conventions...

• To: netcdfgroup@xxxxxxxxxxxxxxxx (netcdfgroup)
• Subject: conventions...
• From: Rich Schramm <scri@xxxxxxxxx>
• Date: Wed, 21 Oct 1992 9:38:33 PDT

I have been following the "conventions" discussion on this mail
and would like to add my few cents worth re variable names etc.

I fall somewhere between Tim Holt and Harry Edmon on names and
attributes.   I think we need to look at what the purpose of the
'name' is.  In other words, I think we are trying to find one
solution to three problems.... 

1) If we want to be able to identify a variable as ocean_temperature
   or ocean_salinity to an application such as a plot program
   then a simple data type or name is sufficient as long as it uniquely
   identifies the variable.  Whether it was measured by CTD or a
   current meter or whatever is irrelevant.

2) Temperature is not only Temperature.  What are the units and
   which defining temperature scale (IPTS-1948, IPTS-1968 or IPTS-1990)
   For salinity also ... PPT or IPS-1978?  Most users will only want to
   know they have temperatures or salinities of the same type.  A robust
   application sould be able to make the appropriate conversions required
   if provided with these pieces of information.
      1) The unique identifier datatype (ie. ocean_temperature),
      2) The units (such as degrees C)
      3) The defining scale (ie. is it on IPTS-68 or IPTS-90)
      4) other required attributes...

  In physical oceanography the 3d piece of information is critical!
  The scale which defines temperature and salinity MUST be known
  if one is to use the data to compute any other parameters such as
  density.

  Item 4 can be critical depending on the variable.  For example,
  conductivity measurements (required to compute salinity) are usually
  reported as Siemens/meter (or mS/cm).  It is impossible to compute
  salinity properly without knowing the value of conductivity at
  a known point we call C(35,15,0) which was used when the instrument
  was calibrated.  This absolute value is not agreed upon internationally
  so salinity is defined in terms of conductivity ratio which side-steps
  that issue but requires us to drag whatever value we pick around with us.
  
3) If we want to be able to use netCDF files to archive our datasets
   in a self-describing way, we need a great deal of additional meta-data
   to describe them.  This is where I think Tim Holt is headed with
   his XMIDAS format and is a great example of the potential of netCDF
   to solve a nasty problem.  However I think its fair to say there will
   always be variable attributes which are application specific which
   most of us will have little use for.  Maybe what is needed is an
   attribute which is something like 'contributors_datatype' which
   can handle things like Tim's "SBE_Sea_Surface_Temperature1" which
   still has data_type_name of 'Temperature', units 'Degrees C' and
   defining_scale of 'IPTS-90'

4) I also am curious as to what is meant by Sigma Coordinate Systems..

   
Rich Schramm.
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