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The Aviation Formulary defines Clairut's formula to compute the intermediate track (bearing) for a GC based upon latitude, but any given GC (ignoring one passing through the poles), crosses a given latitude twice (below the maximum latitude where track = 270 or 90).

For my application I'd easier compute the intermediate track based on longitude, since this will have only one solution, but I'm insufficiently capable at spherical triangle math to re-arrange the available formulae to give that result. Any hints on computing immediate GC tracks based on longitude, or modifying the computation based on latitude to select the 'rising' or 'falling' solution, would be welcome.

For context, this is for a navigation device emulation in a simulator, to show the desired track along a leg. All the other values (eg, cross track error) are provided by existing formulae.

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    $\begingroup$ Maybe try this question at gis.stackexchange.com if you get no answers here. $\endgroup$ – Invariant May 30 '19 at 21:02
  • $\begingroup$ This is not an aviation question, this is a math question, should be moved math.stackexchange.com. $\endgroup$ – Juan Jimenez Jun 1 '19 at 13:22
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@James Turner: considering the date of the question, you've surely moved on. If you found a solution elsewhere, you can post it here and get some bonus points for answering your own question. Anyhow, the Aviation Formulary has the answer to your question: it's not Clairaut's Formula. You can find the latitude of ownship position by entering its longitude and the coordinates of the GC endpoints into the "Latitude of point on GC" formula. Then, knowing lat and long of both current position and destination, you can determine the heading to use leaving that point from the "Course between points" formula.

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