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Article: Don’t be misled by the apparent accuracy of electronic navigation aids

29 January 2019

Maritime navigation has undergone a gradual increase in precision and accuracy, from early astro-navigation through Harrison’s chronometers to modern satellite systems. However, while navigators might assume that the accuracy of Global Navigation Satellite Systems (GNSS) is certain, this is not always the case, and many navigators are placing too much faith in the information that their ECDIS displays. For safe navigation, a mariner must also consider the accuracy and reliability of the source hydrographic data.

The flaws in technology
Electronic Navigation Charts (ENCs) display hydrographic data that, according to The International Hydrographic Office’s S66 ‘ based on the latest source data available to the relevant Hydrographic Office…’.
In many cases the ‘latest source data available’ may have been gathered some time ago, using older less reliable methods. The risk is that this reduces the accuracy of the information being displayed on the ship’s ECDIS, and creates a false sense of confidence for the navigator. Category Zones of Confidence (CATZOCs) enable the mariner to make decisions on the degree of reliance to place on the chart when planning a passage or conducting navigation.

GNSS provides mariners with highly accurate positioning information. Over-reliance on this information can lead to an erosion of the traditional cautious approach with which navigators used to make landfall, because the navigator believes they have a clear picture of where they are, in relation to other objects.

If mariners do not use any other source to confirm their location or the ENC, and one of these is wrong, a casualty is inevitable.

Case study
A cargo vessel grounded in the Mediterranean Sea on a charted shoal.
During the planning of the passage the 2nd officer consulted several sources, such as pilot books, the Admiralty list of radio signals and temporary and preliminary notices to mariners. One thing which was not checked, however, was the value of the CATZOCs for the ENCs to be used along the route.

During the voyage the vessel’s destination was changed, requiring an alteration of the route. The subsequent alteration took the vessel closer to the shoal than the original passage plan. Despite this decrease in clearance, no reference was made to the accuracy/reliability of the ENCs which depicted the shoal. The changes to the track reduced the passing distance between the vessel and the shoal to a few hundred metres. The margin for error had been significantly reduced!

Unfortunately, the depiction of the shoal on the ENCs was inaccurate. They had been compiled by the local hydrographic authority and utilised data from only a single source. This data had been gathered using analogue techniques in the 1960s, the CATZOC value for the area was ‘U = Unassessed’. The age and lack of information regarding the value of the CATZOC data should have served as a red flag for the crew. The failure to check the CATZOCs meant that this red flag went unnoticed.

Post incident the locally produced ENC was compared to data from the BA chart series. The BA charts had been compiled using information from a variety of sources and more accurately depicted the location of the shoal. When the grounding position was plotted on the BA chart it showed the ship’s position within the extent of the shoal, whereas the ECDIS of the ship depicted the vessel in safe, deep water.

The shoal patch was covered by a pair of sectored lights. The respective red and green sectors of these lights overlapped covering the shoal in question. A vessel observing both the red and green sectors would be inside this overlap. Details of the lights were contained in the pilot book for the area. These lights and their significance for navigation were not considered by the 2nd officer during the planning of the passage.

Hydrographic Offices around the world are engaged in an effort to provide mariners with accurate, up to date hydrographic information. Despite these efforts, some ENCs have been compiled from older, less accurate source data. Officers should always factor these considerations into the passage planning process, by consulting the CATZOCs for each leg of the voyage and using pick reports to determine the details of the survey data. Officers should take care to avoid being seduced by the levels of precision that GNSS and ECDIS offer when used together. They should continue to incorporate appropriate margins of safety into their plans whilst employing good judgement on what does and does not constitute an acceptable navigational risk.



The Standard Club thanks Richard Bell for the original draft of this article. For queries, contact the Loss Prevention team.