IMG2GRD(1) Generic Mapping Tools IMG2GRD(1)NAMEimg2grd - Extract region of img in Mercator or geographic form
SYNOPSISimg2grd imgfile -Ggrdfile -Rwest/east/south/north[r] -Ttype [ -C ] [
-D[minlat/maxlat] ] [ -E ] [ -L ] [ -M ] [ -Nnavg ] [ -Sscale ] [ -V ]
[ -Wmaxlon ] [ -mminutes ]
DESCRIPTIONimg2grd is a front-end to img2mercgrd which reads an img format file
and creates a grid file. 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_DATADIR, then img2mercgrd will try to find imgfile in
$GMT_DATADIR; else it will try to open imgfile directly.
-G grdfile is the name of the output grid file.
-R west, east, south, and north 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 r if lower left and
upper right map coordinates are given instead of w/e/s/n. The
two shorthands -Rg and -Rd 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
-R settings (and grid spacing, if applicable) are copied from
the grid.
-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 grid 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-C Set the x and y Mercator coordinates relative to projection cen‐
ter [Default is relative to lower left corner of grid].
Requires -M.
-D Use the extended latitude range -80.738/+80.738. Alternatively,
append minlat/maxlat as the latitude extent of the input img
file. [Default is -72.006/72.006].
-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 furthermore 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. In this case the region set by -R must be
given in multiples of the increment (.e.g, -R 0/45/45/72).
-L With no other arguments, list all *.img files found in the
directory pointed to by $GMT_DATADIR, 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 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 grid
file. [Default is 1.0]. For recent img files: img topo files
are stored in (corrected) meters [-S 1]; free-air gravity files
in mGal*10 [-S 0.1 to get mGal]; vertical deflection files in
microradians*10 [-S 0.1 to get microradians], vertical gravity
gradient files in Eotvos*50 [-S 0.02 to get Eotvos, or -S 0.002
to get mGal/km]).
-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].
-W 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].
EXAMPLES
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 -G merc_grav.grd -R-40/40/-70/-30 -M -T 1 -V
Without the -M option the same command will yield a geographic grid.
SEE ALSOGMT(1), img2mercgrd(1)GMT 4.5.14 1 Nov 2015 IMG2GRD(1)
