The sphgeom::LonLat and sphgeom::UnitVector3d classes provide constructors for converting between the two types. However, the axis convention used for the conversion is left unspecified, limiting the situations where these classes' interoperability can be used.

I propose that the axis convention currently used by the implementations of these classes be made part of their APIs, so that external code knows what results to expect:

(0°, 0°) <--> <1, 0, 0>

(90°, 0°) <--> <0, 1, 0>

(*, 90°) <--> <0, 0, 1>