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  ]  [  -ymin-
       lat/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 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.


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  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 grd
              file. [Default is 1.0].  (img topo files  are  stored  in  (cor-
              rected)  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"].  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
              -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 geographic grid.


SEE ALSO

       gmt(GMTMANSECTION), img2mercgrd(GMTMANSECTION)



                                  1 Jan 2004                        IMG2GRD(l)

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