What is CATZOC in nautical charting? Zone of confidence explained

If you've ever pulled up an ENC and squinted at a little diamond or asterisk tucked into a shaded zone, you've run into CATZOC. It stands for Category Zone of Confidence, and it's the IHO's way of telling a mariner, or a survey planner, how much to trust the depth data underneath a given patch of chart.

CATZOC rates the confidence of the survey behind a charted depth rather than the depth itself. Two charts can show the same sounding, 14.2 meters, and mean very different things depending on whether that number came from a multibeam survey run last year or a lead line cast in 1952.

The six CATZOC categories, from best to worst

The IHO defines six bands, each tied to position accuracy, depth accuracy, and seafloor coverage:

A1: full bottom coverage, systematic survey, tight position and depth tolerances. This is modern multibeam with everything cleared.
A2: full coverage, slightly looser tolerances than A1, but still a thorough, systematic survey.
B: full coverage isn't guaranteed. Depths are reasonably reliable but there could be gaps between survey lines.
C: low accuracy. Sparse or old sounding data, large position uncertainty. Common on legacy charts in less-trafficked waters.
D: worse than C. Data quality is poor enough that even general depiction is in question.
U: unassessed. Nobody has rated the source data, so treat it about the same as D until proven otherwise.

A survey planner reading a chart with B or C zones in the proposed work area is looking at depths that could be decades old, pulled from soundings spaced too far apart to pin down exactly where a shoal patch sits.

Chart adequacy versus chart accuracy

Chart adequacy and chart accuracy get used interchangeably, but they're different questions. Accuracy asks how close a charted number is to the true depth. Adequacy asks whether this chart is good enough to support the activity someone wants to do in this water. A C-rated chart might be perfectly adequate for routing deep-draft transits through a well-known channel and completely inadequate for siting a cable landing or scoping a marina dredge.

This is where CATZOC earns its keep in survey programming. A zone of confidence map, overlaid against planned activity, flags exactly where the office's chart data is thin. Pull the ENC's source diagram alongside the CATZOC layer and you get a quick triage: A1/A2 zones can wait, B zones get queued behind higher priorities, and C/D/U zones go on the shortlist for the next survey season, assuming the budget stretches that far.

The catch is that CATZOC tells you where the data is old or sparse. It doesn't tell you what's actually down there now. A C-rated zone could be perfectly fine, or it could have shoaled since the last lead line pass came through. You won't know until a vessel gets eyes on it, and a lot of offices are sitting on more C/D/U zones than their survey fleet can reach in any given year.

That gap between knowing a chart is weak and knowing where to send the boat first is exactly where an annual depth estimate from high-resolution satellite imagery earns its keep. It won't replace the sounding those zones eventually need, but it gives a planner a rough depth picture across remote or hazardous water before committing a vessel, which is the whole point of the approach behind Satellite Bathymetry.

Where you'll see CATZOC

CATZOC shows up as metadata on individual ENC cells, the M_QUAL object if you're down in S-57 or S-101 layers, and most ECDIS software will display it as a shaded zone or a symbol you can query by clicking. If your office maintains its own source diagrams, cross-referencing them against CATZOC zones is usually the fastest way to spot where survey debt has quietly piled up.

If your backlog list is longer than your survey season, it might be worth seeing what an annual depth estimate layer looks like over your own area of interest.