View Full Version : Projection properties

20th Apr 2004, 06:58
G'day pprune

I'm currently studying for ATPL Nav and for the exam i'll need to know the shape of rhumb lines and great circles etc... on various chart projections.

In past exams I have always (and with success) tried to learn why things are so and the principles behind them rather than simply learn the answer to the question i'm likely to see in the exam. (I find this helps thinking through problems in the exam anyway by giving me a proper understanding of the material)

But cartography and making maps and charts is something I can't get my head around and I have no idea :confused: why a rhumb line might be this on a Lambert's Conformal and that on a Transverse Mercator.

Can anyone explain (or link to a good explanation) of what is going on or do people just remember the answers for the exam?

Hope you can help ;)



WX Man
20th Apr 2004, 19:34
In true Blue Peter fashion, get a globe and a piece of string (and an adult if you need one).

Take two points on your globe, say, London and New York, and stretch the piece of string between the two. You will notice that the line the string traces between the two points doesn't always cut the meridians at the same angle. That is the great circle, i.e. the shortest distance between the two points.

If you make the surface of your globe flat (using whatever method), it is very unlikely that the line that you originally drew would be straight.

Rhumb lines: these always cut meridians at the same angle... so on the globe, look at the equator. It cuts each meridian at 90 degrees. And look at the tropic of capricorn... same deal.

Hope that helps.