Re: [CF-metadata] Quality flag values for missing data

[Lowry, Roy K.]

Hi Randy,

The practice recommended for SeaDataNet is to specify a fill value for the flag 
that is defined in the flag convention as 'missing value'.  I always feel more 
comfortable with explicit semantics.

Cheers, Roy.


From: CF-metadata [cf-metadata-boun...@cgd.ucar.edu] On Behalf Of Randy Horne 
[rho...@excaliburlabs.com]
Sent: 23 August 2012 19:37
To: cf-metadata@cgd.ucar.edu
Cc: aschu...@harris.com; rhorn...@harris.com; ekenn...@aer.com
Subject: [CF-metadata] Quality flag values for missing data

What is the “generally accepted” approach for how a specific quality flag value 
should be assigned for a corresponding data value (in the corresponding data 
variable) that has a _FillValue ?

Is it sufficient that, because the data variable value is _FillValue, the 
corresponding quality flag value can be undefined ?



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Re: [CF-metadata] Quality flag values for missing data

[John Graybeal]

From the QARTOD meetings I attended and the vocabularies in our repository, 
I'm confident the oceanographic community uses a lot more QC flag approaches 
than listed so far.  So it is safe to say that there is no single approach.

Still, the approaches from Randy and Roy certainly are accepted by a whole lot 
of folks. If that meets your definition of the generally accepted approach, 
you're good to go.

John

On Aug 24, 2012, at 00:27, andrew walsh wrote:

 Randy,
 
 The oceanographic community uses a set of integer QC flags (0 to 9)
 See document http://www.nodc.noaa.gov/GTSPP/document/qcmans/MG22rev1.pdf
 and section 2 'Quality Flagging'.
 
 One of the flags is for a 'missing value' = 9. Would that suit your fill 
 value case?
 
 A netCDF sample in CDL is:
 
 double TEMP(TIME, DEPTH, LATITUDE, LONGITUDE) ;
 TEMP:standard_name = sea_water_temperature ;
 TEMP:units = Celsius ;
 TEMP:_FillValue = -99.99 ;
 TEMP:valid_min = -2. ;
 TEMP:valid_max = 40. ;
 TEMP:quality_control_set = 1. ;
 TEMP:ancillary_variables = TEMP_quality_control ;
 byte TEMP_quality_control(TIME, DEPTH, LATITUDE, LONGITUDE) ;
 TEMP_quality_control:long_name = quality control flag for temperature ;
 TEMP_quality_control:standard_name = sea_water_temperature status_flag ;
 TEMP_quality_control:quality_control_convention = IMOS standard using IODE 
 flags ;
 TEMP_quality_control:_FillValue = -9b ;
 TEMP_quality_control:valid_min = 0 ;
 TEMP_quality_control:valid_max = 9 ;
 TEMP_quality_control:flag_values = 0b, 1b, 2b, 3b, 4b, 5b, 6b, 7b, 8b, 9b ;
 TEMP_quality_control:flag_meanings = no_qc_performed good_data 
 probably_good_data bad_data_that_are_potentially_correctable bad_data 
 value_changed not_used not_used interpolated_values missing_values ;
 
 HTH,
 
 Andrew Walsh
 
 - Original Message - From: Randy Horne rho...@excaliburlabs.com
 To: cf-metadata@cgd.ucar.edu
 Cc: aschu...@harris.com; rhorn...@harris.com; ekenn...@aer.com
 Sent: Friday, August 24, 2012 4:37 AM
 Subject: [CF-metadata] Quality flag values for missing data
 
 
 
 What is the generally accepted approach for how a specific quality flag 
 value should be assigned for a corresponding data value (in the 
 corresponding data variable) that has a _FillValue ?
 
 Is it sufficient that, because the data variable value is _FillValue, the 
 corresponding quality flag value can be undefined ?
 
 
 
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John Graybealmailto:jgrayb...@ucsd.edu phone: 858-534-2162
Product Manager
Ocean Observatories Initiative Cyberinfrastructure Project: 
http://ci.oceanobservatories.org
Marine Metadata Interoperability Project: http://marinemetadata.org







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Re: [CF-metadata] Quality flag values for missing data

[Lynnes, Christopher S. (GSFC-6102)]

On Aug 23, 2012, at 2:37 PM, Randy Horne rho...@excaliburlabs.com wrote:

 
 What is the “generally accepted” approach for how a specific quality flag 
 value should be assigned for a corresponding data value (in the corresponding 
 data variable) that has a _FillValue ?
 
 Is it sufficient that, because the data variable value is _FillValue, the 
 corresponding quality flag value can be undefined ?

I do want to mention one variant to consider:  if the quality of a particular 
point is the *reason* that the data value is _FillValue, then it is useful for 
that corresponding quality flag to retain its value.  We use this approach in 
our Data Quality Screening Service, which allows users to filter (some of) our 
satellite data based on the quality flags.  In the output product, we replace 
data values with _FillValues if the quality flag indicates it does not meet 
specified quality criteria, but then we retain the quality flag so that the 
user will know WHY that particular point has a _FillValue (vs. not being able 
to get a retrieval at all).
--
Dr. Christopher Lynnes, NASA/GSFC, Code 610.2, 301-614-5185

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