Vertical position in the context of "Above mean sea level"

Height above mean sea level is a measure of a location's vertical distance (height, elevation or altitude) in reference to a vertical datum based on a historic mean sea level. In geodesy, it is formalized as orthometric height. The zero level varies in different countries due to different reference points and historic measurement periods. Climate change and other forces can cause sea levels and elevations to vary over time.

Height is measure of vertical distance, either vertical extent (how "tall" something or someone is) or vertical position (how "high" a point is). For an example of vertical extent, "This basketball player is 7 foot 1 inches in height." For an example of vertical position, "The height of an airplane in-flight is about 10,000 meters."

When the term is used to describe vertical position (of, e.g., an airplane) from sea level, height is more often called altitude.Furthermore, if the point is attached to the Earth (e.g., a mountain peak), then altitude (height above sea level) is called elevation.

In geodesy, surveying, hydrography and navigation, vertical datum or altimetric datum is a reference coordinate surface used for vertical positions, such as the elevations of Earth-bound features (terrain, bathymetry, water level, and built structures) and altitudes of satellite orbits and in aviation.In planetary science, vertical datums are also known as zero-elevation surface or zero-level reference.

Commonly adopted criteria for a vertical datum include the following approaches:

In geology and geomorphology a base level is the lower limit for the vertical position of an erosion process. The modern term was introduced by John Wesley Powell in 1875. The term was subsequently appropriated by William Morris Davis who used it in his cycle of erosion theory. The "ultimate base level" is the surface that results from horizontal projection of the sea level under landmasses (the geoid). It is to this base level that topography tends to approach due to erosion, eventually forming a peneplain close to the end of a cycle of erosion.

There are also lesser structural base levels where erosion is delayed by resistant rocks. Examples of this include karst regions underlain by insoluble rock. Base levels may be local when large landmasses are far from the sea or disconnected from it, as in the case of endorheic basins. An example of this is the Messinian salinity crisis, in which the Mediterranean Sea dried up making the base level drop more than 1000 m below sea level.

In surveying and geodesy, a marker (also called a mark, monument, or station) is an object firmly installed to indicate points on the Earth's surface with known coordinates. A benchmark is a type of survey marker that indicates elevation (vertical position). Horizontal position markers used for triangulation are also known as triangulation stations. A set of interrelated geodetic markers form a geodetic network. An active station or reference station is occupied permanently with a measurement instrument, such as GPS/GNSS continuously operating reference stations (CORS).Benchmarking or trigpointing is the hobby of searching for these marks.