I am 100% certain I am not confusing the two. What book are you using? The only occasions whereby a great circle forms a curved line is: (a) on a Direct Mercator chart, whereby, owing to the parallel meridians on the chart, a straight line intersecting those lines is a line of constant bearing and as such forms a rhumb line. Nevertheless, the rhumb line is on the Equatorial side of the great circle. This means that the great circle is on the polar side of the rhumb line (as always!) and if the rhumb line is a straight line, the great circle must be a curved line (concave to the Equator - the origin of the Direct Mercator chart). (b) On Lamberts conformal conical charts, other than meridians and the Equator (which are straight lines), great circles are curved lines (very slightly curved), concave to the origin of the chart, which is the parallel of origin (the average of the standard parallels). (c) On polar stereographic charts, other than meridians (which are straight lines), great circles are curved lines (very slightly curved), concave to the origin of the chart, which is the pole. On both Lamberts and Polar charts, rhumb lines are curved lines, concave to the nearer pole, and on the Equatorial side of the great circle. What you are pointing out is a common misunderstanding among students, and usually based on the properties of the Direct Mercator chart, which often causes confusion. Your course book is probably referring to the Direct Mercator chart. If not, I am concerned as to the accuracy of the book. ICAO Annex 4 (Aeronautical Charts) clears it up nicely, as it is a mandatory requirement of ICAO Annex 4 that, on all aeronautical charts the great circle must approximate a straight line. Meridians and the Equator are straight lines in all cases. All other straight lines approximate great circles, within the usable area of the projection, and for practical purposes are taken to be great circles. Hope that helps, but please let me know which training material you are using.
No, definitely not, it depends on the hemisphere. Northern hemisphere - GCT < RLT to left of the track lines and GCT > RLT to right of the track lines; the opposite in the southern hemisphere (as in this question, the GCT > RLT to the left of the track lines - if you drew in the bearings on the right side you would see that the GCT < RLT). The direction of the track lines is irrelevant, easterly or westerly the situation remains the same.
Simples! The convergency formula is applied to find the change in a Great Circle Track between two points along the track line. This varies between a general earth convergency formula and a chart specific chart convergency formula (different for different charts). The conversion angle formula (equal to 1/2 x convergency) is applied to find the difference between the Great Circle Track and the Rhumb Line Track at a fixed point. Hope that helps. :-)
Thanks for your answer! That indeed helps, but as you mention, the Convergency is applied to find a Great Circle Track change, right? But then this type of question comes in: "An aircraft passes position A (60N 120W) enroute to position B (60N 140,3W). What is the Great Circle Track on departure from A? What should I use? One explanation (which gives the correct answer) says Conversion Angle. Why? Thank you so much!
Good day Nick, the rhumb line track is the constant bearing - that is the prime property of a rhumb line. In this case, that is determined by both positions being on the same latitude, which gives us the RLT of 090T. All we need to do to work out the GCT is to figure out whether we add or subtract the conversion angle and that can be determined from a simple diagram. Remember! Ask CARL! Conversion Angle Rhumb Line (or when converting between RL and GC always use CA). Hope that helps.
I use to Draw the lines in a different way, so correct me if I am wrong: You drawing the globe (as long as the Meridians are bent and divergent toward north (so SH) and the straight line is the GC Track). So the curve line means the Rhumb. If for any chance me or someone draw the same but not bending the reference meridians ... that make all wrong, right? I mean .. If you draw two parallel meridians straight north, the straight line now is the Rhumb and the curve line is the GC, right? Just I use to draw like that (curve line is the GC and straight line is the Rhumb). Only bc in most of the questions I found, they give you the Rhumb or is easy to find bc coincide with a Parallel (we flight on same Latitude, like this example) so a track 090 looks perfectly good as straight line to the right.. Sorry my question may be more confusion for some people trying to understand this Convergence and Conversions angle.
Hi Tony, I have the same answer but shouldn't the GC be curved down? To the South Pole I mean. We're on the south (70 deg.) and GC line curves to the nearest pole. It can be even noticed on the picture You've made - your GC line in point A is pointing hdg less than 090 deg instead of pointing greater hdg (132 in this questions). Am I correct? I just want to be sure, anyway your movies are really helpful! Best regards, Simon
Hi Simon, Two important points to note: 1. The great circle in the diagram is the straight line; the rhumb line is the curved line, concave to the pole and on the Equatorial side of the great circle. 2. The diagrams are only meant to show the relationship between the angles, i.e. which angles are greater and which are smaller, and are not to be seen as accurate diagrams. However, the great circle track at A appears greater than 90 degrees. The rhumb line track, being a parallel of latitude (two points are both at S70), follows a bearing of 090T. I hope that helps. Tony
Hi Tony, A quick question to clarify a point. We determine that the bearing of the RLT is 90°. Is it only due to the fact that we are flying easterly ? Would it be 270° if we were going from B to A ? Great explanations !
@cat3c On my ATPL Navigation book the Great Circle is the curved one and the Rhumb Line is the straight one..u sure u are not confusing the 2? Thanks
Go to atplontrack.com and see my other GNAV and other explanations. I will be adding more videos to atplontrack.com in the very near future.
Best explanation and the easiest explanation too!
This is gold, many thanks sir.
I am 100% certain I am not confusing the two. What book are you using?
