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<title>SPHDISTANCE</title>
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<h1 align="center">SPHDISTANCE</h1>
<a href="#NAME">NAME</a><br>
<a href="#SYNOPSIS">SYNOPSIS</a><br>
<a href="#DESCRIPTION">DESCRIPTION</a><br>
<a href="#OPTIONS">OPTIONS</a><br>
<a href="#ASCII FORMAT PRECISION">ASCII FORMAT PRECISION</a><br>
<a href="#GRID VALUES PRECISION">GRID VALUES PRECISION</a><br>
<a href="#EXAMPLES">EXAMPLES</a><br>
<a href="#SEE ALSO">SEE ALSO</a><br>
<a href="#REFERENCES">REFERENCES</a><br>
<hr>
<h2>NAME
<a name="NAME"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em">sphdistance
− Calculate nearest distances from Voronoi
construction of spherical data</p>
<h2>SYNOPSIS
<a name="SYNOPSIS"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em"><b>sphdistance</b>
<i>infiles</i> <b>−G</b><i>grdfile</i> [
<b>−C</b> ] [ <b>−D</b> ] [ <b>−E</b> ] [
<b>−F</b> ] [ <b>−H</b>[<b>i</b>][<i>nrec</i>] ]
[
<b>−I</b><i>xinc</i>[<i>unit</i>][<b>=</b>|<b>+</b>][/<i>yinc</i>[<i>unit</i>][<b>=</b>|<b>+</b>]]
] [ <b>−L</b><i>unit</i> ] [
<b>−Q</b><i>voronoi.d</i> ] [
<b>−R</b><i>west</i>/<i>east</i>/<i>south</i>/<i>north</i>[<b>r</b>]
] [ <b>−V</b> ] [ <b>−:</b>[<b>i</b>|<b>o</b>] ]
[
<b>−b</b>[<b>i</b>|<b>o</b>][<b>s</b>|<b>S</b>|<b>d</b>|<b>D</b>[<i>ncol</i>]|<b>c</b>[<i>var1</i><b>/</b><i>...</i>]]
] [ <b>−m</b>[<b>i</b>|<b>o</b>][<i>flag</i>] ]</p>
<h2>DESCRIPTION
<a name="DESCRIPTION"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em"><b>sphdistance</b>
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
<b><A HREF="sphtriangulate.html">sphtriangulate</A></b>, thus bypassing the memory- and
time-consuming triangularization. <i><br>
infiles</i></p>
<p style="margin-left:22%;">Data files with the point
coordinates in ASCII (or binary; see <b>−b</b>). If no
files are given the standard input is read.</p>
<table width="100%" border="0" rules="none" frame="void"
cellspacing="0" cellpadding="0">
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p><b>−G</b></p></td>
<td width="8%"></td>
<td width="78%">
<p>Name of the output grid to hold the computed
distances.</p> </td></tr>
</table>
<h2>OPTIONS
<a name="OPTIONS"></a>
</h2>
<table width="100%" border="0" rules="none" frame="void"
cellspacing="0" cellpadding="0">
<tr valign="top" align="left">
<td width="11%"></td>
<td width="4%">
<p style="margin-top: 1em"><b>−C</b></p></td>
<td width="7%"></td>
<td width="78%">
<p style="margin-top: 1em">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 <b>−Q</b>.</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="4%">
<p><b>−D</b></p></td>
<td width="7%"></td>
<td width="78%">
<p>Used with <b>−m</b> to skip the last (repeated)
input vertex at the end of a closed segment if it equals the
first point in the segment. Requires <b>−m</b>
[Default uses all points].</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="4%">
<p><b>−E</b></p></td>
<td width="7%"></td>
<td width="78%">
<p>Instead of computing distances, return the ID numbers of
the Voronoi polygons that each grid node is inside [Default
computes distances].</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="4%">
<p><b>−F</b></p></td>
<td width="7%"></td>
<td width="78%">
<p>Force pixel node registration [Default is gridline
registration]. (Node registrations are defined in <b><A HREF="GMT.html">GMT</A></b>
Cookbook Appendix B on grid file formats.)</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="4%">
<p><b>−H</b></p></td>
<td width="7%"></td>
<td width="78%">
<p>Input file(s) has header record(s). If used, the default
number of header records is <b><A HREF="gmtdefaults.html#N_HEADER_RECS">N_HEADER_RECS</A></b>. Use
<b>−Hi</b> 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.</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="4%">
<p><b>−I</b></p></td>
<td width="7%"></td>
<td width="78%">
<p><i>x_inc</i> [and optionally <i>y_inc</i>] is the grid
spacing. Optionally, append a suffix modifier.
<b>Geographical (degrees) coordinates</b>: Append <b>m</b>
to indicate arc minutes or <b>c</b> to indicate arc seconds.
