Dear Nan,
(Wow, I actually typed your name as "NaN" to start with, just shows
how I get buried in code! ;-)
Thanks for this. Yes, you're right, we would need to be careful with
data subsets, either recalculating or omitting these values. As for
"actual_valid_min/max", I see what you are saying but I actually find
that nomenclature more confusing (it seems to imply that the other
valid_min/max is somehow fictitious). Others might disagree.
Anyway, I have posted a Trac ticket on this matter
http://cf-pcmdi.llnl.gov/trac/ticket/31, which I guess is where future
discussion will happen.
Cheers, Jon
On Fri, Jun 20, 2008 at 4:11 PM, Nan Galbraith <ngalbraith at whoi.edu> wrote:
> This "breaks" when you subset data via opendap, for example,
> doesn't it? I agree that it could be useful for discovery and for
> apps like contouring, but it will need to be used carefully.
>
> Although the name would be "actual_min" (or max), the definition
> would need to specify that it include only valid data values, otherwise
> it won't be useful. Maybe actual_valid_min (or max) would be more
> clear, and would save the overhead that the use of valid_min/max
> incurs.
>
> Cheers - Nan
>
>> Since Jon mentions me in this thread I'll put in my brief contribution,
>> although I agree very much with the discussion so far.
>>
>> Despite the limitations I think we really need these attributes. At
>> BADC we are either putting custom attributes in the NetCDF or setting
>> approximate min/max in configuration files. A standard approach would
>> be a great improvement.
>>
>> I too like actual_min/max.
>>
>> Cheers,
>> Stephen.
>>
>> ---
>> Stephen Pascoe +44 (0)1235 445980
>> British Atmospheric Data Centre
>> Rutherford Appleton Laboratory
>>
>> -----Original Message-----
>> From: cf-metadata-bounces at cgd.ucar.edu
>> [mailto:cf-metadata-bounces at cgd.ucar.edu] On Behalf Of John Caron
>> Sent: 10 June 2008 00:54
>> Cc: cf-metadata at cgd.ucar.edu
>> Subject: Re: [CF-metadata] attributes for min/max data valuesfor
>> visualization
>>
>> I also like "actual_min" and "actual_max": short and to the point.
>>
>> One point Ill make is that I see a number of well thought out files
>> using valid_min and valid_max when they really mean actual_min and
>> actual_max. On first glance this may seem harmless, but it triggers (in
>> the netcdf-java library default behavior) a costly scan of the data
>> looking for invalid data, ie values outside that range.
>>
>> So adding actual_min/max as recommended attributes should help users
>> from mistakenly using valid min/max. And its very useful for visualizers
>> to create color scales etc.
>>
>> Julia Collins wrote:
>>
>>> On Mon, 9 Jun 2008, Jon Blower wrote:
>>>
>>>
>>>> I have rediscovered this conversation in the depths of my inbox and
>>>> thought it might be worth resurrecting. To summarize the discussion
>>>> so far:
>>>> 1) I proposed two new standard optional CF attributes (called
>>>> something like minimum_data_value and maximum_data_value) that would
>>>> be attached to a variable in a NetCDF file, redundantly holding its
>>>> actual min and max values in that file. [...] As well as helping with
>>>>
>>
>>
>>>> data mining applications, this would act as a hint to visualization
>>>> packages to provide a first attempt at automatically defining a
>>>> colour scale range for sensible portrayal. Note that this attribute
>>>> pair is distinct in purpose from valid_min and valid_max (which
>>>> contains theoretical extrema, beyond which data is considered
>>>>
>> invalid).
>>
>>>>
>>>>
>>> I support the idea of formalizing these sorts of attributes in some
>>> way. However, I would prefer a simpler name(s). For example:
>>>
>> "actual_min"
>>
>>> and "actual_max." Less typing :-), and more like the existing
>>>
>> "valid_min"
>>
>>> and "valid_max" attributes.
>>>
>
>
> --
> *******************************************************
> * Nan Galbraith (508) 289-2444 *
> * Upper Ocean Processes Group Mail Stop 29 *
> * Woods Hole Oceanographic Institution *
> * Woods Hole, MA 02543 *
> *******************************************************
>
>
>
> _______________________________________________
> CF-metadata mailing list
> CF-metadata at cgd.ucar.edu
> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
>
--
--------------------------------------------------------------
Dr Jon Blower Tel: +44 118 378 5213 (direct line)
Technical Director Tel: +44 118 378 8741 (ESSC)
Reading e-Science Centre Fax: +44 118 378 6413
ESSC Email: jdb at mail.nerc-essc.ac.uk
University of Reading
3 Earley Gate
Reading RG6 6AL, UK
--------------------------------------------------------------
Received on Mon Jun 23 2008 - 07:48:41 BST
