NAME

       img2grd - Extract region of img in Mercator or geographic form

SYNOPSIS

       img2grd imgfile -Ggrdfile -Rwest/east/south/north[r] -Ttype  [ -C ] [ -D[minlat/maxlat] ] [ -E ] [ -L ] [
       -M ] [ -Nnavg ] [ -Sscale ] [ -V ] [ -Wmaxlon ] [ -mminutes ]

DESCRIPTION

       img2grd  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  topography  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 center [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 ALSO

       GMT(1), img2mercgrd(1)

GMT 4.5.11                                         5 Nov 2013                                      IMG2GRD(1gmt)