Allow Flag 0 in _FLAG_W columns #28
Description
While not part of the WOCE format, which is basically fixed at this point. The need for a sentinel value indicating that "no QC" has been performed has come up several times.
The value of 0 seems like the natural choice for this since 0 is used in several other schemes to indicate the lack of QC including:
- Argo
- SeaDataNet
- ESEAS
- OceanSITES
Note that at least three of these are netCDF specific standards (and I don't know what ESEAS uses).
I propose that we change the Exchange format (and CF/netCDF) to allow for 0 value to exist in any of the FLAG_W columns. The actual meaning is somewhat ambiguous, in most situations it will indicate that no QC has been performed and is a "no flag" flag. In some other situations, e.g. GLODAP. This indicates a calculated (or possibly interpolated) value.
Because of the amount of flag 0 in the wild already, the flag definition tables should stay the same, the ability to use a 0 value in the flag column should be a format specific extension and be defined as a "user flag" that can be used for an additional flag that needs definition in the file itself. With no definition in the file, a default meaning of "NO FLAG" or "NO QC" should be assumed.