A Great Circle Course is drawn first on a Gnomonic Chart. This GC course is a straight line on the Gnomonic chart because of the Projection of the chart. Thereafter you need to transfer this course on to the Mercator Chart which results in plotted course appearing as a curved route.
How to transfer the GC route to the Mercator Chart?
It needs to be decided by the Master as after how many degrees of longitude would he require to alter the course of the vessel. Also, he needs to decide what is the maximum latitude he wants to reach and limit his route. Basically, he does not want to go higher than a certain latitude to avoid bad weather, ice limits etc
This highest latitude is called the vertex of the GC course.
So after these two things are decided, we can start transferring the waypoints from the gnomonic chart to mercator chart.
Let us assume that Master decides the course alteration interval to be carried out every 5 degrees of longitude.
So, first pick up the way points from the Gnomonic chart and not down the latitude and position where the GC Route drawn on Gnomonic chart crosses longitudes at every 5 degree interval. For example, if you have noted down the latitude at 030 - 00W then note the latitude passing through 025-00W longitude on your course. Similarly, the next will be latitude passing through the 020-00W. So the interval remains constant at 5 degress of longitude.
Now as we know the limiting latitude of the course. You shall ensure that the route does not go beyond that latitude.
After noting down these way points plot these waypoints on the mercator chart and you will observe that the Course drawn appears to be curved.
The Advantages of GC Route is -
1. Saving of distance,
2. Saving of time
3. Saving of fuel
Disadvantages of GC Course
1. Takes you to higher latitudes which may result in encountering heavy weather resulting in drop in speed
2. Practically, no course is a pure GC Course. Mostly a combination of Rhumb Line course and GC Course is used for transatlantic voyages. Hence, the distance saving is slightly less than anticipated.