The point where the direction of the plum-line extended below the horizon meets the celestial sphere. The nadir is directly opposite the zenith. The point on the ground which is vertically underneath the perspective center of an airborne-camera lens system. The point on a map corresponding to the ground nadir. The point at which a vertical line through the perspective center of the camera's lens system pierces the plane of the photograph. Also called nadir point. See Datum, National Geodetic Vertical. Those tide stations of the U.S. National Ocean Service that provide the basic tidal datums for coastal boundaries and for chart datums of the United States of America. Geodetic leveling is not required between stations of the network. A convention, adopted by the United States and effective November 28 1980, that made changes (a) through (g), below, to the previously established rules governing tidal datums. (a) Establishment of one uniform, continuous tidal datum for all tidal waters of the United States (including Commonwealths, territories, and United Nations trust territories under jurisdiction of the U.S.A.); (b) a tidal datum independent of computations based on the type of tide provided; (c) chart datums lowered from mean low water to mean lower low water along the Atlantic coast of the United States; (d) the National Tidal Datum Epoch from the epoch 1941 through 1959 changed to the epoch 1960 to 1978; (e) the name "Gulf Coast Low Water Datum" changed to "mean lower low water"; (f) the tidal datum of mean higher high water introduced in regions of predominately diurnal tides; (g) mean high water lowered in regions of predominately diurnal tides. A period of 19 years adopted by the National Ocean Service as the period over which observations of tides are to be taken and reduced to average values for tidal datums. The epoch is designated by the years the period began and ended, e.g., National Tidal Datum Epoch of 1941 through 1959. Note that the word "Epoch" as used here is at variance with the astronomic and geodetic conventions. The determination of the course and position of a vehicle, vessel, or aircraft over an interval of time. Determination of where the vehicle is at a particular instant is called positioning . A set of equipment and techniques used for navigation is called a navigation system; a set of equipment used for positioning is called a positioning system. Navigation in which the course is maintained along a circular arc by reference to radio signals from a transmitter at the center of the arc. (1) Navigation using the shift in frequency (Doppler shift) of sound waves reflected from the ocean bottom to determine the velocity of the vessel. (2) Navigation using the Doppler shift in frequency of radio waves reflected from the ground to determine the velocity of the aircraft. (3) Navigation using the shift in frequency of radio waves from an orbiting radiotransmitter to determine the location of the vessel or aircraft. See navigation system, hyperbolic. Any method of navigation in which location or velocity is inferred from measurements on radio waves. The term is generally applied only to one of the following methods of navigation: (a) measuring direction or distance to two or more radio transmitters, (b) measuring differences of distance to two or more pairs of radio transmitters, (c) measuring the Doppler shift in frequency of a signal from an orbiting beacon or beacons. A set of equipment and techniques by which the location of a moving vehicle, vessel, or aircraft can be determined and made known sufficiently quickly so the information can be used for navigation. In general, any navigation system which makes use of the measured shift in frequency of a signal of known frequency to determine the velocity of the receiving system relative to the signal source and, from these measurements, the location of the receiver. Two different kinds of Doppler navigation system are used at present: that in which the system itself is both source and receiver of the signal, and the velocity is with respect to the surface producing the echo; and that in which the system receives a signal from a beacon. Systems of the first kind take two forms which are very different instrumentally but share much the same mathematics. These forms are systems in which the signal consists of sound waves reflected from the ocean bottom, and those in which the signal consists of radio waves reflected from the ground. The first is used almost exclusively by ships or water craft, the second almost exclusively by aircraft. Because neither the ocean bottom nor the ground is simple in shape nor accurately known, velocities inferred from the reflected signals are degraded by lack of complete knowledge of these shapes. Most systems of the second kind have the beacon in orbit about the Earth. The orbit of the beacon is determined by measuring the beacon's radial velocity with respect to a small number of receivers at fixed locations and solving the resulting observation equations for the parameters of the orbit. Navigating systems then use this known orbit, together with the frequency shift they measure, to determine the location of the receiver with respect to the orbit. A navigation system using the differences in distance (measured in wavelengths) of a mobile unit from three or more fixed stations to determine location. The locus of points all of which have the same difference of distance is a hyperbola. If the difference in distance from two pairs of fixed points (one point of which may be common to the two) is determined, two intersecting hyperbolas result and the mobile unit is located at one of those intersections. Only the fractional part of one wavelength is actually measured. Most hyperbolic navigation systems keep count of the changes of difference by a whole wavelength, so that once the entire distance is known, the system continues to indicate the total difference, regardless of the motion of the mobile unit. Any navigation system in which gyroscopes or accelerometers are used to provide a coordinate system which has a fixed orientation with respect to the distant galaxies. A gyroscopic compass is a particularly simple form of inertial navigation system. A more complicated form, called SINS (Ship's Inertial Navigation System), gives not only orientation but location. It is used for ship navigation. A navigation system which determines the distances of a mobile unit from two or more fixed stations