Determine the plane that a given curve resides in and return its normal vector. Ask the curve for its start and end points and some point in the middle. The latter can be obtained by asking the curve for its parameter range and evaluating it in the middle, or by tessellation. In case of tessellation, you could iterate through the tessellation points and use each one together with the start and end points to try and determine a valid plane. Once one is found, you can add debug assertions to ensure that the other tessellation points (if there are any more) are in the same plane. In the case of the line, the tessellation only returns two points. I once heard that that is the only element that can do that, all non-linear curves return at least three. So you could use this property to determine that a line is a line (and add an assertion as well, if you like). Update, later: please note that the Revit API provides an overload of the NewPlane method taking a CurveArray argument.