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Sailing reference
Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.
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The angle from straight overhead down to the body
Equal to 90 degrees minus the true altitude. Expressed in minutes of arc it converts directly to nautical miles of distance from the geographical position, one minute equalling one nautical mile. Zenith distance underpins the noon latitude calculation, where latitude equals zenith distance combined with declination according to whether the body was north or south of the observer. It is also the radius of the circle of equal altitude used in the intercept method.
Read definition →A chart rating for how trustworthy the survey data really is
CATZOC is an ENC meta-attribute grading survey quality from A1, the best, through A2, B and C to D, with U for unassessed. Category A1 implies position accuracy of about five metres and depth accuracy of roughly half a metre plus one per cent of depth, with full seafloor search; the lower categories widen these limits sharply and may involve no systematic search for isolated dangers. Poor zones demand greater under-keel clearance.
Read definition →A chart rating showing how good the underlying survey was
The IHO scheme classifies survey quality as A1, A2, B, C, D and U for unassessed, each defining position and depth accuracy and the expected seabed coverage. A1 implies full seafloor search with position accuracy of about 5 metres plus 5 percent of depth, while C and D indicate large uncertainties and possible undetected hazards. ZOC is shown on paper charts as a diagram and in electronic charts as the CATZOC attribute, and should be checked before relying on charted depths.
Sailing reference
Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.