Provided by: gmt-common_5.4.3+dfsg-1_all 
NAME
img2grd - Extract subset of img file in Mercator or Geographic format
SYNOPSIS
img2grd imgfile -Ggrdfile
-Rregion
-Ttype [ -C ] [ -D[minlat/maxlat] ] [ -E ] [ -Iminutes ] [ -M ] [ -Nnavg ] [ -S[scale] ] [
-V[level] ] [ -Wmaxlon ] [ -nflags ]
Note: No space is allowed between the option flag and the associated arguments.
DESCRIPTION
img2grd reads an img format file, extracts a subset, and writes it to a grid file. The -M option dictates
whether or not the Spherical Mercator projection of the img file is preserved or if a Geographic grid
should be written by undoing the Mercator projection. If geographic grid is selected you can also request
a resampling onto the exact -R given.
REQUIRED ARGUMENTS
imgfile
A Mercator 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_DATADIR, then img2grd will try to find imgfile in $GMT_DATADIR; else it will try to open
imgfile directly.
-Ggrdfile
grdfile is the name of the output grid file.
-Rwest/east/south/north[/zmin/zmax][+r][+uunit]
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 for grid
creation, give Rcodelon/lat/nx/ny, where code is a 2-character combination of L, C, R (for left,
center, or right) and T, M, B for top, middle, or bottom. e.g., BL for lower left. This indicates
which point on a rectangular region the lon/lat coordinate refers to, and the grid dimensions nx
and ny with grid spacings via -I is used to create the corresponding region. Alternatively,
specify the name of an existing grid file and the -R settings (and grid spacing, if applicable)
are copied from the grid. Appending +uunit expects projected (Cartesian) coordinates compatible
with chosen -J and we inversely project to determine actual rectangular geographic region. For
perspective view (-p), optionally append /zmin/zmax. In case of perspective view (-p), a z-range
(zmin, zmax) can be appended to indicate the third dimension. This needs to be done only when
using the -Jz option, not when using only the -p option. In the latter case a perspective view of
the plane is plotted, with no third dimension.
OPTIONAL ARGUMENTS
-C Set the x and y Mercator coordinates relative to projection center [Default is relative to lower
left corner of grid]. Requires -M.
-D[minlat/maxlat]
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]. Not usually required since we
can determine the extent from inspection of the file size.
-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, -R0/45/45/72).
-I Indicate minutes as the width of an input img pixel in minutes of longitude. [Default is 2.0]. Not
usually required since we can determine the pixel size from inspection of the size.
-M Output a Spherical Mercator grid [Default is a geographic lon/lat grid]. The Spherical Mercator
projection of the img file is preserved, so that the region -R set by the user is modified
slightly; the modified region corresponds to the edges of pixels [or groups of navg pixels]. The
grid file header is set so that the x and y axis lengths represent distance from the west and
south edges of the image, measured in user default units, with -Jm1 and the adjusted -R. By
setting the default PROJ_ ELLIPSOID = Sphere, the user can make overlays with the adjusted -R so
that they match. See EXAMPLES below. The adjusted -R is also written in the grid header remark, so
it can be found later. See -C to set coordinates relative to projection center.
-Nnavg 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[scale]
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 [-S1]; free-air gravity files in
mGal*10 [-S0.1 to get mGal]; vertical deflection files in micro-radians*10 [-S0.1 to get
micro-radians], vertical gravity gradient files in Eotvos*10 [-S0.1 to get Eotvos, or -S0.01 to
get mGal/km]). If no scale is given we try to determine the scale by examining the file name for
clues.
-Ttype 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 [Default is 1].
-V[level] (more …)
Select verbosity level [c]. Particularly recommended here, as it is helpful to see how the
coordinates are adjusted.
-Wmaxlon
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].
-n[b|c|l|n][+a][+bBC][+c][+tthreshold] (more …)
Select interpolation mode for grids.
-^ or just -
Print a short message about the syntax of the command, then exits (NOTE: on Windows just use -).
-+ or just +
Print an extensive usage (help) message, including the explanation of any module-specific option
(but not the GMT common options), then exits.
-? or no arguments
Print a complete usage (help) message, including the explanation of all options, then exits.
