arc measurement-A survey method used to determine the size of the earth. A long arc is measured on the earth's surface and the angle which subtends this measured arc is determined. By assumptions and mathematical formula the size and shape of the earth can then be determined.

arc navigation—A navigation system in which the position of an airplane or ship is maintained along an arc measured from a control station by means of electronic distance-measuring equipment, such as shoran. See also hiran; loran.

arc triangulation-A system of triangulation of limited width designed to progress in a single general direction. Arc triangulation is executed for the purpose of connecting independent and widely separated surveys, coordinating, and correlating local surveys along the arc, furnishing data for the determination of a geodetic datum, providing a network of control points for a country-wide survey, etc. arc of parallel-A part of an astronomic or geodetic parallel.

Arctic Circle-The geographical parallel having

a north latitude equal to the complement of the declination of the summer solstice. The obliquity of the ecliptic is steadily changing so that the summer solstice is not a point of fixed declination, and the Arctic Circle, as defined, is not a line of fixed position. When the Arctic Circle is to be shown on a map, however, it is desirable that it be treated as a line of fixed position, and that a conventional value be adopted for its latitude. For this value 66° 33′ north latitude is used. Also called north polar circle.

area (surveying)-In general, an area is any aggregate of plane spaces to be considered in an investigation; especially the quantity projected on a horizontal plane inclosed by the boundary of any polygonal figure.

area analysis intelligence-The production of intelligence data relative to a specific geographic area.

area coverage-Complete coverage of an area by aerial photography having parallel overlapping flight lines and stereoscopic overlap between exposures in the line of flight. When

applied to shoran, the term implies that recorded shoran distances are available for each exposure.

area delimiting line—A line fixing the boundary of an area.

Area Radar Prediction Analysis (ARPA)A series of graphics having a scale of 1:50,000, designed to provide radar significant data in concise graphic and tabular form to facilitate production of related target materials, mission plans, and target studies. It consists of a mosaic or chart base, a radar intelligence/surface height (RI/SH) overlay, and a radar prediction data table (RPDT). area survey-A survey of areas large enough to require loops of control.

area target-A target consisting of an area rather than a single point. Contrasted with pin-point target.

area triangulation-A system of triangulation designed to progress in every direction. Area triangulation is executed to provide survey control points over an area, as of the city or county; or for filling in the areas between arcs of triangulation which form a network extending over a county or state.

Area Weighted Average Resolution (AWAR)— A single average value for the resolution over the picture format for any given focal plane. areodesy-That branch of mathematics which determines by observation and measurement, the exact positions of points and the figures and areas of large portions of the surface of the planet Mars, or the shape and size of the planet Mars.

areodetic-Of or pertaining to, or determined by areodesy.

argument—In astronomy, an angle or arc, as in argument of perigee.

argument of latitude-In celestial mechanics, the angular distance measured in the orbit plane from the ascending node to the orbiting object; the sum of the argument of perigee and the true anomaly.

argument of perigee-An orbital element defined as the angle at the center of attraction from the ascending node to the perigee point

measured in the direction of motion of the orbiting body.

arithmetical mean-The value obtained by dividing the sum of a series of values by the number of values in the series.

artificial asteroid. A manmade object placed in orbit about the sun.

artificial earth satellite-A manmade earth satellite, as distinguished from the moon. artificial horizon-A device for indicating the horizontal, as a bubble, gyroscope, pendulum, or the flat surface of a liquid.

artificial monument-A relatively permanent object used to identify the location of a survey station or corner. Objects include manmade structures such as abutments, stone markers, concrete markers, and railroad rails. art-work prediction-See experience radar prediction.

Arundel method-A combination of graphical and analytical methods, based on radial triangulation, for point-by-point topographic mapping from aerial photographs.

ascending node-That point at which a planet, planetoid, or comet crosses the ecliptic from south to north, or a satellite crosses the equator of its primary from south to north. The opposite is descending node.

ascending vertical angle-See angle of elevation.

ascensional difference The difference between right ascension and oblique ascension. aspects The apparent positions of celestial bodies relative to one another; particularly the apparent positions of the moon or a planet relative to the sun.

asperical lens-A lens in which one or more surfaces depart from a true spherical shape. assault-landing model-A special form of assault model designed specifically for planning amphibious landings. Also called amphibiousassault landing model. See also assault models. assault models-Large-scale models giving a particular representation of vegetation, lesser landforms, prominent man-made features,

and a detailed representaton of specific or sensitive objectives such as airfields, radar installations, and the like. These models emphasize the aspects of objects as seen from surface approach.

associated Legendre function-A solution of the Legendre Equation, which is a special case of the Laplace Equation, in the form of a power series of a special kind; used in the spherical harmonic expansion of the gravitational potential.

assumed ground elevation-The elevation assumed to prevail in the local area covered by a particular photograph or group of photographs. Used especially to denote the elevation assumed to prevail in the vicinity of a critical point, such as a peak or other feature having abrupt local relief.

