I've found some online calculators that compute (in 3 dimensions)  ECEF x,y,z to LLA and back.

And I would like to do the same thing using boost geometry.

I watched Adam's 2019 Fos4G in Bucharest and I think I saw that only 2D is supported.

I'm new to GIS so all of the vocabulary terms are quite overwhelming.  

I have seen the example (transform_2) in the docs of going from degree (lon-lat) to 3d on the unit circle.  Even with that example i don't understand how (theta 5.0 and phi = 15.0) map to lat/long.  

Thanks for your patience and help.