# How should I interpret the difference in flight direction (azimuth) for great circle and rhumb line?

I am trying to understand the image above. If you fly in the Northern Hemisphere going west. The image said it's 150 degree if you fly Great Circle while 270 degree flying rhumb line.

If you think about this on a map, how is it possible to be 150 degree? Shouldn't the degree be between north(360) and west(270) as you fly west? I'm so confused.

• Welcome. Don't think at it on a map, think at it on a sphere. the heading depends on how the two end points of the path are located related to each other. – mins May 31 '15 at 12:45
• It's not just that 150°. Most of the green numbers are wrong, really. – Jan Hudec Jul 29 '16 at 7:03

I have no idea where the image comes from, but it seems that you have spotted an inconsistency/copy-paste typo.

Let's remember that the heading is always measured w.r.t. the north, increases in a cloclwise fashion and indicated the direction the aircraft is flying to, as shown in the following figure:

(Image Source: www.allstar.fiu.edu)

On the Rhumb line both East and West directions are shown both with arrows ($\rightarrow \;\; \leftarrow$) and with the initial headings ($90^o \;\; 270^o$)

On the Great Circle lines only the westerly arrows appear and only the starting headings from the West are correct. These seems to have been copied to the East side, but they should have also been augmented by $180^o$, i.e. as you were expecting, you should have $330^o$ for the northern hemisphere (in lieu of the $150^o$) and $210^o$ instead of $30^o$ for the southern.

After some thoughts and ask around, finally figure out the answer to this.

It can be depicted by the image below:

(Image Source: Own Work - Author: tipsywacky)

If you are flying great circle, the angle is measured against the meridian. If it starts from left to right, it will be 30 degree increasing and then reach 150 on the other side and vice versa. Drawing it out on the orange is the key.

• Did you make this picture yourself? – SentryRaven Jun 1 '15 at 11:19
• Just something I discovered this week: On th PC Version of Google maps, there is a distance measuring tool. While it looks like a ruler and draws straight black lines, what happens when you zoom out and measure Paris - New York? Have fun! – sweber Jun 1 '15 at 11:26
• @SentryRaven Yeah I made it myself. haha. and sweber thanks for the info. I am gonna try google earth as well. It's always good to have something being able to give us a better understanding. – tipsywacky Jun 2 '15 at 5:21
• and vice versa not really, as when you go fromright to left the angles are larger than 180 (yes, you measure against the meridian, but heading angles are always measured clockwise) – Federico Jun 2 '15 at 6:09
• Ah, good call @Federico. I'll keep a note to it. I guess there are different ways of viewing it. The image uses the 180 degree benchmark or measure against the equator. Thanks for pointing it out. – tipsywacky Jun 3 '15 at 4:40