Zone of Confidence – Are Your Chart Depths Accurate?

Zone of Confidence

Seventy per cent of the earth is covered with water. Hydrographic offices around the world conduct regular surveys, but the task is ongoing and has been for a long, long time.

The method of conducting these surveys has, of course, changed over the years. At the start, they were completed with lead lines by hand and single beam echo sounds. Later came the wire-drag method, with the obstruction detected with the wire stretching. The modern technique uses SONAR multi-beam waves to record the depths. This dated is processed with other data, e.g. tides.

It’s worth remembering that a hydrographic surveyor can only physically see the parts that rise above the sea surface. For the rest, the surveyor puts their confidence in their systems.

The major shipping routes have priority for surveying which means that mariners must have the skills to interpret the quality indicators that should be on every chart (and if they’re not, I’d bin that chart).

The Zone of Confidence (ZOC) has replaced the Reliability Diagram (aka source diagrams).

Older charts may still have the Reliability Diagram which requires knowledge of past and present hydrographic surveying practices to fully understand. This is why the ZOC was developed by the Australian Hydrographic Office, and is now in use worldwide.

ZOC categories warn mariners which parts of the chart are based on good or poor information and which areas should be navigated with caution.

Chart AUS252 ZOC

ZOC diagram AUS 252

ZOC table AUS 252


Possible errors in depths

When a depth is measured during a survey, there can be two errors to this depth.

  • Depth may not be accurate.
  • The position at which this depth is marked may not be accurate

You must apply the maximum possible errors wherever required.

Proper Look-Out

Nothing replaces a proper look-out following the COL REGS, and approaching a port by day. Give shallow areas of a questionable sounding a wider berth

And use every available resource and skill you have. You’re the Skipper you’re responsible.

From the Australian Hydrographic Office’s Website

‘All charts, whether paper or electronic, contain data which varies in quality due to the age and accuracy of individual surveys. In general, remote areas away from shipping routes tend to be less well surveyed, and less frequently, while areas of high commercial traffic are re-surveyed frequently to very high levels of accuracy, particularly where under-keel clearances are small. It is quite accurate to consider a chart as a jigsaw of individual surveys pieced together to form a single image. Having the necessary skills to determine how much confidence should be placed in the surveys which combine to form a chart should be essential for any mariner venturing into unfamiliar waters.’