refDistance is used to define the vector direction of the offset curve from the basis curve. It can be omitted in the 2D case, where the distance can be positive or negative. In that case, distance defines left side (positive distance) or right side (negative distance) with respect to the tangent to the basis curve. In 3D the basis curve shall have a well defined tangent direction for every point. The offset curve at any point in 3D, the basis curve shall have a well-defined tangent direction for every point. The offset curve at any point (parameter) on the basis curve c is in the direction - - - - s = v x t where v = c.refDirection() and - t = c.tangent() - For the offset direction to be well-defined, v shall not on any point of the curve be in the same, or opposite, direction as - t. The default value of the refDirection shall be the local co-ordinate axis vector for elevation, which indicates up for the curve in a geographic sense. NOTE! If the refDirection is the positive tangent to the local elevation axis ("points upward"), then the offset vector points to the left of the curve when viewed from above.