There are no fixed rules on how to use CATZOC information. To know how we must use the CATZOC, we need to look into the navigation manual of the company and look for company specific information.
Below are one of the most comprehensive instructions in the navigation manual of a company that I have come across.
The UKC available should be more than the CATZOC accuracy.
Let us say the calculated maximum possible CATZOC error (based on the star rating of the ENC) is 2.0 meters.
Now if the available UKC is more than 2.0 meters, vessel can proceed with the voyage.
These instructions are based on the fact that the CATZOC error is the maximum error and not the definite error. So in the above example, CATZOC error can be any value between -2.0 meters to +2.0 meters.
Even if at some place the error is at the maximum, the vessel will still be safe (though at that point vessel will not comply with the company's UKC policy).
In these instructions CATZOC and UKC policy are taken as two separate subjects.
Now let us say that available UKC is 1.9 meters as per UKC calculation sheet (without taking into account CATZOC value). The vessel complies with UKC policy of the company but now the possible maximum CATZOC error is more than the available UKC.
In this case, vessel need to inform the company and take measures as defined by the company's navigation manual.
These measures are
- Get the exact depth from the past experiences (like last time vessel passed that with similar or more draft).
- Get the exact depth from the agent, pilots or different nautical publications.
If information from these resources suggests that there was no error in the charted depth shown, then vessel can proceed with the voyage after written permission from the company.
These are only one of the possible instructions related to the handling of CATZOC information. Your company could have different kind of instructions and you must follow the company's instructions.
Now based on above instructions let us say that the UKC available in the dredged channel is 0.5 meters.
Minimum UKC required as per company policy is 0.4 meters. ZOC is 6 stars (A1) and depth of the dredged channel is 15 meters. In this case, the maximum possible CATZOC error will be 0.65 meters (0.5 + 1% of depth).
In this case vessel need to take written permission from the company for proceeding with the voyage.
Finally, the answer to your question is "yes, we need to consider the CATZOC value in the dredged channel too.
But the procedure to consider the CATZOC value can differ from company to company and we must follow the instructions in the SMS manual of the company.