I don't know where you ever got the idea that a great circle is any more accurate than a rhumb line, they are both equally accurate,...
So, how can a rhumb line EVER be AS ACCURATE as a well laid out great circle, unless right along the equator? It can't - simply because a rhumb line really amounts to flying a curve (if above or below the equator) - therefore its a bit deceiving to the casual observer - and accuracy is in fact lost. Of course as we both know, for shorter distances over land where landmarks are much more prevalent, a rhumb line is a fine thing - there's little lost, and certainly no more lost than is easily made-up by the use of occasional landmarks.
I hate to be this blunt Jeff, but you are just plain wrong. We can argue about the facts of the Earhart case but you can't argue that two plus two does not equals four. Rhumb line navigation is the standard way that ships have always been navigated and to a much higher level of precision than aircraft. I posted an experiment you can do
on Google Earth here, and the example I used had both points at the same latitude. The rhumb line course between those point is exactly 90.00000000000000000000000000°, straight east from Milwaukee to the end point in Asia and if you can fly a perfect heading of 90.00000000000000000000000000000° without any errors or cross wind then you will end up at the end point. The course is exact. And this goes for every other rhumb line too. Of course
flying either the great circle or the rhumb line will have the same level of errors in attempting to stay on the chosen line.
What course line, great circle or rhumb line, appears as a straight line depends on the projection used to create your chart. On Mercator charts a rhumb line is straight and the great circle appears as a curved line. On a Gnomic chart it is just the opposite, the GC is straight and the RL is curved. You use Sectional Charts when you fly. Those charts are based on the Lambert Conformal Projection and on these charts a straight line APPROXIMATES a great circle. On this type of chart the meridians are not parallel but converge toward the pole. I am sure you were taught, when plotting a course on your sectional, that you should measure the true course with your plotter placed on the meridian line nearest the midpoint of your course line, not at the meridian near your starting point. You didn't know it, but you were actually measuring and then flying the rhumb line course, not the great circle. Take out a sectional and prove it to yourself. Lay out a long course line and then measure the direction at the central meridian as you normally do. Then go back to the meridian nearest the departure and lay off the course direction you just measured. This line will go south of the first, approximate GC, line. Then where that line hits the next meridian lay off the course line again in the direction you had measured and do this with subsequent meridians. You will end up drawing a curved course line between your departure and your destination that is south of the straight line. The easiest way to do this test is to pick two spots at the same whole degree of latitude. The course you measure will be 90 degrees and if you follow the procedure you will end up drawing in the parallel of latitude since that line crosses each meridian at 90 degrees but you don't have to draw it yourself since it is printed on the chart for you already. The parallel is the rhumb line between those two points. On a Lambert chart this curved line is a rhumb line and this is the line you will actually fly without a cross wind and without looking out the window, this is the way the autopilot will take the plane. And now that you are looking at this carefully, it should be obvious that you were measuring and flying a rhumb line and not a great circle. You can't measure a single course line in a single direction and be following a great circle as Williams' careful computations demonstrate. To actually measure a great circle on the Lambert chart would require you to measure a new course at all the meridians that your course line crossed and make up a table like Williams did of the changing courses necessary to define the great circle. So the proof that you can measure and fly a rhumb line as accurately as a great circle is that
you have gotten to your destinations and you have been flying rhumb lines even though you didn't know it. You have been able to get to your destinations, haven't you?
Good, then I'll call you as my first witness.
If Earhart had been able to maintain a true course of 078.1° for 2556 SM then she would have hit Howland (within the approximate 4 SM accuracy available from a course expressed to one-tenth of a degree.)
I have attached several pages from the
American Practical Navigator, U.S. Navy Hydrographic Office Publication Number 9,
the standard reference book used by the Navy and all American surface navigators.
gl
