On a plane surface, contours are what?

• A. Curved and irregular
• B. Concentric only
• C. Circular around a point
• D. Straight and parallel

Answer: (D)

Contours are lines of equal elevation. On a plane surface, the height changes linearly with position, so the elevation at any point can be described by z = ax + by + c. For a fixed elevation k, the contour is ax + by + c = k, which is a straight line. As k varies, these lines stay in the same direction and are parallel to each other, yielding straight and parallel contours. If the surface weren’t a plane, contours could curve or form circular patterns around a peak or sink, which is why the straight, parallel form is specific to a plane with uniform slope.