The axis of tilt could be any of several lines in space (e.g., the isometric parallel or the ground line), but the present definition is the only one which permits the concept of tilting a photograph without upsetting the positional elements of exterior orientation.

axis of the level bubble-See spirit level axis.

azimuth-1. (surveying) The horizontal direc

tion of a line measured clockwise from a reference plane, usually the meridian. Also called forward azimuth to differentiate from back azimuth. 2. (photogrammetry) Azimuth of the principal plane. See also astronomical azimuth; back azimuth; computed azimuth angle; direction method of determining astronomical azimuth; geodetic azimuth; grid azimuth; Laplace azimuth; magnetic azimuth; method of repetitions (determination of astronomical azimuth); micrometer method (determination of astronomical azimuth); normal section azimuth; true azimuth.

azimuth angle-1. (astronomy) The angle less than 180° between the plane of the celestial meridian and the vertical plane containing the observed object, reckoned from the direction of the elevated pole. In astronomic work, the azimuth angle is the spherical angle at the zenith in the astronomical triangle which is composed of the pole, the zenith, and the star. In geodetic work, it is the horizontal angle between the celestial pole and the observed terrestrial object. 2. (surveying) An angle in triangulation or in a traverse through which the computation of azimuth is carried. In a simple traverse, every angle may be an azimuth angle. Sometimes, in a traverse, to avoid carrying azimuths over very short lines, supplementary observations are made over comparatively long lines, the angles between which form azimuth angles. In triangulation, certain angles, because of their size and position in the figure, are selected for use as azimuth angles, and enter into the formation of the azimuth condition equation (azimuth equation).

azimuth bar-See azimuth instrument.

azimuth circle-A ring designed to fit snugly over a compass or compass repeater, and provided with means for observing compass bearings and azimuths.

azimuth equation-A condition equation which expresses the relationship between the fixed azimuths of two lines which are connected by triangulation or traverse. azimuth error of closure-See error of closure, definition 3.

azimuth instrument-An instrument for measuring azimuths, particularly a device which fits over a central pivot in the glass cover of a magnetic compass. Also called azimuth bar; bearing bar.

azimuth line-(photogrammetry) A radial line from the principal point, isocenter, or nadir point of a photograph, representing the direction to a similar point of an adjacent photograph in the same flight line; used extensively in radial triangulation.

azimuth mark-A mark set at a significant distance from a triangulation or traverse station to mark the end of a line for which the azimuth has been determined, and to serve as a starting or reference azimuth for later use. See also astronomical azimuth mark; geodetic azimuth mark; Laplace azimuth mark.

azimuth transfer-Connecting, with a straight line, the nadir points of two vertical photographs selected from overlapping flights.

azimuth traverse-A survey traverse in which the direction of the measured course is determined by azimuth and verified by back azimuth. To initiate this type of traverse it is necessary to have a reference meridian, either true, magnetic, or assumed.

azimuthal chart-A chart on an azimuthal projection. Also called zenithal chart.

azimuthal equidistant chart-A chart on the azimuthal equidistant map projection.

azimuthal equidistant map projection-An azimuthal map projection on which straight lines radiating from the center or pole of projection represent great circles in their true azimuths from that center, and lengths along those lines are of exact scale. This projection is neither equal-area equal-area nor conformal.

azimuthal map projection-A map projection

on which the azimuths or directions of all lines radiating from a central point or pole are the same as the azimuths or directions of the corresponding lines on the sphere. Also called zenithal map projection.

azimuthal orthomorphic map projection-See stereographic map projection.

BC-4 camera-A trade name for the ballistic or geodetic steller camera consisting of a Wild Astrostar lens cone mounted on the modified lower part of the Wild T4 astronomical theodolite. Originally designed for the recording of the trajectory of a rocket but since adapted for the photographic tracking of artificial earth satellites for geodetic purposes.

Bz curve-(photogrammetry) A graphical representation of the vertical errors in a stereotriangulated strip. In a Bz curve, the x-coordinates of the vertical control points, referred to the initial nadir point as origin, are plotted as abscissas, and the differences between the known elevations of the control points and their elevations as read in the stereotriangulated strip are plotted as ordinates; a smooth curve drawn through the plotted points is the Bz curve. The elevation read on any pass point in the strip is adjusted by the amount of the ordinate of the Bz curve for an abscissa corresponding to the x-coordinate of the point.

