Charts, projections and symbols
Navigators work mainly on the Mercator projection, on which meridians are parallel and a straight line is a rhumb line of constant course — ideal for plotting, at the cost of scale distortion that grows toward the poles. Distance is therefore always measured on the latitude scale beside the vessel's position, never the longitude scale. The gnomonic projection, on which a great circle plots as a straight line, is used for ocean great-circle planning and for harbour plans. Every symbol and abbreviation is defined in Admiralty publication NP5011, and the chart's horizontal datum must be checked because a GNSS position on WGS84 may need shifting to match an older chart datum.
Fixing and estimating position
| Type | Meaning | Reliability |
|---|---|---|
| DR (dead reckoning) | Position from course steered and distance run only | Lowest |
| EP (estimated position) | DR corrected for tidal set/rate and leeway | Better |
| Fix | Two or more position lines that cross | Good |
| Running fix | One position line transferred along the ground track to cross a later one | Good when only one object is available |
| The position types, in ascending order of confidence. |
Position lines come from compass bearings, radar range and bearing, horizontal or vertical sextant angles, a line of soundings, or the rising or dipping distance of a light. Three position lines crossing in a small triangle — a tight 'cocked hat' — give confidence; a large cocked hat signals an error to resolve, not a position to accept, and the OOW should treat the vessel as being at the corner nearest any danger.
The four-stage passage plan
Appraisal, Planning, Execution, Monitoring
- Appraisal — gather every source: charts, sailing directions, light lists, tide tables and stream atlas, routeing charts, navigational warnings and the ALRS, and assess the risks of the whole passage.
- Planning — lay off the intended track berth to berth, marking no-go areas, clearing bearings, wheel-over points, contingency anchorages, abort points and the required underkeel clearance.
- Execution — brief the bridge team, confirm the plan against the actual tide and weather, and adjust timings for tidal gates before getting underway.
- Monitoring — fix and cross-check position by primary and secondary means throughout, and act early the moment the vessel is set off the plan.
WARNING — Never trust a single source A confident but wrong GNSS position has grounded many ships. Cross-check the electronic position against visual bearings, radar ranges and the echo sounder, and keep a running EP so a jump in position is obvious.