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<title>IMG2GRD</title>
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<h1 align="center">IMG2GRD</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="#EXAMPLES">EXAMPLES</a><br>
<a href="#SEE ALSO">SEE ALSO</a><br>
<hr>
<h2>NAME
<a name="NAME"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em">img2grd −
Extract region of img in Mercator or geographic form</p>
<h2>SYNOPSIS
<a name="SYNOPSIS"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em"><b>img2grd</b>
<i>imgfile</i> <b>−G</b><i>grdfile</i>
<b>−R</b><i>west</i>/<i>east</i>/<i>south</i>/<i>north</i>[<b>r</b>]
<b>−T</b><i>type</i> [ <b>−C</b> ] [
<b>−D</b>[<i>minlat/maxlat</i>] ] [ <b>−E</b> ]
[ <b>−L</b> ] [ <b>−M</b> ] [
<b>−N</b><i>navg</i> ] [ <b>−S</b><i>scale</i> ]
[ <b>−V</b> ] [ <b>−W</b><i>maxlon</i> ] [
<b>−m</b><i>minutes</i> ]</p>
<h2>DESCRIPTION
<a name="DESCRIPTION"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em"><b>img2grd</b>
is a front-end to <b><A HREF="img2mercgrd.html">img2mercgrd</A></b> which reads an img
format file and creates a grid file. The <b>−M</b>
option dictates whether or not the Spherical Mercator
projection of the img file is preserved. <i><br>
imgfile</i></p>
<p style="margin-left:22%;">An img format file such as the
marine gravity or seafloor topography fields estimated from
satellite altimeter data by Sandwell and Smith. If the user
has set an environment variable <b>$GMT_DATADIR</b>, then
<b><A HREF="img2mercgrd.html">img2mercgrd</A></b> will try to find <i>imgfile</i> in
<b>$GMT_DATADIR</b>; else it will try to open <i>imgfile</i>
directly.</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><i>grdfile</i> is the name of the output grid file.</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p><b>−R</b></p></td>
<td width="8%"></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="3%">
<p><b>−T</b></p></td>
<td width="8%"></td>
<td width="78%">
<p><i>type</i> handles the encoding of constraint
information. <i>type</i> = 0 indicates that no such
information is encoded in the img file (used for pre-1995
versions of the gravity data) and gets all data. <i>type</i>
> 0 indicates that constraint information is encoded
(1995 and later (current) versions of the img files) so that
one may produce a grid file as follows:
<b>−T</b><i>1</i> gets data values at all points,
<b>−T</b><i>2</i> gets data values at constrained
points and NaN at interpolated points;
<b>−T</b><i>3</i> gets 1 at constrained points and 0
at interpolated points.</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="3%">
<p style="margin-top: 1em"><b>−C</b></p></td>
<td width="8%"></td>
<td width="78%">
<p style="margin-top: 1em">Set the x and y Mercator
coordinates relative to projection center [Default is
relative to lower left corner of grid]. Requires
<b>−M</b>.</p> </td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p><b>−D</b></p></td>
<td width="8%"></td>
<td width="78%">
<p>Use the extended latitude range -80.738/+80.738.
Alternatively, append <i>minlat/maxlat</i> as the latitude
extent of the input img file. [Default is
-72.006/72.006].</p> </td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p><b>−E</b></p></td>
<td width="8%"></td>
<td width="78%">
<p>Can be used when <b>−M</b> is not set to force the
final grid to have the exact same region as requested with
<b>−R</b>. By default, the final region is a direct
projection of the original Mercator region and will
typically extend slightly beyond the requested latitude
range, and furthermore the grid increment in latitude does
not match the longitude increment. However, the extra
resampling introduces small interpolation errors and should
only be used if the output grid must match the requested
region and have x_inc = y_inc. In this case the region set
by <b>−R</b> must be given in multiples of the
increment (.e.g, <b>−R</b> 0/45/45/72).</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p><b>−L</b></p></td>
<td width="8%"></td>
<td width="78%">
<p>With no other arguments, list all *.img files found in
the directory pointed to by <b>$GMT_DATADIR</b>, or the
current directory if not defined. Ignored if other options
are present on the command line.</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p><b>−M</b></p></td>
<td width="8%"></td>
<td width="78%">
<p>Output a Spherical Mercator grid [Default is a
geographic lon/lat grid].</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p><b>−N</b></p></td>
<td width="8%"></td>
<td width="78%">
<p>Average the values in the input img pixels into
<i>navg</i> by <i>navg</i> squares, and create one output
pixel for each such square. If used with
<b>−T</b><i>3</i> it will report an average constraint
between 0 and 1. If used with <b>−T</b><i>2</i> the
output will be average data value or NaN according to
whether average constraint is > 0.5. <i>navg</i> must
evenly divide into the dimensions of the imgfile in pixels.
[Default <i>1</i> does no averaging].</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p><b>−S</b></p></td>
<td width="8%"></td>
<td width="78%">
<p>Multiply the img file values by <i>scale</i> before
storing in grid file. [Default is 1.0]. For recent img
files: img topo files are stored in (corrected) meters
[<b>−S</b> 1]; free-air gravity files in mGal*10
[<b>−S</b> 0.1 to get mGal]; vertical deflection files
in microradians*10 [<b>−S</b> 0.1 to get
microradians], vertical gravity gradient files in Eotvos*50
[<b>−S</b> 0.02 to get Eotvos, or <b>−S</b>
0.002 to get mGal/km]).</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p><b>−V</b></p></td>
<td width="8%"></td>
<td width="78%">
<p>Selects verbose mode, which will send progress reports
to stderr [Default runs "silently"]. Particularly
recommended here, as it is helpful to see how the
coordinates are adjusted.</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p><b>−m</b></p></td>
<td width="8%"></td>
<td width="78%">
<p>Indicate <i>minutes</i> as the width of an input img
pixel in minutes of longitude. [Default is 2.0].</p></td></tr>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p><b>−W</b></p></td>
<td width="8%"></td>
<td width="78%">
<p>Indicate <i>maxlon</i> as the maximum longitude extent
of the input img file. Versions since 1995 have had
<i>maxlon</i> = 360.0, while some earlier files had
<i>maxlon</i> = 390.0. [Default is 360.0].</p></td></tr>
</table>
<h2>EXAMPLES
<a name="EXAMPLES"></a>
</h2>
<p style="margin-left:11%; margin-top: 1em">To extract data
in the region <b>−R</b>-40/40/-70/-30 from
<i>world_grav.img.7.2</i> and preserve the Mercator
gridding:</p>
<p style="margin-left:11%; margin-top: 1em"><b>img2grd</b>
world_grav.img.7.2 <b>−G</b> merc_grav.grd
<b>−R</b>-40/40/-70/-30 <b>−M −T</b> 1
<b>−V</b></p>
<p style="margin-left:11%; margin-top: 1em">Without the
<b>−M</b> option the same command will yield a
geographic grid.</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="img2mercgrd.html">img2mercgrd</A></i>(1)</p>
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