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Yes Andrés,

Very seldom did I earlier take some interest in such "EAL's" (Equal Altitudes Lines). Our earlier private exchanges on this matter were no doubt the last time I looked into this topic. Thanks then for bringing them to my memory.

I just checked that our earlier exchanges covered the article : "PRATIQUE DE LA DROITE D'AZIMUT A BORD DES SNLE" - "Practicing Azimuth LOP's on-board SSBN's" from Institut Français de Navigation Vol XXX Jan 17th, 1982.

Nonetheless this article does not cover EAL's in general but primarily their local tangents. It does not either give their "general" shape or their "degraded" shapes and seems far insufficient to solve our new EAL's challenge about them intersecting or not intersecting EAC's (Equal Altitudes Circles), and if so intersecting : how many times ?

Hence it would be really great to get another article mentioned in the above article, i.e. from Volume 116 from the same French Magazine NAVIGATION, October 1981 which apparently extensively covers this subject. That would be an excellent start. Any easy possibility for anybody to get a copy of this October 1981 article ?

The Mathematician on duty might think that for continuity reasons there could be 2 such intersections (or a greater even number of them) in the general case.

It would then follow that we may observe more frequently than currently thought that Dave's initial problem has 2 distinct real world solutions vs. only one.

Worth carefully checking, no ?

Best Friendly Navigational Regards

⚓  Kermit ✈️