Some of these terms are not true geodetic definitions as the earth and its composition is a complex and dynamic system. For this chapter, we will define terms in the context of a homogeneous spherical earth model. This is sufficient for most project based elevation applications.
Refer to Figure A-1 for illustration of general terms.
General Elevation Terms
A line in the direction of gravity. Visually: a freely suspended non-swinging plumb bob.
Over large areas, vertical lines are not parallel because of the earth's shape (they're also not straight, nor do they converge to a single point, but that's more a concern in geodetic leveling)
b. Level line
A line which is perpendicular to the direction of gravity (vertical) along its entire length. Because vertical lines converge, a level line is a curved line.
c. Level surface
A surface which, like a level line, is everywhere perpendicular to gravity. It is a curved surface.
d. Horizontal line
A straight line which is perpendicular to gravity at a single point.
e. Horizontal surface
A flat plane which is perpendicular to gravity at a single point.
Point, line, or surface to which measurements are referenced.
g. Vertical datum
A level surface which serves as a datum for vertical distances.
Data can be formal or assumed. Formal data have some defining mathematical parameters and a network of physical marks (see Benchmark) allowing new points to be easily and accurately referenced.
Examples of national data include Mean Sea Level (MSL), National Geodetic Vertical Datum of 1929 (NGVD28), and North American Vertical Datum of 1988 (NAVD88).
Some regions and cities created and used local formal data before national data were widely available.
An assumed datum is created for convenience and is not dependant on a formal datum. Elevations based on an assumed datum are correct relative to each other but not to elevations on other data.
The vertical distance from a vertical datum.
i. Benchmark (BM)
A physical structure whose elevation with respect to a datum is known. It serves as a local reference to a datum allowing us to quickly determine elevations based on that datum.
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