Definition of headings

True, Magnetic and Needle Course: Definitions

The headings are defined in function of the meridian taken as origin (true, magnetic or needle) and the bow-stern line of the vessel.

• TRUE course (Rv): he angle formed by the true meridian of the place and the bow-stern line.
• MAGNETIC course (Rm): he angle formed by the magnetic meridian of the place and the bow-stern line.
• NEEDLE course (Ra): he angle formed by the needle meridian (determined by the orientation of the needle and differs from the magnetic due to the magnetic materials of the ship) and bow stern line.

Relationship between them

If we look at Figure 9.11, we can deduce that: Rv = Ra + Δ + dm, and since Ct = dm + Δ.

• Rv = Ra + Ct
• Ra = Rv – Ct

From the same figure it is also easy to deduce that:

• Rm = Ra + Δ Rm = Rv – dm

Ways to measure quadrantal and circular heading

• Circulars: from 0o to 360o and clockwise. Its value is always positive.
• Quadrantals (or quadrants): dividing the circumference into four quadrants.

To avoid confusion the directions must be said or state by numbers.

From quadrantal value to circular

Of degrees to quadrantals either “quadrants” to circular degrees:

• Between 000-090 – belong to the 1st quadrant (NE) and correspond to the same number of degrees
• Between 090-180 – belong to the 2nd quadrant (SE) – Se subtract from 180o
• Between 180-270 – belong to the 3rd quadrant (SW) – They 180o remains
• Between 270-360 – belong to the 4th quadrant (NW) – They subtract from 360o

Examples:

• N45oE = 045o (First quadrant).
• S10oE = 180o – 45o = 135th (Second quadrant).
• S10oW = 180o + 45o = 225th (Third quadrant)
• N45oW = 360o – 45o = 315th (Fourth quadrant).