Navigation General · Chart Accuracy
A 'source diagram' or 'zones of confidence' (ZOC) note on a chart tells the navigator about the:
- AReliability and age of the survey data underlying the charted depths✓ Correct
- BTidal range
- CMagnetic variation
- DRadio frequencies
Explanation
Source diagrams and ZOC data indicate how, when, and how thoroughly an area was surveyed. Older, leadline-era surveys may have undetected hazards, so a prudent navigator allows greater clearance where the data are poor.
Authority: NOAA/IHO charting; zones of confidence
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