orthometric error-An error due to the spheroidal form of the earth and the action of centrifugal force; level surfaces at different elevations are not exactly parallel.

orthomorphic chart-A chart on which very small shapes are correctly represented.

orthomorphic map projection-See conformal map projection.

orthophotograph-A photographic copy, prepared from a perspective photograph, in which the displacements of images due to tilt and relief have been removed.

orthophotomap-A photomap made from an assembly of orthophotographs. It may incorporate special cartographic treatment, photographic edge enhancement, color separation, or a combination of these. orthophotomosaic-An assembly of orthophotographs forming a uniform-scale

mosaic.

orthophotoscope-A photomechanical device, used for producing orthophotographs. orthopictomap-A pictomap made from an orthophotomap base.

orthostereoscopy-A condition wherein the horizontal and vertical distances in a stereoscopic model appear to be at the same scale.

oscillation-A double motion, one in each direction, of a pendulum. An oscillation is composed of two successive vibrations.

osculating elements-The elements that define an osculating orbit. See also osculating orbit.

osculating ellipse-An ellipse that is tangent at a point (called the epoch of osculation) to a real orbit.

osculating orbit-The ellipse that a satellite

would follow after a specific time "t" (the epoch of osculation) if all forces other than

central inverse-square forces ceased to act from time "t" on. An osculating orbit is tangent to the real, perturbed, orbit and has the same velocity at the point of tangency.

outer orientation-See exterior orientation.

outer planets-The planets with orbits larger than that of Mars, i.e., Jupiter, Saturn, Uranus, Neptune, and Pluto.

outline map-A map which presents just sufficient geographic information to permit the correlation of additional data placed upon it.

overcharging-Applying excessive additional information (aeronautical or navigational) to a map or chart resulting in clutter.

overhang (aerial photography) The additional exposures beyond the boundary of an area to be photographed, usually two exposures at the ends of each strip to assure complete stereoscopic coverage.

overlap-1. The amount by which one photograph includes the same area as covered by another, customarily expressed as a percentage. See also forward lap; side lap. 2. That area of a map or chart which overlaps the same geographical area on an adjoining map or chart. 3. An area included within two surveys of record, which by record are described as having one or more common boundary lines with no inclusion of identical parts.

overlapping grid-A military grid which is

extended beyond its normal limits to map sheets located in areas bordering grid junctions and spheroid junctions. Normally, large scale maps that fall within approximately 25 miles of a grid, zone, or spheroid junction will bear an overlapping grid, depicted by ticks emanating from the map neatline. Overlapping grids are used primarily for extension of fire control and survey operations. See also major grid; secondary grid.

panoramic field of view. Contrasted with frame camera.

panoramic distortion-The displacement of ground points from their expected perspective positions, caused by the cylindrical shape of the negative film surface and the scanning action of the lens in a panoramic camera system.

panoramic photograph-Photography obtained from a panoramic camera.

pantograph-An instrument for copying maps, drawings, or other graphics at a predetermined scale. Pantographs capable of adjustment for several scales are known as fixed-ratio pantographs. See also twodimensional pantograph.

paper-strip method-(rectification) A graphical method of making a point-by-point rectification based on the invariance of the cross ratio. A modification of this technique permits map detail to be revised. from an oblique aerial photograph based on the projectivity of straight lines.

parallactic aberration-See differential aberration.

parallactic angle-1. (astronomy) The angle between a body's hour circle and its vertical circle. Also called position angle. 2. (photogrammetry) The angle subtended by the eye base of the observer at the object viewed. Also called angle of convergence; angular parallax.

parallactic error-An error caused by personal or instrument parallax.

parallactic grid-(photogrammetry) A uniform. pattern of rectangular lines drawn or engraved on some transparent material,

usually glass, and placed either over the photographs of a stereoscopic pair or in the optical system of a stereoscope, in order to provide a continuous floating-mark system.

parallactic inequality-A secondary effect in solar perturbations in the moon's longitude due to the ellipticity of the earth's orbit.

parallax-1. The apparent displacement of the position of a body, with respect to a reference point or system, caused by a shift in the point of observation. 2. The apparent displacement between objects on

the earth's surface due to their difference in elevation. See also absolute stereoscopic parallax; age of parallax inequality; angular parallax; annual parallax; equatorial horizontal parallax; geocentric parallax; horizontal parallax; instrument parallax; linear parallax; lunar parallax; residual parallax; solar parallax; want of correspondence; y-parallax.

parallax age-See age of parallax inequality.

parallax bar-See stereometer.

parallax difference-The difference in the absolute stereoscopic parallaxes of two points imaged on a pair of photographs. Customarily used in determination of the difference in elevation of objects.

parallax in altitude-Geocentric parallax at any altitude. The expression is used to distinguish the parallax at the given altitude from the horizontal parallax when the body is in the horizon.

parallax inequality-The variation in the range

of tide or in the speed of tidal currents because of the continual change in the distance of the moon from the earth. The range of tide and speed of tidal currents tend to increase as the moon approaches perigee and to decrease as it approaches

apogee.

parallel-A circle on the surface of the earth,

parallel to the plane of the equator and connecting all points of equal latitude, or a

parametric latitude-The angle at the center of a sphere which is tangent to the spheroid along the geodetic equator, between the plane of the equator and the radius to the point intersected on the sphere by a straight line perpendicular to the plane of the equator and passing through the point on the spheroid whose parametric latitude is defined. Parametric latitude is an auxiliary latitude used in problems of geodesy and cartography. In astronomical work, when the term reduced latitude is used, geocentric latitude is meant. Also called geometric latitude; reduced latitude.