Martin,
The two subfields are independent. You can have very bad quality data
and very bad weather at the same time. And that's how the flag masks and
flag values are supposed to work. The mask splits off bit regions that
are independent of one another. There is no ambiguity.
The possible options and the values masked by the flag masks of 3
(binary 0011) and 12 (binary 1100) are:
Weather Quality
Binary Value
Binary value & 3
Binary value & 12
very bad
very bad
0000
0
0
very bad
bad
0001
1
0
very bad
good
0010
2
0
very bad
very good
0011
3
0
bad very bad 0100
0
4
bad bad 0101
1
4
bad good 0110
2
4
bad very good 0111
3
4
good very bad 1000
0
8
good bad 1001
1
8
good good 1010
2
8
good
very good 1011
3
8
very good very bad 1100
0
12
very good bad 1101
1
12
very good good 1110
2
12
very good very good 1111
3
12
Grace and peace,
Jim
On 11/21/18 12:03 PM, Martin Juckes - UKRI STFC wrote:
> Hello Jim, Julien,
>
>
> I'm not sure .. I think the conformance might be right here and your flag_values should be 0,1,2,3, 4, 8,12,16, and flag_masks 3,3,3,3,28,28,28,28
>
>
> If, for instance, you very_bad_quality and very_bad_weather, then "var" should have value 4 = '00100000` in binary. Masked with 3 (11000000) gives zero, and masked with 28 (00111000) gives 4. Re-using the zero value would make zero ambiguous, so you need to start the 2nd sequence at 4.
>
>
> regards,
>
> Martin
>
> ________________________________
> From: CF-metadata <cf-metadata-bounces at cgd.ucar.edu> on behalf of Jim Biard <jbiard at cicsnc.org>
> Sent: 20 November 2018 16:51:24
> To: cf-metadata at cgd.ucar.edu
> Subject: Re: [CF-metadata] Multiple zeros in flag_values allowed?
>
>
> Julien,
>
> That's fine. The conformance document probably needs a better statement of the requirement when flag masks are used.
>
> Grace and peace,
>
> Jim
>
> On 11/20/18 11:40 AM, Julien Demaria wrote:
> Hi,
>
> We want to define a flags variable defining like that:
> var:flag_masks = 3, 3, 3, 3, 12, 12, 12, 12 ;
> var:flag_values = 0, 1, 2, 3, 0, 4, 8, 12 ;
> var:flag_meanings = ?very_bad_quality bad_quality good_quality very_good_quality
> very_bad_weather bad_weather good_weather very_good_weather? ;
>
> I understand from http://cfconventions.org/Conformance/conformance.html that it is not allowed to use several time the same value (here zero) in flag_values:
>
> Requirements:
>
> ? The flag_values attribute values must be mutually exclusive among the set of flag_values attribute values defined for that variable.
> So it means that for each new ?bits combination? in the flags definition we lost one of the combination because we cannot use zero more than one time?
> Do you confirm this? What is the reason?
>
> Thanks in advance,
> Julien
>
>
>
>
> _______________________________________________
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>
>
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