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)