[FAQ] Do You Know What CatZoc Is?

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Credit: Ruben Christen/Unsplash

Category Zone of Confidence (CATZOC) is the value used to highlight the accuracy of data presented on charts. Learn Marine’s Blog explains CATZOC in depth. Here’s an excerpt from that.

CATZOC in IHO S-4 standard

Before the ECDIS era, mariners used to assess paper chart accuracy via Source diagrams. 

However, this was changed with the wider introduction of ENCs in the industry and the consequent introduction of CATZOC, which is described in the IHO S-4 standard. 

The latter allows accessing the ship’s UKC numerically with regard to a given ZOC.

Accuracy of charted data and CATZOC

Whenever mariners are asked, they respond that it is the Zone of Confidence and that the accuracy of data depends on the number of stars. But very few say exactly how the accuracy of charted data depends on CATZOC.

3 main factors

Apparently, there are three main factors: 

  1. Sounding Depth Accuracy, 
  2. Sounding Position Accuracy and 
  3. Quality of the Survey.

To access the reliability of paper chart bathymetry data mariner shall have a look at a source diagram, it can be stated that the earlier a survey was conducted, the less accurate the soundings data on the chart. Often, on the same chart, the data of several bathymetric surveys are compiled.

It is quite obvious that if the depth survey was done in 1832 by a leadline and using visual bearings for position fixing, we can’t really say that the usual 10% of your maximum static draft is enough for UKC, as we can’t really trust the data on the chart.

CATZOC table

There is a more accurate tool available on ENCs in comparison to paper charts. On the modern ECDIS S-57 compliant ENC (Electronic Navigational Chart) you can turn on the layer called “Accuracy”, “Data quality”, or “Accuracy pattern”, etc. 

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). 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.

This layer doesn’t need to be displayed all the time. 

Passage planning

However, during passage planning, or when one wants to access the present situation and possible navigational risks the deck officer shall be able to understand the meaning of the accuracy pattern. A detailed description of these accuracy symbols is given in the table below in a slightly modified way, based on the REGULATIONS OF THE IHO FOR INTERNATIONAL (INT) CHARTS and CHART SPECIFICATIONS OF THE IHO.

How does ZOC affect Passage Planning, Safety Depth setting and UKC?

First of all: 

SAFETY DEPTH = MAX DRAFT + REQUIRED UKC

So, firstly, we are concerned about the error in sounding given on the chart. For instance, if the route leg is going through ZOC A1, how will it affect our Safety Depth?

We should find the minimum depth in the vicinity of a planned track and apply correction to it.

One of the components of the REQUIRED UKC is ZOC Allowance. From the CATZOC table for ZOC A1:

ZOC Allowance (A1) = 0.5 + 0.01 × Depth

ZOC Allowance (A1) = 0.5 + 0.01 × 18.8 = 0.688 ≈ 0.7 m

If for ZOC A1, A2 and B sounding position accuracy is relatively high (position error is less than 50 m for ZOC B), for ZOC C and D this value is 500 m and bigger. 

In this case, you have to access not only the ZOC Allowance for UKC but also the positions of soundings when you are plotting tracks on the chart. 

In the picture below the wreck is located right in the middle of the Traffic Separation Lane. For ZOC C:

ZOC Allowance (C) = 2.0 + 0.05 × 17.3 = 2.865 ≈ 2.9 m

This means that the real depth may be between 14.4 – 20.2 m anywhere in 500 m radius. Dynamic UKC may be even less if the vessel experiences motion due to waves (rolling, pitching or heaving) in the open ocean area (in this case it is the West Pacific Ocean on the approach to San Francisco).

CATZOC example ZOC C 

Conclusion

In conclusion of the above mentioned, you should always assess the chart data reliability. CATZOC is one of the criteria which allows you to do that. 

During passage planning and execution, you shall not only provide the allowance for the depth accuracy but also the position accuracy of given hazardous soundings.

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Source: Learn Marine

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