Elevation-The vertical distance of a point above or below an established or assumed point
(datum) on a level surface.
Geodetic datum-Datum that forms the basis for the computation of horizontal-control surveys
in geodetic surveying. It consists of five quantities: the latitude and the longitude of an initial
point, the azimuth of a line from this point, and two constants necessary to define the terrestrial
Grade (gradient) -The rate of rise and fall or slope of a line; generally expressed in percent or
as a ratio.
Horizontal angle-The angle formed by two intersecting lines on a horizontal plane.
Horizontal distance-A distance measured along a level line. It is commonly thought of as the
distance between two points. The distance may be measured by holding a tape horizontally or
by measuring the inclined distance between the points. However, the inclined distance is
always reduced to its horizontal length.
Horizontal line-A straight line perpendicular to a vertical line at a given point.
Horizontal plane-A plane tangent to a level surface (also called plane of the horizon).
Instrument adjustment-Adjusting the parts of an instrument to obtain the highest practical
precision. For example, field adjustments of theodolites include adjusting the bubble tube,
circular bubble, line of sight, horizontal axis, telescope bubble tube, vertical circle, and optical
Isogonic line-An imaginary line or a line on a map joining points on the earth's surface at
Legend-A description, explanation, table of symbols, and so on printed on a map or chart for a
better understanding and interpretation of it.
Level surface-A surface that is parallel with the spherical surface of the earth, such as a body of
-A level surface
to which elevations are referred. Generally, the
for leveling in the US is the mean sea level.
Measured angles-Angles that are either vertical or horizontal.
Parallax-An error in sighting that occurs when the objective and/or the crosshairs of a
telescope are improperly focused. In testing the focusing of a telescope, the head of the
observer must move from side to side or up and down while sighting through the eyepiece.
Any apparent movement of the crosshairs in relation to the object image means that parallax is