Thursday, 24 May 2012

Zone of Confidence

Zones of Confidence is an infrequently applied, but important correction to charts when used for port approach. 

Zone of Confidence Table

When a Hydrographer issues a chart, a table showing the Zone of Confidence, or margin of error expected, is placed in the explanatory notes alongside a plan showing the zone boundaries. The most accurate depths will be within the maintained depth channels bounded by buoys or beacons and which are required for deep draft vessels. The higher accuracy permits Masters to use Dynamic Under Keel Clearance (DUKC) to maximise loading based on computer-predicted tide heights. Less navigated areas are sounded with wider spacing and a greater margin of error.

In a recent grounding incident, the vessel was navigated outside the channel to improve the angle of berth approach. Despite being a well trafficked area of the port, the ZoC was B with a depth accuracy of 1.0 metre plus 2% of the depth with hazards not expected but may exist.

The Pilot had calculated an Under Keel Clearance between the vessel's draft, and allowing for squat, the tide height and the charted depths. Had the ZoC been applied, it is arguable that the Master and Pilot would not have considered departing from the channel into the Zone B area.

The Antipodean Mariner is careful to qualify that the ZoC contributed to, but was not the primary factor in the grounding. Pilotage practices have been changed though when berth approach requires the vessel to leave the swept channels and a sounding program is being undertaken to ensure no more surprises are found near the busy terminals.


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