Navigation, Charts and Position Fixing
Chart work in Part A is practical: read a Hong Kong chart, convert a bearing, and fix the vessel. Questions lean on chart datum, the compass and tidal height rather than on celestial navigation.
Chart datum and soundings
- Depths are reduced to chart datum, the low-water reference. A drying height is measured above datum and is shown differently from a sounding.
- Depth of water at any time equals charted depth plus height of tide. The Hong Kong Observatory publishes predicted heights above chart datum.
- Corrections come from Local Notices to Mariners and chart updates. An uncorrected chart is both a hazard and a mark-loser.
Position fixing
- Three-point fix: choose objects subtending wide angles, take bearings quickly, plot, and read the cocked hat. A transit gives one reliable line; combine it with a single bearing.
- GPS positions are on WGS 84. Check the chart's horizontal datum before plotting.
- Dead reckoning needs course, speed, leeway and an estimate of set and drift.
Compass conversion
Compass to magnetic to true: add easterly deviation and variation, subtract westerly. Work backwards when going from chart to compass.
Worked example
A hand-bearing compass gives 095°C on a headland. The deviation card shows 3°E for that heading; the chart notes variation 2°30' W.
Reasoning: compass to magnetic, add 3°E, giving 098°M. Magnetic to true, subtract 2°30' W, giving 095°30'T. Plot 095.5°T from the headland. Had the deviation been forgotten, the plotted line would be three degrees out - over a mile at twenty miles' range.
Traps
- Adding variation when the chart note says west.
- Forgetting the deviation card for the steering compass.
- Comparing an echo sounder reading with the chart without adding height of tide.
- Plotting a GPS position without checking the chart datum.