assumed latitude-The latitude at which an observer is assumed to be located for an observation or computation, as the latitude of an assumed position or the latitude used for determining the longitude by time sight. assumed longitude-The longitude at which an observer is assumed to be located for an observation or computation, as the longitude of an assumed position or the longitude used for determining the latitude by meridian altitude.

assumed plane coordinates-A local plane-coordinate system set up at the convenience of the surveyor. The reference axes are usually assumed so that all coordinates are in the first quadrant. The Y-axis may be in the direction of astronomic (true) north, magnetic north, or an assumed north.

astatized gravimeter-A gravimeter, sometimes referred to as unstable, where the force of gravity is maintained in an unstable equilibrium with the restoring force. The instability is provided by the introduction of a third force which intensifies the effect of any change in gravity from the value in equilibrium.

asteroid-A minor planet; one of the many small celestial bodies revolving around the sun, most of the orbits being between those of Mars and Jupiter. Also called minor planet; planetoid. See also artificial asteroid.

astigmatism—An aberration affecting the sharpness of images for objects off the axis in which the rays passing through different meridians of the lens come to a focus in different planes. Thus, an extra-axial point object is imaged as two mutually perpendicular short lines located at different distances from the lens. astigmatizer-A lens which introduces astigmatism into an optical system. Such a lens is so arranged that it can be placed in or removed from the optical path at will. In a sextant, an astigmatizer may be used to elongate the image of a celestial body into a horizontal line.

astre fictif-Any of several fictitious stars assumed to move along the celestial equator at uniform rates corresponding to the speeds of the several harmonic constituents of the tide-producing force. Each astre fictif crosses the meridian at the instant the constituent it represents is at a maximum. astrocompass-A direction-finding instrument comprising means for sighting on a celestial body and for mechanically solving the astronomical triangle, especially one adapted to night use.

astrodynamics The practical application of celestial mechanics, astroballistics, propulsion theory, and allied fields to the problem of planning and directing the trajectories of space vehicles.

astro-geodetic datum orientation-Adjustment of the ellipsoid of reference for a particular datum so that the sum of the squares of deflections of the vertical at selected points throughout the geodetic network is made as small as possible.

astro-geodetic deflection-The angle at a point between the normal to the geoid and the normal to the ellipsoid of an astro-geodetically oriented datum. Also called relative deflection. astro-geodetic leveling-A concept whereby the astro-geodetic deflections of the vertical are used to determine the separation of the ellipsoid and the geoid in studying the figure of the earth. Also called astronomical leveling. astro-geodetic undulations Separations between the geoid and astro-geodetic ellipsoid.

astrograph-A device for projecting a set of precomputed altitude curves onto a chart or plotting sheet, the curves moving with time such that if they are properly adjusted, they will remain in the correct position on the chart or plotting sheet.

astrograph mean time-A form of mean time, used in setting an astrograph. Astrograph mean time 1200 occurs when the local hour angle of Aries is 0°.

astrographic position-See astrometric position. astro-gravimetric leveling-A concept whereby a gravimetric map is used for the interpolation of the astro-geodetic deflections of the deflections of the vertical to determine the separation of the ellipsoid and the geoid in studying the figure of the earth. astro-gravimetric points-Astronomical positions corrected for the deflection of the vertical by gravimetric methods. The initial point of most geodetic systems are of this type.

astrolabe-1. (general) Any instrument designed to measure the altitudes of celestial bodies. 2. (survey) An instrument designed for very accurate celestial altitude measurements. See also equiangulator; pendulum astrolabe; planispheric astrolabe; prismatic astrolabe.

astrometric position-The position of a heavenly body (or space vehicle) on the celestial sphere corrected for aberration but not for planetary aberration. Astrometric positions are used in photographic observation where the position of the observed body can be measured in reference to the positions of comparison stars in the field of the photograph. Also called astrographic position. Compare with apparent position.

astrometry-The branch of astronomy dealing with the geometric relations of the celestial bodies and their real and apparent motions. The techniques of astronmetry, especially the the determination of accurate position by photographic means, are used in tracking satellites and space probes.

astronomical (astronomic)-Of or pertaining to astronomy, the science which treats of

heavenly bodies, and the arts based on that science. In any combination, such as astronomic coordinates, astronomic is equivalent to astronomical.

astronomical arc-The apparent arc described

above (diurnal arc) or below (nocturnal arc) the horizon by the sun or another celestial body.

astronomical azimuth-The angle between the astronomical meridian plane of the observer and the plane containing the observed point and the true normal (vertical) of the observer, measured in the plane of the horizon, preferably clockwise from north.

astronomical azimuth mark-A signal or target whose astronomic azimuth from a survey station is determined from direct observations on a celestial body. The mark may be a lamp or illuminated target placed especially for the purpose; or it may be a signal lamp at another survey station.

astronomical bearing-See true bearing.

astronomical constants The elements of the orbits of the bodies of the solar system, their masses relative to the sun, their size, shape, orientation, rotation, and inner constitution, and the velocity of light. See also system of astronomical constants.

astronomical control-A network of control stations the positions of which have been determined by astronomical observation. Latitudes and longitudes thus determined will normally differ from the geodetic latitudes and longitudes of the same stations due to the force of gravity.

