3.8.4 Chart Coverage

Even though ENC coverage is increasing rapidly, many vessels will, to some degree, have to operate a dual or even triple system, with a voyage covered by various combinations of ENCs, Paper and Raster charts. The planning and validation of an intended voyage must therefore consider issues such as which chart types are available for the various segments of that voyage.

The IMO performance standards for ECDIS state that where ENCs are not available, RNCs maybe used in ECDIS to meet carriage requirements. However, this is subject to the agreement ofthe flag administration when issuing the vessel-specific approval for use of ECDIS and must besupported with paper charts of relevant areas as back-up.

3.8.5 Unofficial Data

Private vector charts produced by commercial suppliers (e.g. Transas) are considered unofficial as they do not meet IMO/SOLAS standards for chart carriage and can therefore not be relied upon as a primary means of navigation. Only ENCs officially authorised and produced by a National Hydrographic Office conforming to IHO S-57 standard can be considered as fulfilling the chart carriage requirements of SOLAS Chapter V. An ECDIS using unofficial chart data for navigation is classified as an Electronic Chart System (ECS).

When unofficial chart data is used, ECDIS warns mariners that they must navigate by means of an official, up to date, paper chart by a continuous warning on the screen. If unofficial chart data is shown on the ECDIS display, its boundary is identified by a one-sidedred line with the diagonal stroke on the unofficial side of the line.

3.8.6 Survey Accuracy of enc Sources

Survey data can be based on information from different sources. Their accuracy depends on survey frequency, time and method and can vary from very accurate to unassessed. It should be noted that areas which have been surveyed very accurately may lie adjacent to areas which have not.

Above: ENC displaying CATZOC.

The S-57 Standard allows information about the accuracy of the survey data to be displayed for each ENC cell. This function is called ‘Category of Zone of Confidence’ (CATZOC). The table below gives an overview of the different categories of zones of confidence and their related accuracies.

Zones of Confidence (ZOC)	ECDIS Presentation	Position Accuracy	Depth Accuracy	Seafloor Coverage
A1		± 5m+5% depth	=0.5m+1% of depth	Full area search undertaken. Significant seafloor features detected and depth measured.
A2		± 20 m	=1.0m+2% of depth	Full area search undertaken. Significant seafloor features detected and depth measured.
B		± 50 m	=1.0m+2% of depth	Full area search not achieved; uncharted features, hazardous to surface navigation are not expected but may exist.
C		± 500 m	=2.0m+5% of depth	Full area search not achieved; depth anomalies may be expected.
D		worse than figure above		Full area search not achieved; large depth anomalies may be expected.
U		Unassessed, the quantity of bathymetric data has not been assessed.
MDSC		No information is available for corresponding sea area.