Model for Descriptive Geometry by A. Jullien - Angle of a Plane with Planes of Projection

Jullien, A. · ca 1880

Given plane APA’, c’ is a point on the intersection line of the plane with the vertical plane. Point b on the horizontal intersection of the plane is chosen so it that bc’ is perpendicular to PA. Connect b and c’ to from the red string. The horizontal projection of bc’ is bc and the vertical projection is cc’. By rotating bc’ about bc to the horizontal, point C₁ is found. Now angle cbC₁ is the angle of the plane with the horizontal plane. Similarly, the angle bC₁C is the angle with the vertical plane.