The NCDF_OPEN function opens an existing NetCDF file.
Result = NCDF_OPEN( Filename [, /NOWRITE | , /WRITE] )
If successful, the NetCDF ID for the file is returned.
A scalar string containing the name of the file to be opened.
Set this keyword to open an existing NetCDF file as read only. This is the default.
Set this keyword to open an existing NetCDF file for both writing and reading.