CATZOC (Category of Zone of Confidence)

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CATZOC on the ENC in simple terms refers to the Quality and Accuracy of survey data and the applicable error in chart datum that sometimes can greatly affect vessel’s UKC calculations.  

Zone of Confidence (ZOC) has 6 Categories named A1, A2, B, C, D and U as shown below:


Bridge watchkeepers should be aware of the ECDIS function to activate the CATZOC symbol on ENC. This data may be used to ascertain the contour settings on the chart.
For example – if the data is unreliable, vessel may need to keep a larger safety contour margin. 

Keep this ON only during Passage Planning. While monitoring this is to be kept off to avoid unnecessary clutter on the ENC. 

The knowledge CATZOC is all the more important when vessel is passing a shallow depth area.

These values can greatly affect the vessel’s UKC calculation and must be considered where the depths are shallow. As a basic thumb rule the Master should plan vessel’s passage by keeping the vessel inareas shown by the triangles i.e CATZOC – A1, A2 and B and staying away from the areas covered by rectangles i.e CATZOC- C, D and U. 

Where the vessel can’t keep clear from passing close to the rectangles, cautions notes should be posted on the ENC using the User Maps option instructing the navigator to “Keep Echo Sounder on”and “Monitor UKC Continuously”.

Ship’s staff should factor in the value associated with these categories. This can be seen in the example below:

If the depth marked on ENC is 10 meters, the actual depth of that area may differ depending upon the CATZOC shown on ENC. 

For area with CATZOC A1, Actual depth may be:  10-(0.5+0.1) = 9.4 mtr.

For area with CATZOC A2, Actual depth may be:10-(1+0.2) = 8.8 mtr.

For area with CATZOC B, Actual depth may be:10-(1+0.2) = 8.8 mtr.

For area with CATZOC C, Actual depth may be:10-(2+0.5) = 7.5 mtr

(Although A2 and B have the same depth accuracy what differs is the positional accuracy.)

For areas with CATZOC – D & U- The error is not specified or unassessed and should therefore be avoided as far as possible specially when in shallow waters. 

While the correction to the depth is + /- can also assist by providing a greater depth, Master shall always calculate for shallower depth by using “-“ accuracy to err on the safer side.

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