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.

              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_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

       -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

       -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


       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(GMTMANSECTION), img2mercgrd(GMTMANSECTION)

GMT4.0                            1 Oct 2004            IMG2GRD(GMTMANSECTION)

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