astronomical coordinates-Quantities defining

a point on the surface of the earth, or of the geoid, in which the local direction of gravity is used as a reference. Sometimes called geographic coordinates; gravimetric coordinates.

astronomical date-Designation of epoch by year, month, day and decimal fraction. For example the astronomical date of December 21, 1966, 18h U.T. (universal time) is 1966 December 21. 75 U.T. The astronomic date is also used in connection with the other time

systems. The system commences every calendar year at 0" on December 31 of the previous year. This epoch is denoted by January 0.0.

astronomical day-A mean solar day beginning

at mean noon, 12 hours later than the beginning of the civil day of the same date. The civil day is now generally used.

astronomical equator-The line on the surface of the earth whose astronomical latitude at every point is 0°. Due to the deflection of the plumb line, the astronomic equator is not a plane curve. However, the verticals at all points on it are parallel to one and the same plane, the plane of the celestial equator; that is, the zenith at every point on the astronomic equator lies in the celestial equator. When the astronomical equator is corrected for station error, it becomes the geodetic equator. Sometimes called terrestrial equator. See also geodetic equator.

astronomical latitude-The angle between the plumb line and the plane of celestial equator. Also defined as the angle between the plane of the horizon and the axis of rotation of the earth. Astronomical latitude applies only to positions on the earth and is reckoned from the astronomic equator (0°), north and south through 90°. Astronomical latitude is the latitude which results directly from observations of celestial bodies, uncorrected for deflection of the vertical.

astronomical leveling-See astro-geodetic leveling.

astronomical longitude-The angle between the plane of the celestial meridian and the plane of an initial meridian, arbitrarily chosen. Astronomical longitude is the longitude which results directly from observations on celestial bodies, uncorrected for deflection of the vertical.

astronomical meridian-A line on the surface of the earth having the same astronomical longitude at every point.

astronomical meridian plane-A plane that contains the vertical of the observer and is parallel to the instantaneous rotation axis of the earth.

astronomical parallel-A line on the surface of the earth which has the same astronomic latitude at every point. Because the deflection of the vertical is not the same at all points on the earth, an astronomical parallel is an irregular line, not lying in a single plane. See also astronomical equator.

astronomical position-1. A point on the earth whose coordinates have been determined as a result of observations of celestial bodies. The expression is usually used in connection with positions on land determined with great accuracy for survey purposes. 2. A point on the earth, defined in terms of astronomical latitude and longitude.

astronomical refraction-The apparent displacement of an object that results from light rays from a source outside the atmosphere being bent in passing through the atmosphere. This results in all objects appearing to be higher above the horizon than they actually are. The magnitude of this displacement is greater when the object is near the horizon and decreases to zero when the object is at the zenith. Also called astronomical refraction error; celestial refraction; and shortened to refraction. See also atmospheric refraction.

astronomical refraction error-See astronomical refraction.

astronomical station-A point on the earth whose position has been determined by observations on celestial bodies.

astronomical surveying The celestial determination of latitude and longitude. Separations are calculated by computing distances corresponding to measured angular displacements along the reference spheroid. astronomical theodolite-See alt-azimuth instrument.

astronomical tidal constituent-See constituent. astronomical time-Solar time in a day (astronomical day) that begins at noon. Astronomical time may be either apparent solar time or mean solar time.

astronomical transit-See transit, definition 4. astronomical traverse-A survey traverse in which the geographic positions of the sta

tions are obtained from astronomical observations, and lengths and azimuths of lines are obtained by computation. astronomical triangle-The triangle on the celestial sphere formed by arcs of great circles connecting the celestial pole, the zenith, and a celestial body. The angles of the astronomical triangles are: At the pole, the hour angle; at the celestial body, the parallactic angle; at the zenith, the azimuth angle. The sides are: Pole to zenith, the co-latitude; zenith to celestial body, the zenith distance; and celestial body to pole, the polar distance. Also called PZS triangle.

astronomical unit-A unit of length equal to 92,900,000 statute miles used for measuring distances within the solar system. This distance approximates the mean distance of the earth from the sun.

astronomical year-See tropical year. asymmetrical-Not symmetrical.

asymmetry of object (target)-Lack of symmetry in the visible aspect of an object as seen from a particular point of observation. A square or rectangular pole may so face the observer that the line bisecting its tangents does not pass through its geometrical center. With a square cupola or tower, the error resulting from observing tangents and taking a mean may be quite large. The error caused by asymmetry of an observed object is of the same character and requires the same treatment as the error resulting from observing an eccentric object. Asymmetry and phase are closely associated but are not identical.

asymptote-A straight line or curve which a curve of infinite length approaches but never quite reaches.

atlas grid-A reference system that permits the designation of the location of a point or an area on a map, photo, or other graphic in terms of numbers and letters. Also called alphanumeric grid.

atmosphere The envelope of air surrounding

the earth; also the body of gases surrounding or comprising any planet or other celestial body. Compare geosphere; hydrosphere; lithosphere.