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<title>IMG2MERCGRD</title>
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<h1 align=center>IMG2MERCGRD</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>
<a name="NAME"></a>
<h2>NAME</h2>
<p style="margin-left:11%; margin-top: 1em">img2mercgrd
− Extract region of img, preserving Mercator, save as
grd</p>
<a name="SYNOPSIS"></a>
<h2>SYNOPSIS</h2>
<p style="margin-left:11%; margin-top: 1em"><b>img2mercgrd</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>−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>
<a name="DESCRIPTION"></a>
<h2>DESCRIPTION</h2>
<p style="margin-left:11%; margin-top: 1em"><b>img2mercgrd</b>
reads an img format file and creates a grid file. The
Spherical Mercator projection of the img file is preserved,
so that the region <b>−R</b> set by the user is
modified slightly; the modified region corresponds to the
edges of pixels [or groups of <i>navg</i> pixels]. The grid
file header is set so that the x and y axis lengths
represent distance from the west and south edges of the
image, measured in user default units, with
<b>−Jm</b>1 and the adjusted <b>−R</b>. By
setting the default <b><A HREF="gmtdefaults.html#ELLIPSOID">ELLIPSOID</A></b> = Sphere, the user can
make overlays with the adjusted <b>−R</b> so that they
match. See <b>EXAMPLES</b> below. The adjusted
<b>−R</b> is also written in the grdheader remark, so
it can be found later. The <b>−T</b><i>type</i>
selects all data or only data at constrained pixels, and can
be used to create a grid of 1s and 0s indicating constraint
locations. The output grid file is pixel registered; it
inherits this from the img file. <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>$GMT_IMGDIR</b>, then
<b>img2mercgrd</b> will try to find <i>imgfile</i> in
<b>$GMT_IMGDIR</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 style="margin-top: 1em" valign="top"><b>−G</b></p> </td>
<td width="8%"></td>
<td width="78%">
<p style="margin-top: 1em" valign="top"><i>grdfile</i> is
the name of the output grid file.</p></td>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p style="margin-top: 1em" valign="top"><b>−R</b></p> </td>
<td width="8%"></td>
<td width="78%">
<p style="margin-top: 1em" valign="top"><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 valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p style="margin-top: 1em" valign="top"><b>−T</b></p> </td>
<td width="8%"></td>
<td width="78%">
<p style="margin-top: 1em" valign="top"><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; all
more recent files do not support this choice) 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>
</table>
<a name="OPTIONS"></a>
<h2>OPTIONS</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" valign="top"><b>−C</b></p> </td>
<td width="8%"></td>
<td width="78%">
<p style="margin-top: 1em" valign="top">Set the x and y
Mercator coordinates relative to projection center (lon =
lat = 0) [Default is relative to lower left corner of
grid].</p> </td>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p style="margin-top: 1em" valign="top"><b>−D</b></p> </td>
<td width="8%"></td>
<td width="78%">
<p style="margin-top: 1em" valign="top">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 valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p style="margin-top: 1em" valign="top"><b>−N</b></p> </td>
<td width="8%"></td>
<td width="78%">
<p style="margin-top: 1em" valign="top">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 valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p style="margin-top: 1em" valign="top"><b>−S</b></p> </td>
<td width="8%"></td>
<td width="78%">
<p style="margin-top: 1em" valign="top">Multiply the img
file values by <i>scale</i> before storing in grid file.
[Default is 1.0]. (img topo files are stored in (corrected)
meters; gravity files in mGal*10; vertical deflection files
in microradians*10, vertical gravity gradient files in
Eotvos*10. Use <b>−S</b>0.1 for those files.)</p></td>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p style="margin-top: 1em" valign="top"><b>−V</b></p> </td>
<td width="8%"></td>
<td width="78%">
<p style="margin-top: 1em" valign="top">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 valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p style="margin-top: 1em" valign="top"><b>−m</b></p> </td>
<td width="8%"></td>
<td width="78%">
<p style="margin-top: 1em" valign="top">Indicate
<i>minutes</i> as the width of an input img pixel in minutes
of longitude. [Default is 2.0].</p></td>
<tr valign="top" align="left">
<td width="11%"></td>
<td width="3%">
<p style="margin-top: 1em" valign="top"><b>−W</b></p> </td>
<td width="8%"></td>
<td width="78%">
<p style="margin-top: 1em" valign="top">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>
</table>
<a name="EXAMPLES"></a>
<h2>EXAMPLES</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>, run</p>
<p style="margin-left:11%; margin-top: 1em"><b>img2mercgrd</b>
world_grav.img.7.2 <b>−G</b>merc_grav.grd
<b>−R</b>-40/40/-70/-30 <b>−T</b>1
<b>−V</b></p>
<p style="margin-left:11%; margin-top: 1em">Note that the
<b>−V</b> option tells us that the range was adjusted
to <b>−R</b>-40/40/-70.0004681551/-29.9945810754. We
can also use <b><A HREF="grdinfo.html">grdinfo</A></b> to find that the grid file
header shows its region to be
<b>−R</b>0/80/0/67.9666667 This is the range of x,y we
will get from a Spherical Mercator projection using
<b>−R</b>-40/40/-70.0004681551/-29.9945810754 and
<b>−Jm</b>1. Thus, to take ship.lonlatgrav and use it
to sample the merc_grav.grd, we can do this:</p>
<p style="margin-left:11%; margin-top: 1em"><b>gmtset
ELLIPSOID</b> Sphere <b><br>
mapproject −R</b>-40/40/-70.0004681551/-29.9945810754
<b>−Jm</b>1 ship.lonlatgrav | <b>grdtrack
−G</b>merc_grav.grd | <b>mapproject
−R</b>-40/40/-70.0004681551/-29.9945810754
<b>−Jm</b>1 <b>−I</b> >
ship.lonlatgravsat</p>
<p style="margin-left:11%; margin-top: 1em">It is
recommended to use the above method of projecting and
unprojecting the data in such an application, because then
there is only one interpolation step (in <b><A HREF="grdtrack.html">grdtrack</A></b>).
