What is the term for a reference plane used for measuring elevations?

The term used for a reference plane employed for measuring elevations is "Datum." In surveying and engineering, a datum is a base reference point or surface against which measurements are taken. It provides a consistent starting point for determining the elevation of various points on the Earth’s surface. This is essential in ensuring that all measurements are uniform and comparable, particularly in large-scale projects or when communicating elevations in different regions.

Understanding datums is critical because they can vary significantly based on the local geography and the purpose of the survey. For instance, surveying might use a geodetic datum (like the North American Datum) or a local datum specific to a city's engineering needs. In both cases, the datum helps surveyors accurately represent the three-dimensional shape of the Earth's surface in a two-dimensional format, which is fundamental in land development, civil engineering, and construction projects.

The other options relate to surveying concepts but have different definitions. A benchmark refers to a fixed point of known position and elevation used to help determine other elevations, while contour lines represent lines on a map that connect points of equal elevation. An elevation gauge is an instrument for measuring the height above a reference level; it does not define a reference plane itself.