Representation of Petit Cru at point A with the main angles and vectors. The plane of Petit Cru is represented in blue with its basis vectors (R _* = {x _*, y _*, z _*}). is composed of the three vectors {Up, East, North}. Angles α and ε are the azimut and elevation respectively. B _proj is the projection of the magnetic field at point H on the plane of Petit Cru, and η is the apparent angle we want to calculate.