datum

A datum is a mathematical model of the Earth's shape and is used to measure locations and elevations of the earth's surface. There are two types of datums:

1. Horizontal datums are used to measure the position of a point on the Earth's surface using a coordinate system (typically latitude and longitude). It is used in surveying, navigation, and monitoring the movement of the Earth's crust. Examples of horizontal datums include WGS84 (World Geodetic System 1984), which is used globally by GPS systems, and NAD83 (North American Datum 1983), which is primarily used in North America for more localized mapping and surveying.
2. Vertical datums are used to measure land elevations and water depths. It is used in construction, flood modeling, and various engineering applications. Common examples of vertical datums include mean sea level (MSL), which represents the average sea level over time, and ellipsoid (ellipsoidal height), which uses a mathematically defined surface to measure height.