GEOGRAPHIC EXAMPLES
The -M option should be excluded if you need the output grid to be in geographic coordinates. To extract
data in the region -R-40/40/-70/-30 from world_grav.img.7.2 and reproject to yield geographic
coordinates, you can try
img2grd world_grav.img.16.1 -Gmerc_grav.nc -R-40/40/-70/-30 -V
Because the latitude spacing in the img file is equidistant in Mercator units, the resulting grid will
not match the specified -R exactly, and the latitude spacing will not equal the longitude spacing. If you
need an exact match with your -R and the same spacing in longitude and latitude, use the -E option:
img2grd world_grav.img.16.1 -Gmerc_grav.nc -R-40/40/-70/-30 -E -V
MERCATOR EXAMPLES
Since the img files are in a Mercator projection, you should NOT extract a geographic grid if your plan
is to make a Mercator map. If you did that you end of projecting and reprojection the grid, losing
short-wavelength detail. Better to use -M and plot the grid using a linear projection with the same scale
as the desired Mercator projection (see GMT Example 29). To extract data in the region -R-40/40/-70/-30
from world_grav.img.7.2, run
gmt img2grd -M world_grav.img.7.2 -Gmerc_grav.nc -R-40/40/-70/-30 -V
Note that the -V option tells us that the range was adjusted to -R-40/40/-70.0004681551/-29.9945810754.
For scripting purposes we can extract this original region string using grdinfo -Ii. Furthermore, we can
also use grdinfo to find that the grid file header shows its region to be -R0/80/0/67.9666667. This is
the range of x,y we will get from a Spherical Mercator projection using
-R-40/40/-70.0004681551/-29.9945810754 and -Jm1. Thus, to take ship.lonlatgrav and use it to sample the
merc_grav.nc, we can do this:
gmt set PROJ_ELLIPSOID Sphere
gmt mapproject -R-40/40/-70.0004681551/-29.9945810754 -Jm1i ship.lonlatgrav | \
gmt grdtrack -Gmerc_grav.nc | gmt mapproject \
-R-40/40/-70.0004681551/-29.9945810754 -Jm1i -I > ship.lonlatgravsat
It is recommended to use the above method of projecting and unprojecting the data in such an application,
because then there is only one interpolation step (in grdtrack). If one first tries to convert the grid
file to lon,lat and then sample it, there are two interpolation steps (in conversion and in sampling).
To make a lon,lat grid from the above grid we can use
gmt grdproject merc_grav.nc -R-40/40/-70.0004681551/-29.9945810754 -Jm1i -I -D2m -Ggrav.nc
In some cases this will not be easy as the -R in the two coordinate systems may not align well. When this
happens, we can also use (in fact, it may be always better to use)
gmt grd2xyz merc_grav.nc | gmt mapproject \
-R-40/40/-70.0004681551/-29.994581075 -Jm1i -I | \
gmt surface -R-40/40/-70/70 -I2m -Ggrav.nc
To make a Mercator map of the above region, suppose our gmt.conf value for PROJ_LENGTH_UNIT is inch. Then
since the above merc_grav.nc file is projected with -Jm1i it is 80 inches wide. We can make a map 8
inches wide by using -Jx0.1i on any map programs applied to this grid (e.g., grdcontour, grdimage,
grdview), and then for overlays which work in lon,lat (e.g., psxy, pscoast) we can use the above adjusted
-R and -Jm0.1 to get the two systems to match up.
However, we can be smarter than this. Realizing that the input img file had pixels 2.0 minutes wide (or
checking the nx and ny with grdinfo merc_grav.nc) we realize that merc_grav.nc used the full resolution
of the img file and it has 2400 by 2039 pixels, and at 8 inches wide this is 300 pixels per inch. We
decide we do not need that many and we will be satisfied with 100 pixels per inch, so we want to average
the data into 3 by 3 squares. (If we want a contour plot we will probably choose to average the data much
more (e.g., 6 by 6) to get smooth contours.) Since 2039 isn’t divisible by 3 we will get a different
adjusted -R this time:
gmt img2grd -M world_grav.img.7.2 -Gmerc_grav_2.nc -R-40/40/-70/-30 -N3 -V
This time we find the adjusted region is -R-40/40/-70.023256525/-29.9368261101 and the output is 800 by
601 pixels, a better size for us. Now we can create an artificial illumination file for this using
grdgradient:
gmt grdgradient merc_grav_2.nc -Gillum.nc -A0/270 -Ne0.6
and if we also have a CPT called “grav.cpt” we can create a color shaded relief map like this:
gmt grdimage merc_grav_2.nc -Iillum.nc -Cgrav.cpt -Jx0.1i -K > map.ps
gmt psbasemap -R-40/40/-70.023256525/-29.9368261101 -Jm0.1i -Ba10 -O >> map.ps
Suppose you want to obtain only the constrained data values from an img file, in lat/lon coordinates.
Then run img2grd with the -T2 option, use grd2xyz to dump the values, pipe through grep -v NaN to
eliminate NaNs, and pipe through mapproject with the inverse projection as above.
SEE ALSO
gmt
COPYRIGHT
2018, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe
5.4.3 Jan 03, 2018 IMG2GRD(1gmt)