vertical coordinates—The vertical distance of a point above or below a reference datum. Points may be plus or minus according to whether the point is above or below the datum.

vertical curve-A parabolic curve used to connect grades of different slope, and used at the vertex of a grade to avoid the sudden change in direction in passing from one grade to the other. This method of grade change is usually used when there is an algebraic difference of more than 0.2% in the two opposing grades. vertical datum-See vertical-control datum. vertical deformation-In relative orientation, the cumulative model warpage effecting the vertical datum from x-tile error and y-tilt

error.

vertical exaggeration-1. The change in a model surface created by proportionally raising the apparent height of all points above the base level while retaining the same base scale. 2. The increase of the vertical scale over the horizontal scale of a terrain model or plastic relief map.

vertical extension-See extension, definition 2. vertical geodetic datum-See vertical-control datum.

vertical intensity-The strength of the vertical component of the earth's magnetism.

vertical parallax-See y-parallax.

vertical pass point-See supplemental elevation. vertical photograph-An aerial photograph

taken with the axis of the camera being maintained as closely as possible to a truly vertical position with the resultant photograph lying approximately in a horizontal plane.

vertical plane-1. Any plane passing through a point on the earth and containing the zenith and nadir of that point; also a plane containing a plumb line. 2. (surveying) A plane at right angles to a horizontal plane and within which angles and distances are observed. vertical sketchmaster-A type of sketchmaster in which vertical photographs are utilized.

vertical stereotriangulation-That portion of stereotriangulation concerned with the establishment of vertical data. Vertical stereotriangulation is often limited or precluded as an operation due to the more rigid accuracy standards established for vertical positions than for horizontal positions.

vibration-A single movement of a pendulum in either direction, to or fro. See also oscillation.

vignetting—1. (photography) A gradual reduction in density of parts of a photographic image due to the stopping of some of the rays entering the lens. Thus, a lens mounting may interfere with the extreme oblique rays. An antivignetting filter is one that gradually decreases in density from the center toward the edges; it is used with many wide-anglelenses to produce a photograph of uniform density by cutting down the overexposure of the center of the photograph. 2. (lithography) A photographic process which portrays a solid color in a screen which shades off gradually into the unprinted paper. Open water is often shown by this method. virtual gravity-The force of gravity on an atmospheric parcel, reduced by centrifugal force due to the motion of the parcel relative to the earth.

virtual image-An image that cannot be shown on a surface but is visible, as in a mirror. Virtual PPI Reflectoscope (VPR) chart-A type of radar chart.

visible horizon-See apparent horizon.

visibility chart-A special purpose map or other graphic showing which areas can be seen and those which cannot be seen from a given observation point.

visual acuity-A measure of the human eye's ability to separate details in viewing an object. The reciprocal of the minimum angular separation, in minutes of arc, of two lines of detail which can be seen separately.

vulgar establishment-See establishment of the port.

WWV time-WWV broadcasts UTC which is offset from A1 by a variable amount determined annually before the start of the year. The epoch is shifted in increments of 100 milliseconds if it departs too far from UT2. wading rod-A rod, graduated in feet and tenths of feet, used for stream-gaging in shallow water.

wall map-A special-purpose map of a large area designed to be displayed on a wall. wander-See apparent wander.

want of correspondence-See y-parallax. warped model-Any spatial model which, due to photographic distortions or orientation errors, has a model datum which is deformed or otherwise incapable of being leveled. Contrast with flat model.

water leveling-A method of obtaining relative elevations by observing heights with respect to the surface of a body of still water. water-stage recorder-An automatic recording instrument which records the rise and fall of the water surface at a stream-gaging station.

waving the rod-In leveling, a technique whereby the rodman slowly pivots the leveling rod toward and away from the instrument position. The least reading obtainable is the proper one to be recorded.

weather map-A map showing the weather conditions prevailing, or predicted to prevail, over a considerable area. Usually, the map is based upon weather observations taken at the same time at a number of stations. Also called synoptic chart; surface chart.

wedge (optics)-A refracting prism of very small deviation, such as those used in the eyepieces of some stereoscopes. Also called optical wedge.

weight-The relative value of an observation, source, or quantity when compared with other observations, sources, or quantities of the same or related quantities. The value determined by the most reliable method is assigned the greatest weight.

weighted mean-A value obtained by multiplying each of a series of values by its assigned weight and dividing the sum of those products by the sum of the weights.

Werner map projection-A particular case of the Bonne map projection, in which the standard parallel is at the pole, and the tangent cone becomes a tangent plane. Any one geographic meridian is chosen as the central meridian and represented by a straight line, divided to exact scale. The geographic parallels are represented by circular arcs, also divided to exact scale, and the other meridians are curved lines.

westing-See departure (plane surveying). west point-See prime vertical plane. whirler-Equipment used for applying sensitizer to press plates.

wide-angle lens-A lens having an angle of coverage between 75° and 100°. A lens whose focal length is equal approximately to one half the diagonal of the format. wiggling-in-A survey procedure used when it is necessary to establish a point, exactly on line between two control points, neither of which can be occupied. It is essentially a trialand-error technique where repeated fore and back readings are taken and the instrument shifted after each pair of readings until exactly in line with the stations. Also called ranging-in.

wiggling-in on line-See double centering. wind rose-1. A diagram showing the relative frequency of winds blowing from different

directions. It may also show average speed or frequency of occurrence of various speeds from different directions. 2. A diagram showing the average relation between winds from different directions and the occurrence of other meteorological phenomena.

wing photograph-A photograph taken by one of the side or wing lenses of a multiple-lens

camera.

wing point-Three easily identified points along each side of an aerial photograph, one near each corner and one near the middle. Used in the extension of radial control in making controlled mosaics.

winter solstice-1. That point on the ecliptic occupied by the sun at maximum southerly declination. Also called first point of Capricornus; December solstice. 2. That instant at which the sun reaches the point of maximum southerly declination, about December 22. wire drag-A sounding device consisting of weighted wires which are maintained at a given depth by floats, and then dragged over any desired course.

witness corner-1. A marker set on a property line leading to a corner; used where it is impractical to maintain a monument at the corner itself. 2. (USPLS) A monumented survey point usually on the line of survey near a corner established as a reference mark when the corner is so situated as to render its monumentation or ready use impractical.

witness mark-A mark placed at a known distance and direction from a property corner or survey station to aid in its recovery and identification. Also called witness post; witness stake.

witness point-(USPLS) A monumented station on a line of the survey, employed to per

petuate an important location remote from, and without special relation to, any regular

corner.

witness post-See witness mark.

witness stake-See witness mark. witness tree-See bearing tree.

Woodward base-line measuring apparatus—See iced-bar apparatus.

working in on a line-See double centering.

working pendulum-A pendulum which is used (swung) in a determination of the intensity of gravity.

World Aeronautical Chart (WAC)-See Operational Navigation Chart.

world geodetic system-A single datum for the earth which provides the location of all points on the surface of the earth with respect to the earth's center of mass.

World Geographic Reference System (GEOREF)-See GEOREF.

world polyconic grid-A grid system in which a grid network is mathematically derived from elements of a polyconic projection. wrong-reading-A descriptive term for an image, which when viewed through the base, is a reverted or mirror image of the original. Other terms, such as reverse reading, etc. are sometimes used to identify image direction, but are not recommended because of possible confusion in negative-positive relationship.

wye level-A leveling instrument having the telescope, with attached spirit level, supported in wyes (Y's). The telescope can be rotated about its longitudinal axis (collimation axis) in the Y's and it can be lifted and reversed, end for end, for testing and adjustment. Also called Y level.

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-direction. x-direction-An observed 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.