Extract a subset from an img file in Mercator or Geographic format


gmt img2grd imgfile -Ggrdfile -Rregion [ -D[minlat/maxlat] ] [ -E ] [ -Iinc ] [ -F ] [ -M ] [ -Nnavg ] [ -S[scale] ] [ -Ttype ] [ -V[level] ] [ -Wmaxlon ] [ -nflags ] [ --PAR=value ]

Note: No space is allowed between the option flag and the associated arguments.


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


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.

-Goutgrid[=ID][+ddivisor][+ninvalid] [+ooffset|a][+sscale|a] [:driver[dataType][+coptions]]

Give the name of the output grid file. Optionally, append =ID for writing a specific file format (See full description). The following modifiers are supported:

  • +d - Divide data values by given divisor [Default is 1].

  • +n - Replace data values matching invalid with a NaN.

  • +o - Offset data values by the given offset, or append a for automatic range offset to preserve precision for integer grids [Default is 0].

  • +s - Scale data values by the given scale, or append a for automatic scaling to preserve precision for integer grids [Default is 1].

Note: Any offset is added before any scaling. +sa also sets +oa (unless overridden). To write specific formats via GDAL, use = gd and supply driver (and optionally dataType) and/or one or more concatenated GDAL -co options using +c. See the “Writing grids and images” cookbook section for more details.


Specify the region of interest.

The region may be specified in one of several ways:

  1. -Rwest/east/south/north[+uunit]. This is the standard way to specify geographic regions when using map projections where meridians and parallels are rectilinear. The coordinates may be specified in decimal degrees or in [±]dd:mm[][W|E|S|N] format. Optionally, append +uunit to specify a region in projected units (e.g., UTM meters) where west/east/south/north are Cartesian projected coordinates compatible with the chosen projection (-J) and unit is an allowable distance unit; we inversely project to determine the actual rectangular geographic region.

  2. -Rwest/south/east/north+r. This form is useful for map projections that are oblique, making meridians and parallels poor choices for map boundaries. Here, we instead specify the lower left corner and upper right corner geographic coordinates, followed by the modifier +r. This form guarantees a rectangular map even though lines of equal longitude and latitude are not straight lines.

  3. -Rg or -Rd. These forms can be used to quickly specify the global domain (0/360 for -Rg and -180/+180 for -Rd in longitude, with -90/+90 in latitude).

  4. -Rcode1,code2,…[+e|r|Rincs]. This indirectly supplies the region by consulting the DCW (Digital Chart of the World) database and derives the bounding regions for one or more countries given by the codes. Simply append one or more comma-separated countries using the two-character ISO 3166-1 alpha-2 convention. To select a state within a country (if available), append .state, e.g, US.TX for Texas. To specify a whole continent, prepend = to any of the continent codes AF (Africa), AN (Antarctica), AS (Asia), EU (Europe), OC (Oceania), NA (North America), or SA (South America). The following modifiers can be appended:

    • +r to adjust the region boundaries to be multiples of the steps indicated by inc, xinc/yinc, or winc/einc/sinc/ninc [default is no adjustment]. For example, -RFR+r1 will select the national bounding box of France rounded to nearest integer degree.

    • +R to extend the region outward by adding the amounts specified by inc, xinc/yinc, or winc/einc/sinc/ninc [default is no extension].

    • +e to adjust the region boundaries to be multiples of the steps indicated by inc, xinc/yinc, or winc/einc/sinc/ninc, while ensuring that the bounding box extends by at least 0.25 times the increment [default is no adjustment].

  5. -Rjustifylon0/lat0/nx/ny, where justify 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). The two character code justify indicates which point on a rectangular region region the lon0/lat0 coordinates refer to and the grid dimensions nx and ny are used with grid spacings given via -I to create the corresponding region. This method can be used when creating grids. For example, -RCM25/25/50/50 specifies a 50x50 grid centered on 25,25.

  6. -Rgridfile. This will copy the domain settings found for the grid in specified file. Note that depending on the nature of the calling module, this mechanism will also set grid spacing and possibly the grid registration (see Grid registration: The -r option).

  7. -Ra[uto] or -Re[xact]. Under modern mode, and for plotting modules only, you can automatically determine the region from the data used. You can either get the exact area using -Re [Default if no -R is given] or a slightly larger area sensibly rounded outwards to the next multiple of increments that depend on the data range using -Ra.

Optional Arguments


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.


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).


Translate the x and y Mercator coordinates so that they are relative to the lower left corner, which then is (0,0). Requires -M.


Indicate inc as the width of an input img pixel in minutes of longitude [Default is 2]. Append m [Default] or s to indicate unit. Not usually required since we can determine the pixel size from inspection of the size.


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 relative to projection center using a -Jm1 projection. See -F to set coordinates to instead be distances 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.


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].


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.


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].


Select verbosity level [w]. Particularly recommended here, as it is helpful to see how the coordinates are adjusted. (See full description) (See cookbook information).


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 exit (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 exit.

-? or no arguments

Print a complete usage (help) message, including the explanation of all options, then exit.


Temporarily override a GMT default setting; repeatable. See gmt.conf for parameters.

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

gmt img2grd world_grav.img.16.1 -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:

gmt img2grd world_grav.img.16.1 -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 -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 -R-40/40/-99.4333333333/-31.4666666667. 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, we can do this:

gmt set PROJ_ELLIPSOID Sphere

gmt mapproject -R-40/40/-70.0004681551/-29.9945810754 -Jm1i -C ship.lonlatgrav | \
          gmt grdtrack | \
          gmt mapproject -R-40/40/-70.0004681551/-29.9945810754 -Jm1i -I -C > 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 -R-40/40/-70.0004681551/-29.9945810754 -Jm1i -I -D2m

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 | \
    gmt mapproject -R-40/40/-70.0004681551/-29.994581075 -Jm1i -I | \
    gmt surface -R-40/40/-70/70 -I2m

To make a Mercator map of the above region, suppose our gmt.conf value for PROJ_LENGTH_UNIT is inch. Then since the above 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., plot, coast) 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 we realize that 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 -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 -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 begin
  gmt grdimage -Cgrav.cpt -Jx0.1i
  gmt basemap -R-40/40/-70.023256525/-29.9368261101 -Jm0.1i -Ba10
gmt end

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