Navigation & Chart Work Flashcards

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

Answer: True North is geographic north; Magnetic North is where the compass points due to the Earth's field; Compass North includes instrument deviation

True North is the direction of the geographic North Pole. Magnetic North is where the Earth's magnetic field points the compass needle and varies from True North by variation (declination). Compass North is further affected by the ship's own magnetic field, called deviation.

What is 'variation' on an Admiralty chart?

Answer: The angular difference between True North and Magnetic North at a given location and time

Variation is the angular difference between True North (geographic) and Magnetic North at any given point on the Earth's surface. It varies by location and changes slowly over time due to shifts in the Earth's magnetic field. It is shown on Admiralty charts.

What is 'deviation' in compass navigation?

Answer: The error in a ship's compass caused by the vessel's own magnetic field and ferrous material on board

Deviation is the error in a compass caused by the magnetic influence of the vessel's own steel structure, electrical systems, and ferrous items on board. It varies with the ship's heading and must be assessed using a deviation card.

A vessel's compass reads 045°C and deviation for the heading is 3°W. What is the Magnetic heading?

Answer: 042°M

To convert Compass to Magnetic: apply deviation. Westerly deviation is subtracted from compass heading. 045° - 3° = 042°M. The mnemonic 'Compass to True — add East' (or subtract West) guides the conversion.

What is a 'fix' in coastal navigation?

Answer: A positional determination obtained by crossing two or more position lines from observed bearings, landmarks, or electronic systems

A fix is a definite position obtained by crossing two or more position lines simultaneously or near-simultaneously. Multiple bearings of charted objects, radar ranges, or electronic position data are used to determine the vessel's position with confidence.

What is the purpose of a 'running fix'?

Answer: To obtain a position using two bearings of the same object taken at different times, with the ship's track run up between them

A running fix uses two bearings of the same object (or two different objects at different times) with the vessel's estimated track run up between them to obtain a position. It is used when only one charted object is visible.