Dear Martin

I'm still uneasy about it having to be the first bin, in particular, or are
you not set on that? If it can be identified from the coordinate value by
flags, it could be any bin.

I believe that a change to convention would be needed to allow flag values
to be used with coordinates, unless we've already agreed that in some ticket.

Best wishes

Jonathan

----- Forwarded message from martin.juc...@stfc.ac.uk -----

> Date: Wed, 12 Oct 2016 17:14:38 +0000
> From: martin.juc...@stfc.ac.uk
> To: cf-metadata@cgd.ucar.edu
> Subject: [CF-metadata]  Missing data bins in histograms
> 
> Dear Karl, Jonathan, Jim,
> 
> thanks for those comments.
> 
> The CMIP6 variable in question is clmisr 
> (http://clipc-services.ceda.ac.uk/dreq/u/59151ed6-9e49-11e5-803c-0d0b866b59f3.html)
>  with a coordinatte of 16 altitude bins 
> (http://clipc-services.ceda.ac.uk/dreq/u/dim:alt16.html ).
> 
> I'd be happy with Jim's proposed solution, which does not need any change to 
> the convention, though it may be a bit cryptic: all the examples in the 
> convention are for cases in which all array values are intended to match one 
> of the flag_values. Having an array which is a mixture of flags and "normal" 
> values would be a new usage.  We could, perhaps, introduce a consistency 
> problem: ticket 151 (http://cf-trac.llnl.gov/trac/ticket/151) explains how, 
> for variables with standard_name "area_type", flag_values and flag_meanings 
> can be used to encode the data, in which case it is the "flag_meanings" which 
> match the requirements of the standard name. Here, on the other hand, we want 
> the special bin to be the exception which is not described by the standard 
> name (altitude). So .. perhaps it is simpler to introduce a new attribute 
> name?
> 
> Concerning Jonathan and Karl's comments, the idea of calling it a 
> "missing_value" was a mistake I made, but it actually refers to locations 
> where cloud is detected but the height of the cloud cannot be retrieved.
> 
> The current proposal is to have a value of 0.0 in the coordinate and 
> (-99000.0,0.0) in the bounds of the special value "bin". I imagine these need 
> to be present, but I think their values are not going to mean anything.
> 
> It is certainly possible to do as Karl suggests and place an explanation in 
> the variable description. Having the special status of the first bin 
> explicitly flagged in way which can be easily picked up by software brings 
> added value.
> 
> regards,
> Martin
> 
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----- End forwarded message -----
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