Dear Karl

Thanks. I have reformulated principle (1), combining yours and mine, and 
stating the purpose at the start. I think "self-describing" means not using 
anything outside the file itself, which is stronger than what you suggested. Is 
this OK?

In response to your first additional point, I've appended a bit to principle 
(8). Thanks for your second additional point, which is important. I have 
inserted principle (3) about this. Finally, I have added principle (10), which 
is partly a corollary of (9), and partly something we've done for its own sake, 
often advocated by Steve Hankin.

Thus, here is the current proposal:

  (1) CF-netCDF metadata is designed to make each dataset self-describing, 
meaning that it should be interpretable without reference to resources outside 
itself. To achieve this purpose, CF-netCDF does not use codes, but instead 
relies on controlled vocabularies containing terms that are chosen as far as 
practically possible to be self-explanatory (and whose precise definitions are 
provided in CF documents).
  
  (2) The conventions are changed only as actually required by common 
use-cases, and not for needs which cannot be anticipated with certainty.
  
  (3) [New] In order to keep them logical, consistent in approach and as simple 
as possible, the netCDF conventions are devised with and within the conceptual 
framework of the CF data model.
  
  (4) The conventions should be practicable for both producers and users of 
data.
  
  (5) The metadata should be both easily readable by humans and easily parsable 
by programs.
  
  (6) [Slightly reordered] To avoid potential inconsistency within the 
metadata, the conventions should minimise redundancy.
  
  (7) The conventions should minimise the possibility for mistakes by 
data-writers and data-readers.
  
  (8) Conventions are provided to allow data-producers to describe the data 
they wish to produce, rather than attempting to prescribe what data they should 
produce; [new] consequently most CF conventions are optional.
  
  (9) Because many datasets remain in use for a long time after production, it 
is desirable that metadata written according to previous versions of the 
convention should also be compliant with and have the same interpretation under 
later versions.
  
  (10) [New] Because all previous versions must generally continue to be 
supported in software for the sake of archived datasets, and in order to limit 
the complexity of the conventions, there is a strong preference against 
introducing any new capability to the conventions when there is already some 
method that can adequately serve the same purpose (even if a different method 
would arguably be better than the existing one).
  
Cheers
  
Jonathan


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