x-axis-1. A horizontal axis in a system of rectangular coordinates; that line on which distances to the right or left (east or west) of the reference line are marked, especially on a map, chart, or graph. 2. The line joining the opposite fiducial marks in the direction which is most nearly parallel to the line of flight.

x-coordinates-See abscissas.

x-correction-The correction to an x-direc

tion.

x-direction-An observed direction. direction in a triangulation figure for which an approximate value is obtained and treated like an observed direction in the adjustment of the figure. The work of a least-squares adjustment of a triangulation figure sometimes requires the use of an x-direction, for which an approximate value is obtained by an inverse position computation, by the solution of the three-point problem, or by other means, and then using the x-direction in the adjustment and obtaining a correction (x-correction) for it, which makes it consistent with the adjusted values of the observed directions.

x-displacement -A component of image displacement. When a point image is to be located by coordinates with respect to rectangular axes, x-displacement represents the distance moved in the x-direction.

x-motion-In a stereoplotting instrument, that linear adjustment approximately parallel to a line connecting two projector stations; the path of this adjustment is, in effect, coincident with the flight line between the two relevant exposure stations. x-parallax-See absolute stereoscopic parallax. x-scale-In oblique photography, the scale along the horizon parallels. It is constant for any one parallel although each successive parallel will have a different

scale.

x-tilt-The component of tilt about the x-axis, which is most nearly in the direction of flight.

x-y scaler-An instrument that provides x- and y-coordinates in digital form from analogue data.

Y

y-axis-1. A vertical axis in a system of rectangular coordinates; that line on which distances above or below (north or south of) a reference line are marked, especially on a map, chart, or graph. 2. The line which is perpendicular to the x-axis and passes through the origin.

y-coordinates-See ordinates.

y-displacement-A component of image displacement. When a point image is to be located by coordinates with respect to rectangular axes, y-displacement represents the distance moved in the y-direction.

Y level-See wye level.

y-parallax-The difference between the perpendicular distances of the two images of a point on a pair of photographs from the vertical plane containing the air base. The existence of y-parallax is an indication of tilt in either or both photographs, or a difference in flying heights, and interferes with stereoscopic examination of the pair. Also called vertical parallax; want of correspondence.

y-scale-In oblique photographs, the scale along the principal line. It varies from point to point along the line.

y-swing method-See one-swing method. y-tilt-The component of tilt about the y-axis which is perpendicular to the direction of flight. Also called pitch. Sometimes called tip, which is an obsolete term.

yaw-1. (air navigation) The rotation of an aircraft about its vertical axis so as to cause the aircraft's longitudinal axis to deviate from the flight line. Also called crab. 2. (photogrammetry) The rotation of camera or a photograph coordinate system about either the photograph z-axis or the

exterior z-axis.

yaw angle-See angle of yaw.

year-The period of about 365 solar days required for one revolution of the earth around the sun. See also anomalistic year; calendar year; eclipse year; great year; fictitious year; sidereal year; tropical year.