The Major Axis and Surface Contour
Up to this point all variations in surface contours have been created by varying the minor axes and placement of the ellipses that describe those surfaces. Now we consider the role of the major axis.
The major axis of an ellipse is used to determine the sideways incline, or tilt, of a plane. For a surface to appear level and flat, the major axes of the ellipses that describe that surface must be parallel to the horizon. And, the minor axes must gradually decrease as the ellipses recede into the distance.
A tilt in the major axis of an ellipse-meaning it is not parallel to the horizon-indicates a laterally inclined plane. The angle of the tilt describes the angle of the plane.
Some of the major and minor axes of these ellipses have been altered in order to describe a hill and valley. Can you see the changes in contour and how they were created?