double ElevationAt(const glm::dvec3 &p) const noexcept { return glm::length(p) - Radius(); }
/// distance to planet center
double DistanceAt(const glm::dvec3 &p) const noexcept { return glm::length(p); }
double ElevationAt(const glm::dvec3 &p) const noexcept { return glm::length(p) - Radius(); }
/// distance to planet center
double DistanceAt(const glm::dvec3 &p) const noexcept { return glm::length(p); }