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 ] [ -ymin-
lat/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 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
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 com-
mand line.
-M Output a Spherical Mercator grid [Default is a geographic
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 averaging].
-S Multiply the img file values by scale before storing in grd
file. [Default is 1.0]. (img topo files are stored in (cor-
rected) 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"]. 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 -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 geographic grid.
SEE ALSO
gmt(GMTMANSECTION), img2mercgrd(GMTMANSECTION)
VERSION DATE IMG2GRD(GMTMANSECTION)
Man(1) output converted with
man2html