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I would just like to add that in the pdf I posted from Mathematica the
variable LHA is measured East so is negative LHA.

Isoazmuth curves are interesting and
https://en.wikipedia.org/wiki/Isoazimuth gives a formula for such a
curve. As is common the best thing about Wikipedia is it cites useful
reliable sources in this case an article in American Mathematics
Monthly https://www.jstor.org/stable/2304185?seq=1#page_scan_tab_contents

I was thinking though that an Isoazimuth is a curve joining points P
where the great circles to two other fixed points Q and R meet at a
specified angle, but for the special case Q is the north pole.  In
plane rather than spherical geometry, replacing great circles by
straight lines,  this is an arc of a circle.

Another interesting link to follow is the Littrow projection
https://en.wikipedia.org/wiki/Littrow_projection  . On this map
isoazimuths to some fixed point are straight lines.  Not useful for
the single sight fix, but perhaps for position lines for a single high
frequency radio beacon where the DF bearing is taken from a mobile
station.

Bill Lionheart

On Wed, 13 May 2020 at 06:16, Antoine Couëtte  wrote:
>
> Dear Peter and Lars,
>
> Excellent example from you Peter and excellent explanation from you Lars.
>
> -If you follow a great circle from one Departure Point (e.g. 
Санкт-Петербу́рг) at a given Departure Track Heading and over a given 
Distance you will eventually hit your arrival point, which is unique.
>
> - But this does not imply the existence of only one such unique/single 
Departure Point with both same Departure Track Heading and Distance to hit 
this very same unique arrival point !
>
> Since we do not have accurate enough azimuth references on board most craft, 
we are - or maybe better : at least I am - insufficiently familiar with Equal 
Azimuth Lines.
>
> Peter, we have here an excellent example of an Equal Azimuth Line (030° 
towards one unique arrival point) having 2 different intersections with a 
circle of equal altitudes centered on that same and unique arrival point.
>
> I really learnt something here.
>
> Thanks also and again to you Dave for having given us this example to study.
>
> Best Regards,
>
> Antoine
>
> 

   

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