pub fn elevated_tangent(curve: &Elevated3, d: f64) -> Result<DVec3, GeomError>Expand description
Unit tangent of an elevated curve at plan distance d.
The plan tangent is horizontal and the grade lifts it, so the 3D tangent is
(t.x, t.y, g) normalised – the sqrt(1 + g^2) factor by which 3D arc
length runs ahead of plan distance.