by measuring the time needed for a signal to travel between the mobile unit and each of the fixed stations. A measured distance locates the mobile unit on the circle with radius, p-pp (rho, whence the name), equal to the distance. The two distances from the two fixed stations locate the mobile unit at one of the two intersections of the two circles. A navigation system that determines the distance, p-pp, of a mobile unit from a single, fixed station and the direction, p-rp, of the unit from each of a pair of fixed stations (one of which may be at the same point as the first, distance-determining station). The mobile unit determines its distance from the single station by measuring the time of travel of a signal between it and that station. This locates the mobile unit somewhere on a circle whose radius p-pp is the distance determined. It determines the difference in distances from the pair of stations in the same way. However, because the two stations of the pair are very close together, this difference in distance can be converted into an approximate direction to the mid-point of the pair. The combination of distance and direction gives the location of the mobile unit. (1) A navigation system used for the navigation of satellites. (2) A navigation system having beacons or transponders placed on satellites rather than at fixed points on land. The most successful satellite navigation system to date has been the TRANSIT system (also called NNSS or Navy Navigation Satellite System) in which the mobile unit determines its location by measuring the Doppler shift in the frequency of the radio waves from one or more satellites. The location of the satellite must, of course, be known. That information is usually available in the form of an orbit ephemeris, but it is also broadcast together with the fixed-frequency radiation from the satellite. Another system under development (1985) is the Global Positioning System in which the mobile unit determines its location by measuring, almost simultaneously, the times of travel of signals from several satellites whose locations are known, and converting this to distances from the satellites. The plan for this system calls for eighteen satellites in orbit at all times; at least three or four of them will be visible simultaneously from any point on Earth. See also positioning system, satellite and Global Positioning System. A set of satellites developed and operated by the U.S. Navy for Doppler navigation and positioning. Support for this system by the U.S. Navy is scheduled to cease soon after the Global Positioning System becomes operational. This system is popularly known by the term, Transit. See tide, neap. The average, semidiurnal difference in height of water occurring at the time of quadrature. The height of mean neap high water above the chart datum. (photography) A photographic transparency which is dark where the original scene was light and light where the original scene was dark, etc., or whose colors are the complements of those in the original scene. See network. The term net has been used extensively, but is gradually being replaced by network. See triangulation network, base-extension. A set consisting of: (a) stations for which geometric relationships have been determined and which are so related that removal of one station from the set will affect the relationships (distances, directions, coordinates, etc.) between the other stations; and (b) lines connecting the stations to show this interdependence. The lines represent adjusted distances, directions, or angles or the sequence in which measurements were carried out. For example, a triangulation network consists of stations whose horizontal coordinates with respect to each other have been determined, and lines representing distances, directions, or angles between the stations. A level network consists of a network of level lines. A gravity network consists of gravity stations occupied in sequence, together with lines connecting them in the sequence in which they were occupied. Networks contain only closed circuits; all lines connect at least two different points, and each point lies on at least two different lines. This follows from the condition that the values for each station are interdependent. See triangulation network, base-extension. A survey network in which the stations are control stations. A survey network connected to and contained within a survey network of the same or higher order. The distances between stations in a densification network are usually much shorter than distances between stations in the surrounding network. See triangulation network, base-extension. A network whose position is not specified. Angles and distances between points are known, but neither coordinates nor directions are known. A network in the geodetic sense: i.e., a network whose nodes are survey stations or gravity stations. See gravity network. See triangulation network, base-extension. (1) A network lying on an equipotential surface or on a flat surface of limited horizontal extent. (2) See leveling network. See leveling network. See National Tidal Datum Control Network. See survey network. See traverse network. See triangulation network. A unit of force defined as that necessary to give a mass of 1 kilogram an acceleration of 1 meter per second per second. It is equal to 10 ⁵ dynes. See gravitational constant. See lens. (1) (astronomy) One of the two points of intersection of two great circles on the celestial sphere. In geodesy, the most important node is the First Point of Aries, which is the intersection of the ecliptic and the Equator where the Sun apparently moves from south to north. (2) One of the two points of intersection of the celestial sphere with the line in which the plane of the osculating orbit of a satellite intersects the reference plane through the focus. (3) One of the points, on the orbit of a satellite, in which the osculating orbit intersects the plane of reference through the focus. (1) The point on the celestial sphere at which a celestial body apparently crosses the celestial Equator from south to north. (2) The point in which the radius vector from the focus through a satellite intersects the celestial sphere at the instant the satellite is passing through the reference plane in what is defined as the upward direction. (3) The point on the orbit of a satellite occupied by the satellite at the instant it is passing through the reference plane in an upward direction. The