Departure, Earth convergence (24-05-18) Flashcards Preview

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What is departure or rhumb line distance?

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


What is the equation for departure?

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

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


What is the distance along a latitude called?

Departure/rhumb line distance


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



How do you use short distance approximation?

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.


What is Earth convergency?

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


What is the equation for earth convergency?

EC = Chlong x sin(mean(lat))


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

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.


What is conversion angle?

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


What is the equation for Conversion angle?

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


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

It is the same, there is not difference