NAME
img2grd - Extract region of img in Mercator or geographic
form
SYNOPSIS
img2grd imgfile -Ggrdfile -Rwest/east/south/north -Ttype [
-L ] [ -M ] [ -Nnavg ] [ -Sscale ] [ -V ] [ -mminutes ] [
-xmaxlon ] [ -yminlat/maxlat ]
DESCRIPTION
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.
imgfile
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
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 min
utes, use the dd:mm format.
-T type handles the encoding of constraint informa
tion. type = 0 indicates that no such information
is encoded in the img file (used for pre-1995 ver
sions 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 interpolated points.
OPTIONS
-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.
-M Output a Spherical Mercator grid [Default is a geo
graphic lon/lat grid]
-N Average the values in the input img pixels into
navg by navg squares, and create one output pixel
for each such square. If used with -T3 it will
report an average constraint between 0 and 1. If
used with -T2 the output will be average data value
or NaN according to whether average constraint is >
0.5. navg must evenly divide into the dimensions
of the imgfile in pixels. [Default 1 does no aver
aging].
-S Multiply the img file values by scale 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.)
-V Selects verbose mode, which will send progress
reports to stderr [Default runs "silently"]. Par
ticularly 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 -72.006/72.006]
EXAMPLES
To extract data in the region -R-40/40/-70/-30 from
world_grav.img.7.2 and preserve the Mercator gridding, try
img2grd world_grav.img.7.2 -Gmerc_grav.grd
-R-40/40/-70/-30 -M -T1 -V
Wothout the -M option the same command will yield a geo
graphic grid.
SEE ALSO
gmt(l), img2mercgrd(l)
15 Oct 2001 IMG2GRD(l)
Man(1) output converted with
man2html