CATZOC Q&A

Updated: Oct 16, 2026

1. What is CATZOC?

CATZOC: CAT-Category

ZOC-Zone of Confidence

They are the values that used to highlight the accuracy of data presented on charts, which depends on 3 main factors : sounding position accuracy, sounding depth accuracy, and quality of the survey.

CATZOC is divided into six main categories, ranging from highest confidence to unassessed :

2. Where to find / how does CATZOC look like on ENC and paper chart?

In ECDIS S-57 compliant ENC, you can turn on the layer called “Accuracy” or “Data quality” or “Accuracy pattern” do display CATZOC symbols.
ECDIS displays these CATZOC values in ENCs using a triangular or lozenged-shaped symbol pattern. The number of stars contained within these symbols denotes the CATZOC value ( 6 stars ~A1, 2 stars ~ U…).
Only a small number of traditional paper charts feature CATZOC information. Most of them display accuracy via Source Diagram.3. How does ZOC affect Passage Planning, Safety Depth setting and UKC?

CATZOC is one of the criteria allows you to assess the chart data reliability. During passage planning and execution, you shall provide the allowance for the position accuracy and the depth accuracy :

Position accuracy :

For ZOC A1, A2 and B sounding position accuracy is relative high (±50m for ZOC B). But for ZOC C and D, this value is ±500m or bigger. It means you have to take ZOC allowance for position of sounding into account of plotting route on the charts.

Depth accuracy :

Safety Depth = Max Draft+ Require UKC

One of the components of Require UKC is ZOC allowance :

For example : Your route leg with the minimum depth 15m is going through ZOC A1 :

ZOC allowance(A1) = 0.5+ 1% x depth = 0.5 + 1% x15= 0.65m

It means the real depth may be between (15-0.65=14.35m) – (15+0.65=15.65m) anywhere in 5m radius.

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