If one of the units <b>e</b>, <b>k</b>, <b>i</b>, or
<b>n</b> 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
<b><A HREF="gmtdefaults.html#ELLIPSOID">ELLIPSOID</A></b>). If /<i>y_inc</i> is given but set to 0 it
will be reset equal to <i>x_inc</i>; otherwise it will be
converted to degrees latitude. <b>All coordinates</b>: If
<b>=</b> is appended then the corresponding max <i>x</i>
(<i>east</i>) or <i>y</i> (<i>north</i>) 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
<i>number of nodes</i> desired by appending <b>+</b> 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
<b>−R</b><i>grdfile</i> is used then grid spacing has
already been initialized; use <b>−I</b> to override
the values.</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="4%">
<p><b>−L</b></p></td>
<td width="7%"></td>
<td width="78%">
<p>Specify the unit used for distance calculations. Choose
among <b>e</b> (m), <b>k</b> (km), <b>m</b> (mile), <b>n</b>
(nautical mile), or <b>d</b> (spherical degree). A spherical
approximation is used unless <b><A HREF="gmtdefaults.html#ELLIPSOID">ELLIPSOID</A></b> is set to an
actual ellipsoid. <b>−N</b> 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 <b>−Q</b>
is used.</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="4%">
<p><b>−Q</b></p></td>
<td width="7%"></td>
<td width="78%">
<p>Append the name of a file with pre-calculated Voronoi
polygons [Default performs the Voronoi construction on input
data]. For binary data <b>−bi</b> you must specify the
node information separately (via <b>−N</b>).</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="4%">
<p><b>−R</b></p></td>
<td width="7%"></td>
<td width="78%">
<p><i>west, east, south,</i> and <i>north</i> 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
<b>r</b> if lower left and upper right map coordinates are
given instead of w/e/s/n. The two shorthands
<b>−Rg</b> and <b>−Rd</b> 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 <b>−R</b> settings (and
grid spacing, if applicable) are copied from the grid.</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="4%">
<p><b>−V</b></p></td>
<td width="7%"></td>
<td width="78%">
<p>Selects verbose mode, which will send progress reports
to stderr [Default runs "silently"].</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="4%">
<p><b>−:</b></p></td>
<td width="7%"></td>
<td width="78%">
<p>Toggles between (longitude,latitude) and
(latitude,longitude) input and/or output. [Default is
(longitude,latitude)]. Append <b>i</b> to select input only
or <b>o</b> to select output only. [Default affects
both].</p> </td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="4%">
<p><b>−bi</b></p></td>
<td width="7%"></td>
<td width="78%">
<p>Selects binary input. Append <b>s</b> for single
precision [Default is <b>d</b> (double)]. Uppercase <b>S</b>
or <b>D</b> will force byte-swapping. Optionally, append
<i>ncol</i>, the number of columns in your binary input file
if it exceeds the columns needed by the program. Or append
<b>c</b> if the input file is netCDF. Optionally, append
<i>var1</i><b>/</b><i>var2</i><b>/</b><i>...</i> to specify
the variables to be read. [Default is 2 input columns].</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="4%">
<p><b>−bo</b></p></td>
<td width="7%"></td>
<td width="78%">
<p>Selects binary output. Append <b>s</b> for single
precision [Default is <b>d</b> (double)]. Uppercase <b>S</b>
or <b>D</b> will force byte-swapping. Optionally, append
<i>ncol</i>, the number of desired columns in your binary
output file. [Default is same as input].</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="4%">
<p><b>−m</b></p></td>
<td width="7%"></td>
<td width="78%">
<p>Multiple segment file(s). Segments are separated by a
special record. For ASCII files the first character must be
<i>flag</i> [Default is ’>’]. For binary
files all fields must be NaN and <b>−b</b> must set
the number of output columns explicitly. By default the
<b>−m</b> setting applies to both input and output.
Use <b>−mi</b> and <b>−mo</b> to give separate
settings to input and output.</p></td></tr>
</table>
<h2>ASCII FORMAT PRECISION
<a name="ASCII FORMAT PRECISION"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em">The ASCII
output formats of numerical data are controlled by
parameters in your .gmtdefaults4 file. Longitude and
latitude are formatted according to
<b><A HREF="gmtdefaults.html#OUTPUT_DEGREE_FORMAT">OUTPUT_DEGREE_FORMAT</A></b>, whereas other values are
formatted according to <b><A HREF="gmtdefaults.html#D_FORMAT">D_FORMAT</A></b>. 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 (<b>−bo</b> if
available) or specify more decimals using the
<b><A HREF="gmtdefaults.html#D_FORMAT">D_FORMAT</A></b> setting.</p>
<h2>GRID VALUES PRECISION
<a name="GRID VALUES PRECISION"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em">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.</p>
<h2>EXAMPLES
<a name="EXAMPLES"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em">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</p>
<p style="margin-left:11%; margin-top: 1em"><b>sphdistance</b>
testdata.txt <b>−Rg −I</b>1
<b>−G</b>globedist.grd</p>
<p style="margin-left:11%; margin-top: 1em">To generate the
same grid in two steps using <b><A HREF="sphtriangulate.html">sphtriangulate</A></b>
separately, try</p>
<p style="margin-left:11%; margin-top: 1em"><b><A HREF="sphtriangulate.html">sphtriangulate</A></b>
testdata.txt <b>−Qv</b> > voronoi.d <b><br>
sphdistance −Q</b>voronoi.d <b>−Rg −I</b>1
<b>−G</b>globedist.grd</p>
<h2>SEE ALSO
<a name="SEE ALSO"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em"><i><A HREF="GMT.html">GMT</A></i>(1),
<i><A HREF="sphinterpolate.html">sphinterpolate</A></i>(1) <i><A HREF="sphtriangulate.html">sphtriangulate</A></i>(1)
<i><A HREF="triangulate.html">triangulate</A></i>(1)</p>
<h2>REFERENCES
<a name="REFERENCES"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em">Renka, R, J.,
1997, Algorithm 772: STRIPACK: Delaunay Triangulation and
Voronoi Diagram on the Surface of a Sphere, <i>AMC Trans.
Math. Software, 23</i> (3), 416−434.</p>
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