Quality Control

What is QARTOD?

As part of the ongoing OOI effort to improve data quality, OOI is implementing the Quality Assurance of Real-Time Oceanographic Data (QARTOD) on an instrument-by-instrument basis. Led by the United States Integrated Ocean Observing System (U.S. IOOS), the QARTOD effort draws on the broad oceanographic observing community to provide manuals for different instrument classes (e.g. salinity, pH, or waves), which outline best practices and identify tests for evaluating data quality. A common code-base is available on GitHub and actively maintained by IOOS partner Axiom Data Science.

OOI has committed to implementing available QARTOD tests where appropriate. QARTOD is well-documented and actively maintained with an engaged user-base across multiple data collection and repository programs. It has a publicly available code-base with standardized tests and flag definitions that result in simplified, easy-to-interpret results. However, some instruments deployed by OOI, such as seismic sensors and multispectral sensors, are not well-suited towards QARTOD.  For instruments for which there is no existing QARTOD manual, such as seawater pCO₂, OOI is implementing “QARTOD-like” quality control (QC).

How are QARTOD results communicated?

QARTOD utilizes a standardized data-flagging scheme, where each data point for an evaluated variable (e.g. salinity) receives one of the following flags: 1 if the data point passed the test; 2 if a test was not evaluated; 3 if the data point is either suspect or interesting/unusual; 4 if the data point fails the test; 9 if the data point is missing.

In interpreting the flag designations, it is important to consider that a flag of 3 does not necessarily mean a data point is bad – it could also mean something interesting or unusual occurred that resulted in the given data point being outside of the expected test threshold. Climatology test data are an exception to the data-flagging scheme.  Climatology test data are simply classified as “pass” or “suspect.”

Importantly, QARTOD only flags data, it does not remove data. In fact, OOI is committed to delivering all available data; the goal is to provide further information on the possible quality of the data.

How is QARTOD implemented by OOI?

OOI is prioritizing implementation of QARTOD on instruments and variables which are shared across arrays and with broad or high scientific interest, such as CTDs, seawater pH and pCO₂, dissolved oxygen, and chlorophyll/fluorescence. OOI is currently implementing the gross range and climatology tests (see this article for a more detailed description of the specific implementation). Note that the climatology test is a site-specific seasonally varying range test and not a World Ocean Atlas-like climatology. These two tests, in addition to other tests under development, utilize thresholds and ranges which are calculated from existing OOI datasets. The code used to calculate the thresholds is publicly available at the oceanobservatories ooi-data-explorations github repository  and resulting threshold tables are available at the oceanobservatories qc-lookup github repository. The tests executed and results are added to the datasets as variables named _qartod_results and _qartod_executed, with the relevant tested data variable name prepended (e.g. practical_salinity would be practical_salinity_qartod_results and practical_salinity_qartod_executed). The _qartod_executed variable is a list of the individual results of each of the tests applied stored as a string. The tests applied and the order in which they were applied are stored in the variable metadata attributes. The _qartod_results provide a summary result of all the tests applied.

What is the current OOI QARTOD status?

The IOOS QARTOD manuals define the tests for each instrument class as “Required,” “Recommended,” and “Suggested”.  The OOI priority will be the “Required” tests for each instrument class in use by OOI.  “Recommended” and “Suggested” test development will be reviewed, with the support of subject matter experts, in the future. The IOOS QARTOD manuals can be found at Quality Assurance/Quality Control of Real Time Oceanographic Data – The U.S. Integrated Ocean Observing System (IOOS).

Current tests under development are gross range (GRT) and climatology (CT). The gap test (GT) will follow next. The syntax and location tests are considered operational checks and are handled within OOI operations and management systems.  QARTOD test data will be available in specific locations where OOI supplies data, starting with M2M, and then propagated to THREDDS and Data Explorer.  The current status of QARTOD test development and availability is shown in Table 1. Table 2 details QARTOD Rollout Plans by OOI instruments.  Figure 1 shows the timeline of QARTOD testing.

Table 1: QARTOD Test Status & Availability to Users

Table 2: QARTOD Rollout Plan for OOI Instruments

Figure 1: QARTOD “Required” Test Development Timeline