.TH IMG2GRD l "15 Oct 2001"
.SH NAME
img2grd \- Extract region of img in Mercator or geographic form
.SH SYNOPSIS
\fBimg2grd\fP \fIimgfile\fP \fB\-G\fP\fIgrdfile\fP \fB\-R\fP\fIwest/east/south/north\fP
\fB\-T\fP\fItype\fP [ \fB\-L\fP ] [ \fB\-M\fP ] [ \fB\-N\fP\fInavg\fP ] [ \fB\-S\fP\fIscale\fP ] [ \fB\-V\fP ]
[ \fB\-m\fP\fIminutes\fP ] [ \fB\-x\fP\fImaxlon\fP ] [ \fB\-y\fP\fIminlat/maxlat\fP ]
.SH DESCRIPTION
\fBimg2grd\fP is a front-end to \fBimg2mercgrd\fP which reads an img format file and creates a grdfile.
The \fB\-M\fP option dictates whether or not the Spherical Mercator
projection of the img file is preserved.
.TP
\fIimgfile\fP
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
GMT_IMGDIR, then \fBimg2mercgrd\fP will try to find \fIimgfile\fP in $GMT_IMGDIR; else it
will try to open \fIimgfile\fP directly.
.TP
.B \-G
\fIgrdfile\fP is the name of the output grdfile.
.TP
.B \-R
\fIwest, east, south,\fP and \fInorth\fP specify the Region of interest. To specify boundaries
in degrees and minutes, use the dd:mm format.
.TP
.B \-T
\fItype\fP handles the encoding of constraint information. \fItype\fP = 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. \fItype\fP > 0 indicates that constraint information is encoded (1995 and
later (current) versions of the img files) so that one may produce a grd file as follows:
\fB\-T\fP\fI1\fP gets data values at all points, \fB\-T\fP\fI2\fP gets data values at
constrained points and NaN at interpolated points; \fB\-T\fP\fI3\fP gets 1 at constrained
points and 0 at interpolated points.
.SH OPTIONS
.TP
.B \-L
With no other arguments, list all *.img files found in the directory pointed to by
$GMT_IMGDIR, or the current directory if not defined. Ignored if other options are
present on the command line.
.TP
.B \-M
Output a Spherical Mercator grid [Default is a geographic lon/lat grid]
.TP
.B \-N
Average the values in the input img pixels into \fInavg\fP by \fInavg\fP squares, and
create one output pixel for each such square. If used with \fB\-T\fP\fI3\fP it will
report an average constraint between 0 and 1. If used with \fB\-T\fP\fI2\fP the output
will be average data value or NaN according to whether average constraint is > 0.5.
\fInavg\fP must evenly divide into the dimensions of the imgfile in pixels.
[Default \fI1\fP does no averaging].
.TP
.B \-S
Multiply the img file values by \fIscale\fP before storing in grd 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.)
.TP
.B \-V
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.
.TP
.B \-m
Indicate \fIminutes\fP as the width of an input img pixel in minutes of longitude.
[Default is 2.0]
.TP
.B \-x
Indicate \fImaxlon\fP as the maximum longitude extent of the input img file. Versions
since 1995 have had \fImaxlon\fP = 360.0, while some earlier files had \fImaxlon\fP =
390.0. [Default is 360.0]
.TP
.B \-y
Indicate \fIminlat/maxlat\fP as the latitude extent of the input img file. All versions
to date have used -72.006/72.006. [Default is -72.006/72.006]
.SH EXAMPLES
To extract data in the region \fB\-R\fP-40/40/-70/-30 from \fIworld_grav.img.7.2\fP and preserve
the Mercator gridding, try
.br
.sp
img2grd world_grav.img.7.2 \fB\-G\fPmerc_grav.grd \fB\-R\fP-40/40/-70/-30 \fB\-M \-T\fP1 \fB\-V\fP
.br
.sp
Wothout the \fB\-M\fP option the same command will yield a geographic grid.
.SH "SEE ALSO"
.IR gmt (l),
.IR img2mercgrd (l)
