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)

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