Data Quality Control
All our data are subjected to a minimum of real-time quality control tests and the resultant flags are vital to understanding the context and reliability of the data. We highly recommend you familiarise yourself with the basics of the format, particularly with regard to data quality.
Individual profiles will posess either an 'R' or 'D' suffix to establish their status as real-time data or delayed-mode data (best quality; subjected to detailed QC and calibration by a human operator). For applications particularly sensitive to pressure biases, or otherwise in need of best quality data, we recommend using only delayed mode profiles (identified by a 'D' suffix). "Delayed mode" variable names (eg. TEMP_ADJUSTED) should be used in preference to the raw versions (eg. TEMP).
Realtime ('R' files)
Real time data is the first form of Argo data available to the public. Because of the requirement for delivering data to users within 24 hours of the float transmitting its profile data, the real time quality control tests are limited and automatic.
In general these data should be consistent with ocean climatologies even though no climatology tests have been performed. For science applications sensitive to small pressure biases (e.g. calculations of global ocean heat content or mixed layer depth), it is not recommended to use "R" files.
We perform the following automated, realtime, quality control checks on all profile data collected by the UK floats. The tests are conducted in the order listed and quality control flags are assigned.
The first 16 tests are standard in Argo data management and are defined in the Argo Quality Control Manual. However BODC implements two additional tests, the profile envelop and the freezing point test. Further information is provided below.
- Impossible date — tests for sensible observation date and time values.
- Impossible location — tests for sensible observation latitude and longitude values.
- Position on land — tests whether the observation position is on land.
- Impossible speed — tests for a sensible distance travelled since the previous profile.
- Global range — tests that the observed temperature and salinity values are within the expected extremes encountered in the oceans.
- Regional range — tests that the observed temperature and salinity values are within the expected extremes encountered in particular regions of the oceans.
- Deepest pressure — tests that the profile does not contain pressures higher than the highest value expected for a float.
- Pressure increasing — tests that pressures from the profile are monotonically increasing.
- Spike — tests salinity and temperature data for large differences between adjacent values.
- Gradient — tests to see if the gradient between vertically adjacent salinity and temperature measurements are too steep.
- Digit rollover — tests whether the temperature and salinity values exceed a floats storage capacity.
- Stuck value — tests for all salinity or all temperature values in a profile being the same.
- Density inversion — tests for the case where calculated density at a higher pressure in a profile is less than the calculated density at an adjacent lower pressure.
- Grey list — tests whether the sensor identifier is present in a list that has been collated to identify sensors which are experiencing problems.
- Sensor drift — tests temperature and salinity profile values for a sudden and important sensor drift.
- Frozen profile — tests for the case where a float repeatedly produces the same temperature or salinity profile (with very small deviations).
- Profile envelop — tests that temperature and salinity profile values are within an envelope of permitted values within depth ranges. On failure the QC test identifier value 2000000000000H is used.
- Freezing point — tests if the profile temperature at a given pressure and salinity is less than the calculated freezing point temperature. On failure the QC test identifier value 4000000000000H is used.
Delayed-mode ('D' files)
Delayed mode data profiles have been subjected to detailed scrutiny by oceanographic experts and the adjusted salinity has been estimated by comparison with high quality ship-based CTD data and climatologies using the process described by OW, WJO, or Böhme and Send. This process is carried out on a 1 year long data window and so no Delayed Mode observations can be less than 1 year old. These data are appropriate for applications sensitive to small pressure biases. It is particularly important to understand flags and variable names in delayed mode files to extract the best possible data quality from them.
Quality control flags
| Flag value | Meaning | Comment |
|---|---|---|
| 0 | No QC | No quality control performed |
| 1 | Good data | All quality control tests performed |
| 2 | Probably good data | |
| 3 | Bad data. Potentially correctable | Test 14 failed, all other tests passed. Not to be used without scientific correction |
| 4 | Bad data | Data have failed one or more tests, excluding test 14. |
| 5 | Value changed | |
| 6 | Unused | |
| 7 | Unused | |
| 8 | Interpolated value | |
| 9 | Missing value |
References
Owens, W.B. and A.P.S. Wong, 2009: An improved calibration method for the drift of the conductivity sensor on autonomous CTD profiling floats by theta-S climatology. Deep Sea Research Part I: Oceanographic Research Papers, 56, 450-457.
Böme, L. and U. Send, 2005: Objective analyses of hydrographic data for referencing profiling float salinities in highly variable environments. Deep Sea Research Part II - Tropical Studies in Oceanography, 52, 651-664.
Wong, A.P.S., G.C. Johnson and W.B. Owens, 2003: Delayed-mode calibration of
Autonomous CTD profiling float salinity data by Theta-S climatology. Journal of
Atmospheric and Oceanic Technology, 20, 308-318.
