a level surface to which elevations are referred. The generally adopted level datum for leveling in the US is the MSL. For local surveys, an arbitrary level datum is often adopted and defined in terms of an assumed elevation for some physical BM ...
An area of elevated land with a nearly level surface.
Tar Sand:
A sandstone that contains asphalt within its pore spaces.
Talus: ...
A vertical datum is a level surface to which heights are referred. In the United States, the generally adopted vertical datum for leveling operations is the National Geodetic Vertical Datum of 1929.
A third model of the shape of the earth is called a geoid, which is a complex and more or less accurate representation of the global mean sea level surface that is obtained through a combination of terrestrial and satellite gravity measurements.
Once the zone has been flattened onto a level surface it can be divided into a basic set of map sheets of a convenient size. These basic sheets are divided further into sections and each section is published as a map of a larger scale.
Simply put, the geoid is the natural extension of the mean sea level surface under the landmass. We could illustrate this idea by digging an imaginary trench across the country linking the Atlantic and Pacific oceans.
This geoid defines the nominal sea level surface by means of a spherical harmonics series of degree 360 (which provides about 100 km horizontal resolution).
Geoid: The figure of the earth considered as a sea level surface extended continuously through the continents. It is a theoretically continuous surface that is perpendicular at every point to the direction of gravity (the plumb line) (see Datum).
Once the coefficients reach a level where there is very little change with the removal of additional stations, they are used to create the monthly sea level surfaces.
The topography - the physical surface of the earth.
The Geoid - the level surface (also a physical reality).
The Ellipsoid - the mathematical surface for computations.
Traditionally, a vertical geodetic datum defines zero height as the mean sea level at a particular location or set of locations; other heights are measured relative to a level surface passing through this point.
 See also: Surface, Longitude, Coordinate, Map, Information
  
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