datum (pl. datums)
In surveying, a reference system for computing or correlating the results of surveys. There are tow principal types of datums: vertical and horizontal. 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. The horizontal datum is used as a reference for position. The North American Datum of 1927 is defined by the latitude and longitude of an initial point (Meade's Ranch in Kansas), the direction of a line between this point and a specified second point, and two dimensions that define the spheroid. The new North American Datum of 1983 is based on a newly defined spheroid (GRS80); it is an Earth-centered datum having no initial point or initial direction.
datum, national geodetic vertical See: national geodetic vertical datum of 1929

declination
In astronomy, the angular distance of a celestial body above (north, plus) or below (south, minus) the celestial Equator. Magnetic declination is the angular difference between magnetic north and true (geographic) north at the point of observation; it is not constant but varies with time because of the "wandering" of the magnetic north pole.

depth curve
Line on a map or chart connecting points of equal depth below the datum.

diazo process
Rapid method for copying documents in which the image is developed by exposure to ammonia.

dike
Bank of earth or stone used to form a barrier, frequently and confusingly interchanged with levee. A dike restrains water within an area that normally is flooded. See levee.

electronic distance measuring (EDM) device:
Instruments that measure the phase difference between transmitted and reflected or retransmitted electromagnetic waves of known frequency, or that measure the round-trip transit time of a pulsed signal, from which distance is computed.

elevation
Vertical distance of a point above or below a reference surface or datum.

ellipsoid
See: spheroid

engineering map
See: map, engineering

ER-55 plotter
Double-projection plotting instrument utilizing ellipsoidal reflectors for light projection.

erosion
Group of natural processes including weathering, dissolution, abrasion, corrosion, and transportation that remove material from any part of the Earth's surface.

estuary
That portion of a stream influenced by the tide of the body of water into which it flows; an arm of the sea at a river mouth.

feature separation
Process of preparing a separate drawing, engraving, or negative for selected types of data in the preparation of a map or chart.

flood control map
See: map, flood control

flood plain
Belt of low flat ground bordering a stream channel that is flooded when runoff exceeds the capacity of the stream channel.

forestry map
See: map, forestry

formlines
Lines, resembling contour lines, drawn to present a conception of the shape of the terrain without regard to a true datum or regular spacing

geodesy
Science concerned with the measurement and mathematical description of the size and shape of the earth and its gravitational fields. Geodesy also includes the large-scale, extended surveys for determining positions and elevations of points, in which the size and shape of the earth must be taken into account.

geoid
Figure of the Earth visualized as a mean 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 plumbline).
geologic map
See: map, geologic

graticule
Network of parallels and meridians on a map or chart.

graticule, geographic
System of coordinates of latitude and longitude used to define the position of a point on the surface of the Earth with respect to the reference spheroid.

grid
Network of uniformly spaced parallel lines intersecting at right angles. When superimposed on a map, it usually carries the name of the projection used for the map- that is, Lambert grid, transverse Mercator grid, universal transverse Mercator grid.