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16 March 2017

Category Zones of Confidence (CATZOC) –dispelling the myths

​​​​​​​​​​​​​​​​​​​​​​​​​Category Zone of Confidence (CATZOC) values are used to indicate the accuracy of data presented on charts to assist mariners with determining a safe Under Keel Clearance (UKC). This blog aims to answer 2 key questions that we regularly receive at our Living with ECDIS Seminars​ on this important feature: 

  • “Do I need to revert to paper charts in areas where the ENC Category Zone of Confidence is set to C or less?” ​

  • “Why do some paper charts have Source Diagrams and others have CATZOC Diagrams?” 

 What is CATZOC?​

Category Zone of Confidence (CATZOC) values are assigned to geographical areas to indicate whether data meets a minimum set of criteria for position, depth accuracy and seafloor coverage. The Zone of Confidence (ZOC) value is dependent on the positional and depth accuracy of the survey.

By understanding the accuracy limitations of the underlying data in greater detail, the mariner can manage the level of risk when navigating in a particular area.​

ECDIS display these CATZOC values  in ENCs using a triangular or lozenge shaped symbol pattern. The number of stars contained within these symbols denotes the CATZOC value. For example six stars are given to the highest level of data quality (A1) and two stars to the lowest (D). A single star is not used to avoid possible confusion with a rock symbol. Areas that have not been assessed for CATZOC are shown as the symbol (U) for unassessed.

Our Zones of Confidence (ZOC) table outlines the position accuracy, depth accuracy and seafloor coverage for each of these values to help you manage the levels of risk during navigation.


Download the Zones of Confidence (ZOC) table​ 

Do I need to revert to paper charts in areas where the ENC Category Zone of Confidence is set to C or less?

No, you don’t. The same data is used for a paper chart and an ENC, therefore neither is more accurate than the other. In fact, an ENC can give you a greater insight into the data via the ’Quality of Data’ and survey reliability features.

The example below clearly highlights that there is no additional information on the paper chart that would aid the mariner in calculating an under keel clearance, and therefore there is no requirement to switch from navigating with ENCs and ECDIS when the CATZOC value is C or less.​


 GB ENC showing M_QUAL area with CATZOG value of C and source data diagram for paper chart BA1977


Why do some charts have Source Diagrams and others have CATZOC Diagrams?

The Source Diagram on a paper chart is the traditional method of indicating when and how the survey was conducted to collect the hydrographic data. From this information, the mariner must deduce the degree of confidence to place in charted data. 

By including CATZOC Diagrams on paper charts, more detail is provided on the accuracy of the underlying hydrographic information than was previously available in the Source Diagram. Providing this additional detail means that the mariner can make more informed decisions when passage planning and calculating under keel clearances. 

As you can see from the examples below, the UKHO are gradually moving to a new style of Source Diagrams including ZOCs.​

New CATZOC diagram (top) and traditional source diagram (bottom)


This blog was written by Thomas Mellor, the Head of OEM Technical Support and Digital Standards at the UKHO​. He's also speaker at the UKHO's 'Living with ECDIS' Seminars which are designed to help ship managers and officers to understand the latest legal and maintenance issues in relation to ECDIS. You can find out more about these seminars here.