Departure, Earth convergence (24-05-18)

Question 1

What is departure or rhumb line distance?

Answer 1

The distance between 2 positions where the track crosses all meridians at the same angle (on a small circle)

Question 2

What is the equation for departure?

Answer 2

Departure = (Chlong x 60) x cos(mean(lat))

Distance covered as if at the equator but scaled down due to latitude.

Question 3

What is the distance along a latitude called?

Answer 3

Departure/rhumb line distance

Question 4

What is the max distance between 2 positions that short distance approximation can be used?

Answer 4

400nm

Question 5

How do you use short distance approximation?

Answer 5

Make a triangle using minutes of arc from longitude change and departure with mean latitude.

Use pythagoras for the actual distance

Use trig for the bearing.

Question 6

What is Earth convergency?

Answer 6

the angle between 2 meridians, at a particular latitude, altering the angle between great circle tracks and meridians.

Question 7

What is the equation for earth convergency?

Answer 7

EC = Chlong x sin(mean(lat))

Question 8

What is the technique for working out a great circle track at one meridian from another?

Answer 8

Transpose a ‘ghost’ version of the starting meridian onto the second and work out EC.

the transposed ghost meridian will tell you whether to add or subtract EC.

Question 9

What is conversion angle?

Answer 9

The angle between a rhumb line and a great circle between 2 points (measured at departure and arrival points).

Question 10

What is the equation for Conversion angle?

Answer 10

CA = 1/2Chlong x sin(mean(lat)) = 1/2 EC

Question 11

What is the difference between great circle and rhumb line tracks at the half way point between 2 positions?

Answer 11

It is the same, there is not difference