What does a vertical angle in surveying refer to?

A vertical angle in surveying is defined as the angle that is measured from a horizontal plane, such as the horizon, to a line above or below that plane. This concept includes two key references: the zenith, which is the point directly above the observer, and the nadir, which is the point directly below. By measuring vertical angles, surveyors can determine the relative height or depth of various points in relation to a specific reference plane.

Understanding vertical angles is crucial because they play a significant role in establishing the topography of the land and in making elevation calculations. While vertical angles can be tied to elevations and can influence how land surfaces are understood, it is their relationship with the horizon, zenith, or nadir that fundamentally defines them in a surveying context. Therefore, the option that highlights this aspect of vertical angles is the most accurate.