A level surface is defined as which?

• A. A surface everywhere perpendicular to gravity; for ordinary leveling, level surfaces at different elevations can be considered parallel.
• B. A flat plane regardless of gravity.
• C. A curved surface such as a dome.
• D. The surface of a perfectly flat glass plate.

Answer: (A)

A level surface is defined as an equipotential surface of the Earth's gravity field, meaning the surface is everywhere perpendicular to the gravity vector. In practice, gravity doesn’t change much over typical surveying distances, so level surfaces at different elevations can be treated as parallel planes for ordinary leveling. This makes it possible to compare heights using instruments that assume a horizontal reference, even though the true level surface is slightly curved in reality.

The other descriptions don’t fit because they don’t tie the surface to gravity. A flat plane regardless of gravity ignores the orientation set by gravity. A curved dome isn’t perpendicular to gravity everywhere, so it isn’t a level surface. A perfectly flat glass plate is a physical object, not a gravity-defined level surface used for height comparisons.