The dihedral angle, the angle between the two planes, is developed by drawing a line from the eave line that is perpendicular to the hip rafter run leg. The next line, altitude line, is drawn perpendicular to the hip rafter leg. Then swing an arc the length of the altitude line until it meets the hip rafter run leg. Then draw another line that connects with the eave line.
This drawing shows the dihedral angle drawn on the ground plan.
This drawing shows the dihedral angle drawn in 3D.