Methods for characterizing two-dimensional concept drawings are disclosed. The concept drawings comprise cross-sections intersecting at cross-hairs. The method comprises: determining, for each cross-section: a plane on which the cross-section is located, the plane having a normal vector in a three-dimensional coordinate system; and, for each cross-hair on the cross-section, a tangent vector in the three-dimensional coordinate system which is tangent to the cross-section at the cross-hair. For each cross-hair comprising ith and jth intersecting cross-sections, one or more constraints are satisfied, the constraints comprising: the normal vector ni of the plane on which the ith cross-section is located is at least approximately orthogonal to the normal vector nj of the plane on which the jth cross-section is located; and the tangent vector tij to the ith cross-section at the cross-hair is at least approximately orthogonal to the tangent vector tji to the jth cross-section at the cross-hair.