Bz curve method-A method utilizing characteristics of the Bz curve for finding the displacement of true photo plumb points from indicated projector plumb points in multiplex strip orientation. The method also provides a means of strip leveling using only the barometric-altimeter readings of the aircraft-flying height.

Bache-Wurdeman base-line measuring apparatus-A compensating base-line measuring apparatus having a measuring element composed of a bar of iron and a bar of brass, each a little less than 6 meters in length, held together firmly at one end, with the free ends so connected by a compensating lever as to form a compensating apparatus.

back azimuth-If the azimuth of point B from point A is given, the back azimuth is the azimuth of point A from point B. Because of the convergence of the meridians, the forward and backward azimuths of a line do not differ by exactly 180°, except where A and B have the same geodetic longitude or where the geodetic latitudes of both points are 0°. See also azimuth, definition 1.

back bearing-1. A bearing differing by 180°, or measured in the opposite direction from a given bearing. Also called reciprocal bearing. 2. (USPLS) The reverse direction of a line as corrected for the curvature of the line from the forward bearing at the preceding station. Important on control lines such as the secant and tangent and on long triangulations. Thus, the forward bearing of the secant at a township corner in latitude 45° is N 89° 57′ 24′′ W (when the line is running westerly); at 3 miles from the starting point the back bearing is east, the forward bearing, west; at the next township corner, 6 miles from the starting point, the back bearing is N 89° 57′ 24′′ E. These are the true bearings in angular measure from the meridian at the point of record.

back focal distance-See back focal length.

back focal length-The distance measured

along the lens axis from the rear vertex of

the lens to the plane of best average definition. Also called back focal distance; back focus.

back focus-See back focal length.

backsight-1. A sight on a previously established survey point or line. 2. (traverse) A sight on a previously established survey point, which is not the closing sight of the traverse. 3. (leveling) A reading on a rod held on a point whose elevation has been previously determined and which is not the closing sight of a level circuit; any such rod reading used to determine height of instrument prior to making a foresight. Also called plus sight.

backstep-The method of determining the offsets for the bottom latitude of a projection by measuring the appropriate distances down from the top latitude of a

chart.

backup-An image printed on the reverse side

of a map sheet already printed on one side. Also the printing of such images.

balancing a survey-Distributing corrections through any traverse to eliminate the error of closure, and to obtain an adjusted position for each traverse station. Also called traverse adjustment. See also compass rule; distance prorate rule; transit rule.

Baldwin solar chart-A chart designed for orienting a planetable by means of the sun's shadow.

ballistic camera-A precision terrestrial

camera, usually employing glass plates, used at night to photograph such objects as rockets, missiles, or satellites against a star background. Also called tracking camera. See BC4 camera.

band-Any latitudinal strip, designated by accepted units of linear or angular measurement, which circumscribes the earth. Also called latitudinal band.

Barlow leveling rod-A speaking rod marked with triangles each 0.02 foot in height. barometer-An instrument for measuring atmospheric pressure. See aneroid barometer; cistern barometer; mercury barometer; siphon barometer.

barometric altimeter-An instrument that indicates elevation or height above sea level, or some other reference height, by measuring the weight of air above the instrument. Also called aneroid altimeter; pressure altimeter; sensitive altimeter.

barometric elevation-An elevation determined with a barometer.

barometric hypsometry-The determination of elevations by means of either mercurial or aneroid barometers.

barometric leveling-A method of determining differences of elevation from differences of atmospheric pressure observed with a barometer. A type of indirect leveling.

bar scale-See graphic scale.

barycenter-The center of mass of a system of masses; as the barycenter of the earthmoon system.

basal coplane-(photogrammetry) The condition of exposure of a pair of photographs in which the two photographs lie in a common plane parallel to the air base. If the air base is horizontal, the photographs are said to be exposed in horizontal coplane.

basal orientation-The establishment of the position of both ends of an air base with respect to a ground system of coordinates. In all, six elements are required. These are essentially the three-dimensional coordinates of each end of the base. In practice, however, it is also convenient to express these elements in one of two alternative ways: (1) The ground rectangular coordinates of one end of the base and the difference between these and the ground rectangular coordinates of the other end of