments
Print a complete usage (help) message, including the explanation
of all options, then exits.
ASCII FORMAT PRECISION
The ASCII output formats of numerical data are controlled by parameters
in your gmt.conf file. Longitude and latitude are formatted according
to FORMAT_GEO_OUT, absolute time is under the control of FOR-
MAT_DATE_OUT and FORMAT_CLOCK_OUT, whereas general floating point val-
ues are formatted according to FORMAT_FLOAT_OUT. Be aware that the for-
mat in effect can lead to loss of precision in ASCII 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 FORMAT_FLOAT_OUT set-
ting.
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 preci-
sion once GMT operates on the grid or writes out new grids. To limit
loss of precision when processing data you should always consider nor-
malizing 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
gmt sphdistance testdata.txt -Rg -I1 -Gglobedist.nc
To generate the same grid in two steps using sphtriangulate separately,
try
gmt sphtriangulate testdata.txt -Qv > voronoi.txt
gmt sphdistance -Qvoronoi.txt -Rg -I1 -Gglobedist.nc
SEE ALSO
gmt(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.
COPYRIGHT
2017, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe
5.4.2 Jun 24, 2017 sphdistance(1)
gmt5 5.4.2 - Generated Thu Jun 29 16:31:33 CDT 2017