Module 4
Leveling, Traversing, Area, and Volume
Leveling
Vertical line. A line that follows the local direction of gravity as indicated by a plumb
line. A curved surface that at every point is perpendicular to the local plumb line. Level surfaces
are approximately spheroidal in shape. Level surfaces are also known as equipotential surfaces
since, for a particular surface, the potential of gravity is equal at every point on the surface. A
plane perpendicular to the local direction of gravity. In plane surveying, it is a plane
perpendicular to the local vertical line. Horizontal line.
A line in a horizontal plane. In plane surveying, it is a line perpendicular to the local
vertical. Vertical datum. Any level surface to which elevations are referenced. This is the surface
that is arbitrarily assigned an elevation of zero. This level surface is also known as a reference
datum since points using this datum have heights relative to this surface. Elevation. The distance
measured along a vertical line from a vertical datum to a point or object.
If the elevation of point A is 802.46 ft., A is ft. above the reference datum. The elevation
of a point is also called its height above the datum and optometric height. A particular level
surface that serves as a datum for all elevations and astronomical observations. MSL was defined
as the average height for the surface of the seas for all stages of tide over a 19-year period as
determined by the National Geodetic Vertical Datum of 1929. Due to flattening of the Earth in
the polar direction, level surfaces at different elevations and different latitudes are not truly
concentric. Tidal datum. The vertical datum used in coastal areas for establishing property
boundaries of lands bordering waters subject to tides. A tidal datum also provides the basis for
locating fishing and oil drilling rights in tidal waters, and the limits of swamp and overflowed
lands.
Various definitions have been used in different areas for a tidal datum, but the one most
commonly employed is the mean high water line. Interpretations of a tidal datum, and the
methods by which they are determined, have been, and continue to be, the subject of numerous
court cases. A relatively permanent object, natural or artificial, having a marked point whose
elevation above or below a reference datum is known or assumed. Common examples are metal
