[CF-metadata] attributes for min/max data values for visualization

[Stephen Pascoe]

I would also like to express some reservations about the usefulness of 
simple  min/max attributes for the purpose John suggests (calculating 
appropriate colourbar ranges in visualisations).  My experience is that 
a single pair of values is only relevant at a particular scale.  Once 
you start subsetting a domain there's a good chance the actual min/max 
will be substantially different.

For instance, taking an example from the IPCC data distribution centre, 
we have a diurnal temperature range field with a min--max of ca. 1--40 
deg_c.  However, half of this range is due to the variation over 
Greenland during the winter.  Subset anywhere else and the max is more 
like 20 deg_c.  Similarly, the maximum temperature field varies between 
ca. -50 and +45 deg_c but most subselections in time or space only cover 
a fraction of this range.

There is no harm in having optional CF attributes for min and max but 
I'm not convinced it will solve the problem.  I like Steve's approach of 
providing the extrema in auxiliary variables.  In CF min/max can be 
specified using the cell_methods attribute.  What would be needed 
something like Steve's "parent" attribute to specify two variables 
represent the same field (with different cell_methods).

Cheers,
Stephen.

---
Stephen Pascoe  01235 445980
British Atmospheric Data Centre
Rutherford Appleton Laboratory, CCLRC


Steve Hankin wrote:
> Hi Jon,
>
> Can you really get away with simple attributes to contain the guidance 
> on extrema?  For example, if this is 3D data (has a Z axis) and you 
> are interested in visualizations at different depths (heights), then 
> the "recommended" contour ranges might well need to be different for 
> each depth (illustrating why we have tended to back away from this 
> problem for such a long time).
>
> Might it make sense to think more in terms of min/max values stored in 
> new variables and identified by standard names.  Here is a conceptual 
> example for discussion (not a formal proposal, so please cut me slack):
>
>     variables:
>       float temperature(time,pres,lat,lon) ;
>       float temp_min(pres) ;
>         temp_min:parent = "temperature" ;
>         temp_min:standard_name = "minimum_over_domain" ;
>       float temp_max(pres) ;
>         temp_max:parent = "temperature" ;
>         temp_max:standard_name = "maximum_over_domain" ;
>
> This approach offers a lot more flexibility.  Does the scope of the 
> problem that needs to be solved require this flexibility?
>
>     - Steve
>
> ==================================================
>
> Jon Blower wrote:
>> Dear Jonathan,
>>
>> OK, that sounds fine too.  How do we move forward to incorporate this
>> into the CF standard?
>>
>> Thanks, Jon
>>
>> On 3/28/07, Jonathan Gregory <j.m.gregory at reading.ac.uk> wrote:
>>   
>>> Dear Jon and Phil
>>>
>>> I'd suggest actual_min and actual_max, because they would complement the
>>> already defined (Unidata standard) valid_min and valid_max.
>>>
>>> Cheers
>>>
>>> Jonathan
>>>
>>>     
>>
>>
>>   
>
> -- 
> Steve Hankin, NOAA/PMEL -- Steven.C.Hankin at noaa.gov
> 7600 Sand Point Way NE, Seattle, WA 98115-0070
> ph. (206) 526-6080, FAX (206) 526-6744
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