Rhumbline track question - please help
 

I had this yesterday and now I can't fathom it! Please help if you can - thanks.

A rhumbline track between 45 degrees N 010 degrees, W and 48degrees 30 minutes N, 015 degrees W is approximately.....?

330, 315, 345 or 300.

Thanks

If I remember rightly...

Draw the line from point A to B, which becomes the hypotenuse of a right angled triangle.

Remember that going North to South and vice versa that: 1' = 1 nm
and that the East to West part of the triangle you have drawn is the DEPARTURE.

This is where they try and catch you out! http://www.arrse.co.uk/images/smiles/icon_hump.gif
(Remember: Departure = Change of Longitude in minutes x cosine latitude)

These should give you the horizontal and vertical dimensions of the triangle, which you can then punch into the basic trigonometrical equations. (SOH, CAH, TOA)

I guarantee that one or two of these will crop up in your General Navigation exam. Just remember to use the Departure formula for the horizontal bit of the triangle!

Let me know if it works! :ok:

For General Navigation learn the formulas off by heart and be nifty with the whizz wheel.

I work better at remembering formulas than functions on the whizz wheel, so I went down that route. Other people are a demon with the CRP-5. You do have to be proficient with the whizz wheel for certain parts of the exam where there is no way of using an equation.

Get yourself on the Bristol Question Bank and use your school pass papers (Bristol were excellent.). I'd even suggest buying the General Navigation Manual off Bristol. Give them a shout. They also hold open days just for General Navigation and Meteorology. Well worth it if you think you need it.

In all honesty, the General Navigation exam is nowhere near as bad as what it first looks. You are actually only tested on a very small amount of the content. PM if you want anymore advice. I passed General Navigation and Meteorology at Oxford back in January with 87% and 78%, respectively. It's still fairly fresh! ;)

P.S Be good at drawing your diagrams of Rhumb Lines etc.

Mutley,

Remember. "East/West distance is Departure." This is the key to why you are getting stuck on both of these.

Thus the East/West elements of those question are equal to the Departure.

Departure = change of longitude x cosine latitude

(Remember that the change of longitude is in minutes in the departure equation above, not degrees as it is in the Convergency equation.)

Just rearrange the departure equation to find the change in longitude along the East/West bits... and Robert's your Father's Brother. PM if you want more help! :ok:

This problem can be solved by approximating the situation to a right angled triangle.

The base of this triangle will be the line from 45N 010W to 45N 015W.

The distance from 45N 010W to 45N 015W can be found using the departure equation:

Departure in nm = Change in longitude (in minutes) x Cos Latitude.

From 010W to 015W is 15 degrees. Which is 15 x 60 = 300 minutes change of longitude.

So between 45N 010W and 45N 015W the
departure 300 x Cos 45 = = 212.132 nm.

So the base of the triangle represents a distance of 212.132 nm.

The vertical left side of the triangle is the line from 45N 015W to 48 30N 015W.

Moving up or down any meridian each minute of latitude is equal to 1 nm. The change of latitude from 45N to 48 30N is 3 degrees 30 minutes, which is 210 minutes. So the distance from 45N to 48 30 N is 210 nm.

So the vertical left side of the triangle represents a distance of 210 nm.

Use the two distances above to draw a right angle triangle joining the point 45N 010W, 45N 015W and 48 30N 015W.

The hypotenuse of this triangle is the rhumb line from 45N 010W to 48 30N 015W.

In this triangle the track from 45N 010W to 45N 015W is 270 degrees.

To calculate the Rhumb Line from 45N 010W to 48 30N 015W we must add the internal angle at 45N 010W.

From the triangle it can be seen that the Tangent of this angle = 210nm / 212.132 nm, which is 0.9899.

So the internal angle at 45N 010W is 44.71 degrees.

Adding this to 270 gives the rhumb line track of 314.7 degrees.

The closest answer to this in the question is 315 degrees.

I have made this explanation very detailed because the important thing is to get the method sorted out in your head. This will enable you to solve any new problems of this type when they appear in your exam.