When using Electronic Chart Display and Information Systems (ECDIS) onboard a vessel, understanding the accuracy of Electronic Navigational Charts (ENCs) is critical for safe navigation. The accuracy of ENC data varies, much like traditional paper charts, as it is only as precise as the original survey data from which it was created. Mariners need to approach the use of ENCs with caution, knowing that the accuracy of the original survey data will never be 100%. This is essential for the watchkeeping officer to comprehend and integrate into their navigational decisions.
Before navigating with ECDIS, it is mandatory for watchkeeping officers to undergo an approved generic ECDIS course. However, it’s equally important to receive onboard familiarization training specific to the ECDIS system installed on the vessel. This ensures that officers are not only familiar with the general principles of ECDIS but are also proficient in using the particular system they are working with.
The International Hydrographic Organization’s (IHO) S-57 standard governs the data used in ECDIS systems, dictating how hydrographic data is exchanged between nations. This includes measures to reduce overlaps in ENC data between countries. Unlike paper charts, ENCs must be maintained by the country whose territory they cover. However, overlapping data between producing nations can lead to anomalies, which the navigator must be aware of.
ENC data accuracy can vary dramatically, much like paper charts. For example, paper charts provide notes on data sources, GNSS accuracy, and other relevant information. This data helps the deck officer during voyage planning to establish safety margins. However, ENCs lack such detailed notes, with their primary method of displaying accuracy being through Zones of Confidence (CATZOC).
ENCs are divided into areas where data accuracy is assessed based on positional accuracy, depth accuracy, and seafloor coverage. These factors are combined and represented as a meta-feature called M_QUAL, which is then assigned a confidence level using CATZOC zones. There are six possible CATZOC zones, ranging from high to low accuracy, represented by stars in symbols on the ENC. More stars indicate higher accuracy, while fewer stars suggest that the data is older or less reliable.
Planning CATZOC symbols play a crucial role in passage planning, helping navigators determine appropriate safety margins. A low CATZOC rating may prompt the officer to increase under keel clearance or adjust the passage route. It is vital for mariners to refer to the Safety Management System (SMS) for guidance on how to incorporate CATZOC zones into their voyage plans. Commercial factors, such as loading to a vessel’s maximum draft, can sometimes challenge these safety margins, making it important for the master to follow the guidance provided in the SMS.
The IHO S-52 standard specifies how ENCs are displayed in ECDIS systems. This includes guidelines for issuing and updating ENC data. Notably, the S-52 Presentation Library Edition 4.0 introduced significant changes, including the ability for mariners to independently select or deselect CATZOC features. These changes highlight the need for mariners to be familiar with S-52 standards before using ECDIS systems.
Data In addition to CATZOC zones, ENC producers can include further data related to accuracy. The “Pick Report” function on ECDIS allows mariners to access more detailed information, such as the reliability of individual surveys (M_SREL) and additional notes on the data’s accuracy (M_ACCY). This information helps the navigator make more informed decisions during voyage planning.
ENCs are built using three main datums: horizontal, vertical, and sounding. Each of these datums affects the accuracy of the navigational data. For example, the sounding datum impacts under keel clearance calculations, while the vertical datum is important for determining air draft clearances. Navigators must ensure that they are aware of these datums, especially when transitioning between areas where datums may change. Monitoring the “Pick Report” function on ECDIS will help officers identify these variations and adjust their passage plans accordingly.
Depths in ENCs On paper charts, soundings are marked in italics to indicate reliable sources, while unreliable sources are non-italicized. On ENCs, unreliable soundings are circled instead. This differentiation is crucial for the mariner to understand, as it affects the trustworthiness of the data used for navigation.
ECDIS provides numerous advantages for modern navigation, but it requires a deep understanding of the underlying data, including ENC accuracy, CATZOC zones, and the impact of different datums. By ensuring thorough familiarization with their ECDIS system and integrating the information provided by CATZOC symbols and other M_QUAL data, mariners can navigate more safely and effectively.
