A horizontal line is one that is:

A horizontal line is one that is:

• A. Perpendicular to the direction of gravity at a point.
• B. Parallel to the direction of gravity at a point.
• C. Perpendicular to the equator at a point.
• D. Parallel to the equator at a point.

Answer: (A)

Horizontality is defined by the local horizontal plane, which is perpendicular to the gravity vector through the point. Since gravity pulls downward, the true horizontal line lies in the plane that is perpendicular to that direction. A line drawn in that plane has no vertical component and remains at the same height locally, so it is horizontal. This is the basis for leveling and establishing horizontal layouts in surveying. The other ideas—being parallel to gravity or related to the equator—do not define horizontality.

Horizontality is defined by the local horizontal plane, which is perpendicular to the gravity vector through the point. Since gravity pulls downward, the true horizontal line lies in the plane that is perpendicular to that direction. A line drawn in that plane has no vertical component and remains at the same height locally, so it is horizontal. This is the basis for leveling and establishing horizontal layouts in surveying. The other ideas—being parallel to gravity or related to the equator—do not define horizontality.