## NAME

           img2grd  - Extract region of img in Mercator or geographic
           form

## SYNOPSIS

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

## DESCRIPTION

           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 min
                  utes, use the dd:mm format.

           -T     type  handles  the  encoding of constraint informa
                  tion. type = 0 indicates that no  such  information
                  is  encoded in the img file (used for pre-1995 ver
                  sions 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.

## OPTIONS

           -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 geo
                  graphic 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 aver
                  aging].

           -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"].  Par
                  ticularly 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]

## EXAMPLES

           To  extract  data  in  the  region  -R-40/40/-70/-30  from
           world_grav.img.7.2 and preserve the Mercator gridding, try

           img2grd         world_grav.img.7.2         -Gmerc_grav.grd
           -R-40/40/-70/-30 -M -T1 -V

           Wothout the -M option the same command will yield  a  geo
           graphic grid.

## SEE ALSO

           gmt(l), img2mercgrd(l)

                               15 Oct 2001                 IMG2GRD(l)