Navigation concepts. Course and heading

In this article we discuss navigation concepts and look at the differences between course and heading.

Course, true course, magnetic course y desired track

We will call course the trajectory to follow, it is the planned or desired route between two points A and B. It could be identified as the straight line that joins both points.

The true course (TC: True Course) would be the angle in degrees between the true or geographic north and the course. It is measured from the geographic (or magnetic) north in a clockwise direction.
Since the meridians on a map all point to true north, they can be used as 0 degree references to measure angles to true north.

Since the geographic north and the magnetic north pole do not coincide (see the article "Magnetic declination" in the "Additional information, curiosities and glossary of terms" section of this site) we will also define the magnetic course (MC: magnetic course) as the angle in degrees between magnetic north and the course. It is measured from magnetic north and also clockwise.

In this case, the meridians cannot be used as references of 0 degrees, instead we will use the compass.

In some environments, the Desired Track (DTK) refers to the true course and the magnetic course interchangeably , depending on the north used as reference (geographic or magnetic).
 

Heading, true heading, magnetic heading, drift y drift correction angle

We will call heading (Heading) the angle formed by the north (geographic or magnetic) and the longitudinal axis of the aircraft (in the case of an airplane it would be where its nose points). It will be called true heading (TH: True Heading) if the angle is taken with respect to geographic north and magnetic heading (MH: Magnetic Heading) if it is taken with respect to magnetic north.

The heading does not necessarily coincide with the course since the aircraft or ship can change its orientation to counteract a crosswind or water currents. In the case of airplanes it is usual that, to maintain a constant course, the aircraft turns its nose slightly towards the wind.

The angle formed between the desired course and the heading of the aircraft will be called the drift correction angle (WCA: Wind Correction Angle) since, by definition, drift is the deviation of a ship from the established heading, for effect of wind, sea or current. 

It's basically the difference between where the ship is pointing and where it's actually moving. This angle is usually calculated by the flight computer. If the wind comes from the left, the correction angle will be negative. If it comes from the right it will be positive.