program read_ssh
c
c   Sample program showing read statements used to read in Topex data.
c
c   The Topex data is on a 540 X 270 element grid with the origin at
c   90N latitude and 0 degrees longitude.  There are 12 monthly data
c   files for 1993
c
c





      real*4 ssh(540,270)       ! Topex data in cm.
                                ! -9999 indicates land
                                ! 9999 indicates no data
      integer*2 iyr,imon        ! year and month of data set

 

      real j_to_lat, i_to_lon      ! conversion functions
      integer lat_to_j, lon_to_i



      open(8,file='ssh9301.dat',form='unformatted')
      read(8) iyr,imon
      write(*,*) iyr,imon	  ! should be 93 and 1 for first Topex file
      read(8) ssh
      close(8)

      stop
      end



c
c
c  The following four functions can be used convert    grid  <----> lat/lon.
c
c

 
      integer function lat_to_j(lat)
c
c     Function takes a latitude and returns the index of the grid row that
c     latitude falls in.  NOTE: assumes 270 grid cells over 180 degrees and
c     that latitude varies from -90 to 90.
c
      real lat
      lat_to_j=int((90.-lat)*3.0/2.0)+1
      return
      end




      integer function lon_to_i(lon)
c
c     Function takes a longitude and returns the index of the grid column
c     that the longitude falls in.  NOTE: assumes 540 grid cells over 360
c     degrees and that longitude varies from 0 to 360.
c
      real lon
      lon_to_i=int(lon*3.0/2.0)+1
      return
      end



      real function j_to_lat(j)
c
c     Function takes the index of a grid row and returns the latitude of the
c     center of the grid cells in that row.  NOTE: assumes 270 grid cells
c     over 180 degrees and that latitude varies from -90 to 90.
c
      integer j
      j_to_lat=90.-((float(j)-.5)*2.0/3.0)
      return
      end



      real function i_to_lon(i)
c
c     Function takes the index of a grid column and returns the longitude of
c     the center of the grid cells in that column.  NOTE: assumes 1080 grid
c     cells over 360 degrees and that longitude varies from 0 to 360.
c
      integer i
      i_to_lon=(float(i)-.5)*2.0/3.0
      return
      end