Packing data: scale_factor add_offset

• To: "netcdfgroup@xxxxxxxxxxxxxxxx" <netcdfgroup@xxxxxxxxxxxxxxxx>
• Subject: Packing data: scale_factor add_offset
• From: Choo Heng Kek <hengkek@xxxxxxxxxx>
• Date: Mon, 13 May 2002 17:13:03 +0800

Hi

I'm a relatively new netCDF user.  Having read the netCDF guide plus
some entries in the support and user forum, I have a burning question
regarding data packing using 'scale_factor' and 'add_offset'.  I've seen
fortran code which implement the packing with code like the following
sample snippet:

        INTEGER*2 X_PACKED
        REAL X, OFFSET, SCALE

        X = 10.23456
        OFFSET = 9.0
        SCALE = 0.23123

        X_PACKED = (X-OFFSET)*SCALE
        ! Then proceed to store X_PACKED in netCDF form.
        . . .

What I don't understand is how this can be "correct".  When you assign a
REAL to an INTEGER, the value of the REAL gets truncated before being
stored in the INTEGER.  Thus, the original value of X cannot be
'reconstructed' from the corresponding INTEGER*2 variable (X_PACKED) in
the netCDF file.

I understand the intent is to store only the 2 bytes that represents the
scaled-down REAL value.  But as far as I know, this is not possible in
Fortran -- as far as simply assigning values to variables is concerned.

Or is the above fortran implementation of packing in netCDF completely
off track?  Am I missing anything?
What is the common practice of netCDF packing in Fortran?  
Any sample Fortran code for netCDF data packing?

Thanks for any explanations or tips.

-Kek
Tropical Marine Sc Institute
Singapore

• Follow-Ups:
  • Packing data: scale_factor add_offset
    • From: Choo Heng Kek