same as the ascending node except that passages are from north to south or downward. In an elliptical orbit, the ascending and descending nodes are on the same straight line through the foci of the ellipse. The time required for the nodes of the Moon's orbit to regress through 360^o of longitude, a period of about 18.6 years. A factor whose value depends on the longitude of the node of the Moon's orbit and, when applied to the average value of the coefficient of a tidal constituent, will make that coefficient usable for the prediction of tides for a particular year. The line connecting the ascending and descending nodes. (1) The part of any radiation that does not convey intelligible or wanted information to the recipient. The term is commonly used in this sense by electronic engineers to denote the randomly varying portion of radio waves, voltages, etc. (2) The random portion of any varying quantity. Noise received from sources other than that emitting the desired signal while that signal is being received. A diagram relating the values on two scales so that a line drawn from a point (value) on one scale through a known second point intersects the other scale at the corresponding point (value). Nomograms are used for solving equations graphically. The accuracy is generally less than that obtainable by numerical computation. See nomogram. A disc having concentric, graduated circles drawn or engraved on it, and used for determining angles. It is named for a Portuguese mathematician and geographer, Nunez (1492 - 1577). The nonius was later replaced by the vernier. The instant at a point on the Earth when the apparent Sun--i.e., the true or actual Sun--is over the upper branch of the local meridian. The mean sea level at the tide gauge in the harbor of Amsterdam which provides a vertical datum for the Netherlands and for the European combined leveling network. The vertical datum for the leveling adjustment of 1970 is designated NAP-REUN-1970. (gravimetric geodesy) Standard, when part of a name. e.g., normal gravity formula, normal height, etc. Normal gravity means standard value of gravity when used for gravimetric geodesy, and average gravity when used for astronomy. (noun) (1) (general) A straight line perpendicular to a surface or to another line. Also, the condition of being perpendicular to a surface or line. (2) (geodesy) A straight line perpendicular to the surface of a particular spheroid or ellipsoid. While it is correct to use the term "normal" to designate a line perpendicular to the surface of the geoid, use of the term "vertical" is preferred. See Normaal Amsterdams Peil. See under equation. The curve in which a plane through the perpendicular at a given point of a surface intersects that surface. In general, for each pair of points there are two normal sections, because the plane through the perpendicular at one of the points will not, in general, also contain the perpendicular at the other point. On a sphere, there is only one normal section through such a pair. See azimuth, normal section. A line connecting two points on an ellipsoid and lying in the plane through the perpendicular at one of the points. It differs from the normal section in being terminated by the two points. (1) The positive direction of a line lying in a plane through the Earth's axis of rotation and tangent to the geoid or to an equipotential (gravity) surface at a point. (2) The positive direction of a line lying in a plane through the minor axis of an ellipsoid and perpendicular to the normal to that ellipsoid at a point. (3) The direction indicated by the positive end of a magnetic needle suspended so as to rotate freely. See north, magnetic. A direction perpendicularly to the left of an observer facing in the direction of the Earth's rotation. In particular, (a) the positive direction of that line parallel to the Earth's axis of rotation and perpendicularly to the left of an observer facing in the direction of the Earth's rotation, or (b) the positive direction of a similar line tangent to the (gravity) equipotential surface at the observer. The direction indicated by the north-seeking end of the needle or other magnetic component of a magnetic compass. It differs from magnetic north, the direction of a magnetic line through the point of observation, by the amount that the supports and housing of the magnetic compass affect the needle's movement. The direction indicated by the north-seeking end of an unconstrained, magnetized needle. See north, compass. See north, astronomic. A direction within 22.5^o of northeast. A direction within 22.5^o of north. (1) A linear distance, in the coordinate system of a map grid, northwards from the east-west line through the origin (or false origin). (2) See latitude, difference of. A value assigned the east-west line through the origin on a map grid to avoid the inconvenience of negative northings at some points. Equivalently, a constant value added to the northing of points on a map grid with origin at (0,0). (1) The northern intersection of the celestial meridian with the horizon. If the observer is close enough to the North Pole that the Pole is included within his horizon, the north point is the intersection closest to the Pole. (2) An arrow-like symbol indicating the direction north on a map or chart. See Polaris. A direction within 22.5^o of northwest. See dynamic number. See geopotential number. See dynamic number, normal. A quasi-periodic motion of the Earth's instantaneous axis of rotation about an average position, i,e., about the position corrected for precession. The period of the major component is approximately 18.6 years. Nutation moves the equinox by as much as 17" ahead of or behind its average position. Astronomers conventionally distinguish between nutation as the periodic movement of the instantaneous axis of rotation, and the "Eulerian nutation" which consists of the long-period oscillation of the axis of figure about the instantaneous axis plus a very small diurnal oscillation. The value adopted by the International Astronomical Union in 1976 is 9."2109 at epoch 2000. This is the amplitude of the longest term in the Fourier-series representation of nutation and depends on the longitude of the node of the Moon's orbit. See nutation. The right ascension of the mean equinox referred to the true Equator and equinox. It is the difference between mean and apparent sidereal time. Since 1960, this has been called the equation of the equinoxes.