img2grd - Extract region of img in Mercator or geographic form
img2grd imgfile -Ggrdfile -Rwest/east/south/north -Ttype [ -E ] [ -L ]
[ -M ] [ -Nnavg ] [ -Sscale ] [ -V ] [ -mminutes ] [ -xmaxlon ] [
img2grd is a front-end to img2mercgrd which reads an img format file
and creates a grdfile. The -M option dictates whether or not the
Spherical Mercator projection of the img file is preserved.
An img format file such as the marine gravity or seafloor topog-
raphy fields estimated from satellite altimeter data by Sandwell
and Smith. If the user has set an environment variable
GMT_IMGDIR, then img2mercgrd will try to find imgfile in
$GMT_IMGDIR; else it will try to open imgfile directly.
-G grdfile is the name of the output grdfile.
-R west, east, south, and north specify the Region of interest. To
specify boundaries in degrees and minutes, use the dd:mm format.
-T type handles the encoding of constraint information. type = 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. type > 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: -T1 gets data values at all
points, -T2 gets data values at constrained points and NaN at
interpolated points; -T3 gets 1 at constrained points and 0 at
-E Can be used when -M is not set to force the final grid to have
the exact same region as requested with -R. 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 futhermore 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.
-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
-M Output a Spherical Mercator grid [Default is a geographic
-N Average the values in the input img pixels into navg by navg
-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.
-m Indicate minutes as the width of an input img pixel in minutes
of longitude. [Default is 2.0].
-x Indicate maxlon as the maximum longitude extent of the input img
file. Versions since 1995 have had maxlon = 360.0, while some
earlier files had maxlon = 390.0. [Default is 360.0].
-y Indicate minlat/maxlat as the latitude extent of the input img
file. All versions to date have used -72.006/72.006. [Default is
To extract data in the region -R-40/40/-70/-30 from world_grav.img.7.2
and preserve the Mercator gridding:
img2grd world_grav.img.7.2 -Gmerc_grav.grd -R-40/40/-70/-30 -M -T1 -V
Without the -M option the same command will yield a geographic grid.
GMT4.0 1 Oct 2004 IMG2GRD(GMTMANSECTION)
Man(1) output converted with