Figure: True Course initial vs. midpoint vs. ending, from Eddie's notes.
Your plotting chart is based on a Lambert Conformal projection, the lines of longitude converge near the poles. Except for the equator, the lines of latitude are not straight, they curve toward the equator. The measurement of your true course depends on where you place the center of your plotter and it does make a difference. In the figure shown, flying from 33°N 160°W to 33°N 150°W should, intuitively, require a 090° true course. The actual course, however, depends on what you want: the starting, mid, or ending course.
Why is this important?
Figure: Course line in pencil, from Eddie's notes.
In the example we draw a line from 50°N 030°W and 51°N 020°W.
Figure: Course line with plotter on initial point, from Eddie's notes.
Placing the center of the plotter compass on our initial point and the course line on top of the endpoint we see our initial true course is 077°.
Figure: Course line with plotter on midpoint, from Eddie's notes.
Placing the center of the plotter compass on the midpoint, 025°W in our example, we see the true course will be 081°.
We can check this mathematically using 10 degree tables at 50°N. See: International Operations / True Course 10 Degree Tables / 50°.
Portions of this page can be found in the book Flight Lessons 1: Basic Flight, Chapter 23.
Portions of this page can be found in the book International Flight Operations, Part II, Chapter 3.