Water levels, sometimes called dumpy levels, consist of a telescope with a spirit level attached to it, and they are most commonly used in civil engineering and land surveying. These instruments are used to determine the elevation of points on the Earth's surface or the height difference between two points.

The process of using a water level involves leveling the instrument (making sure the spirit level is centered and the telescope is horizontal), sighting a target (usually a rod with markings indicating the elevation), and reading the elevation or height at the line of sight. The basic mechanism relies on the principle of geometric leveling, where the horizontal line of sight, defined by the optical axis of the telescope, serves as a reference for determining elevations.

Levels are particularly useful for surveying applications that require precise measurements of elevation changes. They are most effective when used over relatively short distances, as errors can accumulate over longer lines of sight due to factors like curvature of the Earth and atmospheric refraction.

In summary, water levels, or dumpy levels, are surveying instruments that combine a telescope and a spirit level to establish horizontal lines of sight and determine elevations. They are used to establish reference points, measure height differences, and perform leveling tasks in various construction, engineering, and surveying projects.