img2grd - Extract region of img in Mercator or geographic form

img2grd imgfile -Ggrdfile -Rwest/east/south/north -Ttype [ -E ] [ -L ] [ -M ] [ -Nnavg ] [ -Sscale ] [ -V ] [ -mminutes ] [ -xmaxlon ] [ -yminlat/maxlat ]

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 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.

-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 futhermore 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.

-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 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 (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"]. 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].

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 -Gmerc_grav.grd -R-40/40/-70/-30 -M -T1 -V

Without the -M option the same command will yield a geographic grid.

gmt(1gmt), img2mercgrd(1gmt)