CATZOC and passage plan

Hello everybody!
Could you please help me understand what is the meaning and importance of CATZOC in passage plans?


Hello there Rosencratz,

CATZOC is a whole chapter by itself and in many times confuses the officers on board, but actually it is a very simple & fast way to check the accuracy of the depth.

The ENC cells include data fields with information about the reliability of the “objects”. CATZOC ( “Category of Zone of Confidence”) gives an estimate of the reliability of the data related to five quality categories for assessed data (CATZOC A1, A2, B, C and D) and a sixth category “U” for data which has not yet been assessed.
CATZOCs must be comprehensively reviewed during the creation of the passage plan to identify the accuracies of positions and depths. This should be taken into account later at the planning stage, when safety margins are established. Experience indicates that the less accurate the position of charted features and depths, the wider the safety margin required.
CATZOC is based on the position accuracy, depth accuracy, seafloor coverage area and the survey method. The CATZOC also indicates the quality of the production process of the ENC, and not simply the quality of hydrographic survey data. It is very important to understand that these values are approximate and indicate the lowest category of confidence that can be claimed by a survey or group of surveys.
The data from surveys is often dated, and the categorization of accuracy may be based primarily on the reputation of the original surveying authority. The ECDIS operator should be aware that survey quality may be less than that indicated by the CATZOC level. CATZOCs may also be downgraded at a later date, via ENC update, where survey quality is found to contain inaccuracies.
Where the CATZOC quality is rated as “U” for “Unassessed”, ECDIS operators should investigate further. Many ENCs have simply not been assessed for CATZOC because the Hydrographic Office producing the ENC has not been able to complete the task, and “Unassessed” may not necessarily mean that the ENC is of poor accuracy. In these cases, ENCs may still be used, but a documented risk assessment may be necessary depending on each company policy. The Navigator should consult the company SMS and determine if,

  • An additional UKC safety margin is required.
  • An additional clearance distance from navigational hazards is required.

Always to remember:

  1. Lower CATZOC ratings indicate that ECDIS operators should allow a wider safety margin between the ship’s track and hazards or dangers. Crosschecking of the ship’s position and echo sounder comparison should be performed with more frequency when navigating in areas with a lower CATZOC.
  2. Despite advances in hydrographic technology, many ENCs are still derived from the source data of the equivalent paper chart and are no more accurate. Navigators must be mindful that there is always potential for error and the presence of undetected dangers.
  3. The CATZOC must always be included in your UKC calculations.
  • We have calculated the CATZOC for an area with accuracy A1, which according to CATZOC table gives an accuracy of 0.5 m + 1% of depth. Depth: 21 m
    The calculation: 21 m x 1% ⇒ 0.21 m + 0.5 m ⇒CATZOC = 0.71 m
    So your UKC calculation should include the following,
    UKC = Depth of Water + Tide - Vessel’s draft - Squat - Swell - CATZOC.

**CATZOC must always be deducted (-) during the UKC calculation.

Hope it helps,

All the best,


Great reply!


Much appreciated @Captain_Tim :slight_smile: