SVOP Navigation and Chartwork 2 — Questions and Answers

Question 1: What is the difference between True North, Magnetic North, and Compass North?

True North is the geographic North Pole; Magnetic North is where the compass points (varies by location due to declination); Compass North is the actual compass reading (affected by magnetic deviation from the vessel) (Correct answer)
True North and Magnetic North are the same — both point to the geographic pole
Compass North is always identical to Magnetic North on a properly calibrated compass
Magnetic declination and compass deviation describe the same error from the same cause

Correct answer: True North is the geographic North Pole; Magnetic North is where the compass points (varies by location due to declination); Compass North is the actual compass reading (affected by magnetic deviation from the vessel)

True North: geographic pole (grid/chart north). Magnetic North: where the compass needle actually points (differs from True by declination/variation, which changes by location and year). Compass North: what your specific compass reads (differs from Magnetic by deviation caused by the vessel's metal and electronics).

Understanding the three 'Norths' is fundamental to navigation: True North is the direction of the geographic North Pole, used on charts. Magnetic North is the direction a free-floating compass needle points, pulled by the Earth's magnetic field. The angle between True and Magnetic North is called Variation (or Declination), which differs by geographic location and changes slowly over time (values are printed on chart compass roses). Compass North is what your specific compass aboard reads, influenced by the vessel's own magnetic field from engines, electronics, and metal structures. The difference between Magnetic North and Compass North is called Deviation, which varies with the vessel's heading. The formula TVMDC: True + Variation = Magnetic + Deviation = Compass is used to convert between them.

Question 2: On a nautical chart, what do contour lines (depth contours or isobaths) represent?

Lines connecting points of equal water depth, helping mariners identify shallow areas, underwater ridges, and safe water routes (Correct answer)
Lines showing equal magnetic variation across a chart area
The high water mark along the shoreline at spring tides
Currents of equal velocity shown on tidal atlas charts

Correct answer: Lines connecting points of equal water depth, helping mariners identify shallow areas, underwater ridges, and safe water routes

Depth contours (isobaths) connect points of equal depth below chart datum (lowest normal tide level). They help mariners identify the shape of the seabed, locate shoals and banks, determine safe draft clearance, and plan routes away from hazards.

Nautical charts produced by the Canadian Hydrographic Service (CHS) show the seabed topography using depth contours. Depths are measured from chart datum, which is typically the Lowest Normal Tidal Level — meaning actual depths at the time of sailing will usually be greater than chart values. Common depth contours are the 2 m, 5 m, 10 m, 20 m, and 50 m lines. Areas inside the 2 m or 5 m contour are generally shallow and hazardous for most vessels. Closely spaced contours indicate a steep underwater slope; widely spaced contours indicate a gentle grade. Operators use contour lines to navigate safely, stay in deep water channels, and identify anchoring areas with appropriate depth and holding ground.

Question 3: What is a 'range' or 'transit' in coastal navigation?

When two charted objects (lights, towers, beacons) are observed in line with each other, providing a precise position line (Correct answer)
The maximum distance a vessel's radar can detect a target
The rated range of a VHF radio in statute miles
An area designated for military or commercial target practice marked on charts

Correct answer: When two charted objects (lights, towers, beacons) are observed in line with each other, providing a precise position line

A range (or transit) occurs when two known charted objects align visually. The observer must be on the line through those two objects, giving a precise position line (LOP). Charted leading lights or marks are specifically placed to guide vessels along safe channels.

A range or transit is one of the most accurate and simple position-finding tools in coastal navigation. When a mariner sees two known charted objects (for example, a lighthouse and a church steeple) in perfect vertical alignment, they know they are on the straight line passing through both objects. This line of position (LOP), transferred to the chart, shows the mariner's line of position. Intersecting two such ranges gives a very accurate fix. Navigational ranges (also called leading lights when lit) are purposely established in harbour approaches to guide vessels along the safe deep-water axis of a channel. Day marks use coloured stripes; night marks use lights of specific colours and sequences. They are more reliable than GPS in harbour approaches due to their passive, fail-safe nature.

Question 4: What is the purpose of a 'DR plot' (Dead Reckoning plot) on a nautical chart?

To estimate the vessel's current position by projecting from the last known fix using course, speed, and elapsed time (Correct answer)
To record confirmed GPS positions at regular intervals in the ship's log
To plot the depth of water along the intended track using echo sounder data
To show the predicted set and drift of tidal currents on a chart

Correct answer: To estimate the vessel's current position by projecting from the last known fix using course, speed, and elapsed time

Dead Reckoning (DR) is the practice of advancing a known position forward by applying the vessel's course and speed over time. It provides an estimated position when GPS is unavailable or needs to be confirmed by other means.

Dead Reckoning is a fundamental navigation technique predating GPS. Starting from a known position (the 'fix'), the navigator applies: (1) the course steered (true, corrected for leeway); (2) the speed through the water; (3) the elapsed time. The result is a DR position — where the vessel should be geometrically. However, DR does not account for current or wind drift. An 'estimated position' (EP) improves on DR by adding a vector for estimated current set and drift. DR plots become critical when GPS fails or is unreliable and when navigating in poor visibility. The DR position should be updated whenever a new fix is obtained. Monitoring the difference between DR position and actual GPS fix reveals the strength and direction of current affecting the vessel.

Question 5: What does a lateral buoy with a red triangular topmark indicate in the IALA 'B' buoyage system used in Canada and North America?

A starboard-hand mark — keep it on your starboard (right) side when proceeding from sea into harbour (returning) (Correct answer)
A port-hand mark — keep it to port when entering harbour
A danger mark — a hazard lies directly beneath the buoy
A fairway buoy indicating the centre of a navigable channel

Correct answer: A starboard-hand mark — keep it on your starboard (right) side when proceeding from sea into harbour (returning)

In the IALA Region B system (Canada, USA, Japan), red marks with even numbers are kept to starboard when returning from sea ('Red Right Returning'). Green marks with odd numbers are kept to port. The triangular topmark confirms it is a starboard-hand (red) mark.

Canada uses the IALA Maritime Buoyage System Region B, which differs from Region A (Europe, Africa, Australia) in the assignment of colours to lateral marks. In Region B: Red buoys (typically can-shaped in the US, but either conical or can in Canada) mark the starboard side of a channel when proceeding from open water into port (returning from sea). Green buoys mark the port side. The mnemonic 'Red Right Returning' helps remember this: when heading upstream or into port, red marks are kept to the right. When leaving port and heading seaward, the colours reverse — red is to port. Operators must know whether they are 'returning' or 'proceeding to sea' to correctly interpret lateral buoyage.

Question 6: What is a 'Cardinal Buoy' and what does a north cardinal mark indicate?

Cardinal marks are placed to indicate safe water lies on the named cardinal side of the mark — a north cardinal indicates safe water to the north of the hazard (Correct answer)
Cardinal marks indicate the direction to the nearest harbour from any cardinal direction
A north cardinal mark means the vessel should head north to avoid the danger
Cardinal buoys mark the edges of the shipping lane closest to the named direction

Correct answer: Cardinal marks are placed to indicate safe water lies on the named cardinal side of the mark — a north cardinal indicates safe water to the north of the hazard

Cardinal marks are yellow and black buoys placed near hazards, indicating that the safest water lies on the named cardinal side. A north cardinal mark: safe water is north of the buoy. Pass to the north of it.

Cardinal buoys are part of the IALA system and use yellow and black colours with distinctive topmarks (two black cones pointing in various directions) to indicate which side of a hazard has safe water. The four cardinal marks: North Cardinal — black over yellow, cones pointing UP (both up) — safe water to the NORTH; South Cardinal — yellow over black, cones pointing DOWN — safe water to the SOUTH; East Cardinal — black-yellow-black bands, cones pointing away from each other (base to base) — safe water to the EAST; West Cardinal — yellow-black-yellow bands, cones pointing toward each other (point to point) — safe water to the WEST. They are placed to the north, south, east, or west of a hazard. Their light sequences are based on a clock: north = continuous, east = 3 flashes, south = 6 flashes + long, west = 9 flashes.

SVOP Navigation and Chartwork 2

What is the difference between True North, Magnetic North, and Compass North?