(09-11-2024, 02:51 PM)PeterB Wrote: I was checking out some practice questions for great circle calculations at this site:

https://www.starpath.com/cgi-bin/ubb/ult...6;t=000452

I do not know if the questions are U.S.C.G. or Starpath generated, but in the thread David Burch offers a link to a YouTube video where he shows their solutions by a plotting program called QTVLM  and there he points out that his answers vary slightly from those of the practice questions because QTVLM uses an ellipsoid model of the Earth while the Coast Guard uses a spherical model for these questions -- so I suppose the questions are U.S.C.G.

In the forth question, 5-615 , they list a starting point, an end point, and a distance between the two along a great circle track. They then give you a zone time of the start of the journey and zone time descriptors for both the start and end locations.  You are asked to figure out the estimated local zone time date and time of arrival at the destination based on a steady speed of 13 knots.

In the YouTube video Burch uses the automated program to generate times in UTC and then converts the UTC time of arrival into a local zone time. It seemed to be a fairly involved "work around" with the software to get the software to do what he wanted, but he did get to an acceptable answer. One that was close enough to the "correct" answer that you would definitely pick that one.

When I tried the question (before viewing the YouTube video) I simply took the provided great circle distance and divided it by 13 knots to get the time en-route in hours. Then I converted that into days/hours/ minutes.  This I added to the UTC time of the start of the journey, derived a UTC time of arrival at the end, and finally converted that to a local zone time -- and that didn't work out to be close to any of the provided answers.

I scratched my head, checked my math and it still didn't work out.

Knowing the "correct" answer I was able to back calculate the time en-route for that answer and see how much distance that would cover. The answer was far less than the distance of 4245 nmi offered as a given information in the question.

I then went ahead and solved the great circle problem for initial course and distance using H.O. 208 tables and found the distance by that method to be 4163 nmi.

Lastly I used MarineWaypoints.com great circle calculator to cross check and got a distance of 4164 nmi

So the upshot is the information listed in the question was incorrect. Using that incorrect information there is no way to get the correct answer except by tossing out their numbers and starting from the very beginning.

I attempted to reply to the thread at Starpath to point this out since it seems even David Burch isn't aware of this error, but even though the forum is a "public discussion" I was unable to log into the discussion to post.

PeterB