The only occasions whereby a great circle forms a curved line is:
(a) on a Direct Mercator chart, whereby, owing to the parallel meridians on the chart, a straight line intersecting those lines is a line of constant bearing and as such forms a rhumb line. Nevertheless, the rhumb line is on the Equatorial side of the great circle. This means that the great circle is on the polar side of the rhumb line (as always!) and if the rhumb line is a straight line, the great circle must be a curved line (concave to the Equator - the origin of the Direct Mercator chart).
(b) On Lamberts conformal conical charts, other than meridians and the Equator (which are straight lines), great circles are curved lines (very slightly curved), concave to the origin of the chart, which is the parallel of origin (the average of the standard parallels).
(c) On polar stereographic charts, other than meridians (which are straight lines), great circles are curved lines (very slightly curved), concave to the origin of the chart, which is the pole.
On both Lamberts and Polar charts, rhumb lines are curved lines, concave to the nearer pole, and on the Equatorial side of the great circle.
What you are pointing out is a common misunderstanding among students, and usually based on the properties of the Direct Mercator chart, which often causes confusion. Your course book is probably referring to the Direct Mercator chart. If not, I am concerned as to the accuracy of the book.
ICAO Annex 4 (Aeronautical Charts) clears it up nicely, as it is a mandatory requirement of ICAO Annex 4 that, on all aeronautical charts the great circle must approximate a straight line. Meridians and the Equator are straight lines in all cases. All other straight lines approximate great circles, within the usable area of the projection, and for practical purposes are taken to be great circles.
Hope that helps, but please let me know which training material you are using.
So is a great circle track always greater than rhumb line track?
No, definitely not, it depends on the hemisphere. Northern hemisphere - GCT < RLT to left of the track lines and GCT > RLT to right of the track lines; the opposite in the southern hemisphere (as in this question, the GCT > RLT to the left of the track lines - if you drew in the bearings on the right side you would see that the GCT < RLT). The direction of the track lines is irrelevant, easterly or westerly the situation remains the same.
How do we know if they are asking about Convergency or Conversion Angle? That confuses me.
Simples!
The convergency formula is applied to find the change in a Great Circle Track between two points along the track line. This varies between a general earth convergency formula and a chart specific chart convergency formula (different for different charts).
The conversion angle formula (equal to 1/2 x convergency) is applied to find the difference between the Great Circle Track and the Rhumb Line Track at a fixed point.
Hope that helps.
:-)
Thanks for your answer!
That indeed helps, but as you mention, the Convergency is applied to find a Great Circle Track change, right? But then this type of question comes in: "An aircraft passes position A (60N 120W) enroute to position B (60N 140,3W). What is the Great Circle Track on departure from A?
What should I use? One explanation (which gives the correct answer) says Conversion Angle. Why?
Thank you so much!
Hi Tony,
Can i just ask why the constant bearing not he rhumb line track is 090T?
Many Thanks,
Nick
Good day Nick, the rhumb line track is the constant bearing - that is the prime property of a rhumb line. In this case, that is determined by both positions being on the same latitude, which gives us the RLT of 090T. All we need to do to work out the GCT is to figure out whether we add or subtract the conversion angle and that can be determined from a simple diagram. Remember! Ask CARL! Conversion Angle Rhumb Line (or when converting between RL and GC always use CA).
Hope that helps.
I use to Draw the lines in a different way, so correct me if I am wrong:
You drawing the globe (as long as the Meridians are bent and divergent toward north (so SH) and the straight line is the GC Track).
So the curve line means the Rhumb.
If for any chance me or someone draw the same but not bending the reference meridians ... that make all wrong, right? I mean .. If you draw two parallel meridians straight north, the straight line now is the Rhumb and the curve line is the GC, right?
Just I use to draw like that (curve line is the GC and straight line is the Rhumb). Only bc in most of the questions I found, they give you the Rhumb or is easy to find bc coincide with a Parallel (we flight on same Latitude, like this example) so a track 090 looks perfectly good as straight line to the right..
Sorry my question may be more confusion for some people trying to understand this Convergence and Conversions angle.
very good videos. thank you. greetings from germany
Hi Tony,
I have the same answer but shouldn't the GC be curved down? To the South Pole I mean.
We're on the south (70 deg.) and GC line curves to the nearest pole.
It can be even noticed on the picture You've made - your GC line in point A is pointing hdg less than 090 deg instead of pointing greater hdg (132 in this questions).
Am I correct?
I just want to be sure, anyway your movies are really helpful!
Best regards,
Simon
Hi Simon,
Two important points to note:
1. The great circle in the diagram is the straight line; the rhumb line is the curved line, concave to the pole and on the Equatorial side of the great circle.
2. The diagrams are only meant to show the relationship between the angles, i.e. which angles are greater and which are smaller, and are not to be seen as accurate diagrams. However, the great circle track at A appears greater than 90 degrees. The rhumb line track, being a parallel of latitude (two points are both at S70), follows a bearing of 090T.
I hope that helps.
Tony
+Cat3C ok, great, Thank You for reply! :-)
Hi Tony,
A quick question to clarify a point.
We determine that the bearing of the RLT is 90°.
Is it only due to the fact that we are flying easterly ?
Would it be 270° if we were going from B to A ?
Great explanations !
Great explanation!
Love u sir !
You just example like a butter 😜