Example Fortran program that reads a GrADS binary file

read_grads_example.f90

program read_grads_example

  ! Example program that reads a flat binary file
  ! 
  ! With GNU Fortran, compile as:
  !
  !   gfortran -fconvert=big-endian read_grads_example.f90
  !
  ! With Intel Fortran, compile as:
  !
  !   ifort -convert big_endian -assume byterecl read_grads_example.f90

  ! use portable 4-byte reals
  use iso_fortran_env, only: real32

  implicit none

  character(len=*), parameter :: filename = 'taucw.grd' ! file to read
  integer, parameter :: im = 2880, jm = 1440, nm = 60 ! grid dimensions
  integer :: fileunit
  integer :: record_length

  real(kind=real32) :: field(im,jm,nm) ! 3-d array to store data

  record_length = storage_size(field) / 8 * size(field)
  open(newunit=fileunit, file=filename, access='direct', recl=record_length)
  read(unit=fileunit, rec=1) field
  close(fileunit)

  write(*,*) 'Min / max: ', minval(field), maxval(field)

end program read_grads_example

Worked great! Thank you :)

Great contribution!
One short question on line 25: I don't see the declaration of storage_size. What is it? And why is record_length = storage_size(field) / 8 * size(field)?
Best
Roberto

Thank you @robetatis. I'm happy you found it useful.

I don't see the declaration of storage_size. What is it?

storage_size is a built-in function that returns the size of its argument in bits. Pay special attention to "The result value is the size expressed in bits for an element of an array that has the dynamic type and type parameters of A", from the description on that page. Even if you pass an array to storage_size, the result is the number of bits occupied by one element of that array.

And why is record_length = storage_size(field) / 8 * size(field)?

Here the record length is expressed in bytes. As storage_size returns the number of bits and there are 8 bits in a byte, storage_size(field) / 8 is the storage size of one element in bytes, and then storage_size(field) / 8 * size(field) is the storage size of the whole array in bytes. We store one array per record, so that's our record length.

Note that by default gfortran assumes the value of recl in bytes, whereas ifort assumes it in number of elements. This is why we specify -assume byterecl flag with ifort--to have a portable solution between two compilers.

Thanks @milancurcic !