Navigation and Chartwork Flashcards

How is tidal current (set and drift) applied to a course to steer calculation?

Answer: Set the current vector on the chart by advancing the DR position using current set and drift; the course to steer is adjusted into the current so the vessel tracks along the intended track

To find course to steer when current is present: from the DR position, draw a vector for set (direction current flows toward) and drift (distance current moves in the timed interval). The course to steer is the direction from the start point to the end of the drift vector, corrected back to the intended track.

What is a 'soundings plot' and when is it useful for navigation?

Answer: A technique where the vessel's echo sounder readings are matched against chart depth contours to confirm position — useful in fog when other references are unavailable

A soundings (or depth) plot involves recording the vessel's actual echo sounder readings at intervals and comparing them to depth contours on the chart. When a sequence of depths matches a contour pattern on the chart, the vessel's position can be estimated — particularly useful in fog or when GPS is unavailable.

What is the meaning of 'Chart Datum' on a Canadian Hydrographic Service chart?

Answer: The tidal reference level from which depths are measured — the Lowest Normal Tidal Level, ensuring charted depths are a minimum at normal tides

Chart Datum on Canadian charts is typically the Lowest Normal Tidal Level (LNTL), ensuring that the water will rarely be shallower than the charted depth. Actual depth = charted depth + height of tide. Operators add the current tide height from tide tables to find the real depth over a shoal.

What is 'leeway' and how is it plotted on a chart?

Answer: The angular difference between the course steered and the course made good through the water, caused by wind — plotted by offsetting the water track from the vessel's heading by the leeway angle into the wind

Leeway is the sideways push of wind on the vessel, causing it to track to leeward of the course steered. On the chart, the water track is drawn at the leeway angle to leeward (downwind) of the heading, before current is then applied to find the ground track.

What are 'leading lines' on a nautical chart and how are they used?

Answer: Charted lines connecting two aligned marks (leading lights) that indicate the safe axis of a channel — when the two marks are in line, the vessel is on the safe track

Leading lines (shown as magenta lines on CHS charts) connect two charted marks — typically paired lights or daymarks — placed specifically so that when observed in alignment they guide vessels along the safe axis of a harbour approach or channel. They are the most accurate tool for staying on track in a narrow entrance.

What is 'pilotage' and when is it required for commercial vessels in Canadian waters?

Answer: Navigation of a vessel through coastal or harbour waters using local knowledge of hazards, tides, and channels — compulsory pilotage areas require a licensed pilot aboard certain vessel classes

Pilotage is specialised navigation using local knowledge. Under the Pilotage Act, compulsory pilotage areas (e.g., St. Lawrence, Pacific Coast, Atlantic) require certain classes of foreign and domestic vessels to take on a licensed pilot. Commercial small vessels may be exempt but must navigate the same hazardous waters with due care.