If one first tries to convert the grid file to lon,lat and
then sample it, there are two interpolation steps (in
conversion and in sampling).</p>
<p style="margin-left:11%; margin-top: 1em">To make a
lon,lat grid from the above grid we can use</p>
<p style="margin-left:11%; margin-top: 1em"><b><A HREF="grdproject.html">grdproject</A></b>
merc_grav.grd
<b>−R</b>-40/40/-70.0004681551/-29.9945810754
<b>−Jm</b>1 <b>−I −F −D</b>2m
<b>−G</b>grav.grd</p>
<p style="margin-left:11%; margin-top: 1em">In some cases
this will not be easy as the <b>−R</b> in the two
coordinate systems may not align well. When this happens, we
can also use (in fact, it may be always better to use)</p>
<p style="margin-left:11%; margin-top: 1em"><b><A HREF="grd2xyz.html">grd2xyz</A></b>
merc_grav.grd | <b>mapproject
−R</b>-40/40/-70.0004681551/-29.994581075
<b>−Jm</b>1 <b>−I</b> | <b>surface
−R</b>-40/40/-70/70 <b>−I</b>2m
<b>−G</b>grav.grd</p>
<p style="margin-left:11%; margin-top: 1em">To make a
Mercator map of the above region, suppose our .gmtdefaults4
<b><A HREF="gmtdefaults.html#MEASURE_UNIT">MEASURE_UNIT</A></b> is inch. Then since the above
merc_grav.grd file is projected with <b>−Jm</b>1 it is
80 inches wide. We can make a map 8 inches wide by using
<b>−Jx</b>0.1 on any map programs applied to this grid
(e.g., <b><A HREF="grdcontour.html">grdcontour</A></b>, <b><A HREF="grdimage.html">grdimage</A></b>, <b><A HREF="grdview.html">grdview</A></b>),
and then for overlays which work in lon,lat (e.g.,
<b><A HREF="psxy.html">psxy</A></b>, <b><A HREF="pscoast.html">pscoast</A></b>) we can use the above adjusted
<b>−R</b> and <b>−Jm</b>0.1 to get the two
systems to match up.</p>
<p style="margin-left:11%; margin-top: 1em">However, we can
be smarter than this. Realizing that the input img file had
pixels 2.0 minutes wide (or checking the nx and ny with
grdinfo merc_grav.grd) we realize that merc_grav.grd used
the full resolution of the img file and it has 2400 by 2039
pixels, and at 8 inches wide this is 300 pixels per inch. We
decide we don’t need that many and we will be
satisfied with 100 pixels per inch, so we want to average
the data into 3 by 3 squares. (If we want a contour plot we
will probably choose to average the data much more (e.g., 6
by 6) to get smooth contours.) Since 2039 isn’t
divisible by 3 we will get a different adjusted OPT(R) this
time:</p>
<p style="margin-left:11%; margin-top: 1em"><b>img2mercgrd</b>
world_grav.img.7.2 <b>−G</b>merc_grav_2.grd
<b>−R</b>-40/40/-70/-30 <b>−T</b>1
<b>−N</b>3 <b>−V</b></p>
<p style="margin-left:11%; margin-top: 1em">This time we
find the adjusted region is
<b>−R</b>-40/40/-70.023256525/-29.9368261101 and the
output is 800 by 601 pixels, a better size for us. Now we
can create an artificial illumination file for this using
<b><A HREF="grdgradient.html">grdgradient</A></b>:</p>
<p style="margin-left:11%; margin-top: 1em"><b><A HREF="grdgradient.html">grdgradient</A></b>
merc_grav_2.grd <b>−G</b>illum.grd
<b>−A</b>0/270 <b>−N</b>e0.6</p>
<p style="margin-left:11%; margin-top: 1em">and if we also
have a cpt file called "grav.cpt" we can create a
color shaded relief map like this:</p>
<p style="margin-left:11%; margin-top: 1em"><b><A HREF="grdimage.html">grdimage</A></b>
merc_grav_2.grd <b>−I</b>illum.grd
<b>−C</b>grav.cpt <b>−Jx</b>0.1 <b>−K</b>
> map.ps <b><br>
psbasemap −R</b>-40/40/-70.023256525/-29.9368261101
<b>−Jm</b>0.1 <b>−B</b>a10 <b>−O</b>
>> map.ps</p>
<p style="margin-left:11%; margin-top: 1em">Suppose you
want to obtain only the constrained data values from an img
file, in lat/lon coordinates. Then run <b>img2mercgrd</b>
with the <b>−T</b>2 option, use <b><A HREF="grd2xyz.html">grd2xyz</A></b> to dump
the values, pipe through grep -v NaN to eliminate NaNs, and
pipe through <b><A HREF="mapproject.html">mapproject</A></b> with the inverse projection
as above.</p>
<a name="SEE ALSO"></a>
<h2>SEE ALSO</h2>
<p style="margin-left:11%; margin-top: 1em"><i><A HREF="GMT.html">GMT</A></i>(1),
<i><A HREF="grdproject.html">grdproject</A></i>(1), <i><A HREF="mapproject.html">mapproject</A></i>(1)</p>
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