Simple explanation on CATZOC (Zone of Accuracy).

The word CATZOC is might be a new word for you but when it comes to ZONE OF ACCURACY is not meant to be new. We sailors are using this phrase since paper chart is in use. The below picture will let you understand more clearly.

Here, you see Alphabet ‘a’ ‘b’  ‘c’  ‘d’ used for survey details e.g. which year the survey was done and by which authority.

Now a question might come into your mind, this survey information is for which portion of the chart?  Also, is this for the whole chart too?

No worries, normally this information came to the chart as a block. Bellow picture will take you out of this confusion.


You see on a RASTER Chart it comes as a block and it shows a diagram of your full chart hydrographic information.

Now let’s come to our discussion, in the ECDIS we are using an electronic chart that is either ENC (Which has multi-layer system) or another one is RASTER Chart (it is just a scanned version of normal RASTER Chart). Our today’s subject “CATZOC” is related with ENC, here it is showing you the accurate information of that Chart and of course, it is having a separate layer (which means you can choose to show or not to show this information.

So, we can say that: 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 displays 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).  Areas that have not been assessed for CATZOC are shown as the symbol (U) for unassessed.

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

Now you must be thinking of the minimum standard for accuracy level. It practically says there is no minimum level for such. Because ENC charts are published based on same hydrographic information as RASTER Chart. Yes, you can one thing that is normal seamanship practices “Navigate with more caution”.

Although somebody says that when CATZOC level is less than ‘C’ shift to RASTER Chart mode but it’s not practically benefitted and also not from the legal source information.

Before I come to the finish line for this subject I must say you read your companies policy about Electronic navigation carefully, it drives you to safe navigation at all times. I have uploaded the Admiralty CATZOC table.

You can  CATZOC Table – Download Here.

Have safe navigation.

About marinelegends 20 Articles
I am a Muslim Master Mariner MH Rahman, living in Dhaka, Bangladesh.


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