nontilting lens rectifier-A class of rectifier wherein the lens is constrained to move in the direction of its fixed axis.

nontilting-negative-plane rectifier-A class of rectifier which contains a nontilting negative carrier. In this class of rectifiers, the negative-carrier plane remains hori

zontal.

normal-1. A straight line perpendicular to a surface or to another line. 2. A condition of being perpendicular to a surface or line. 3. In geodesy, the straight line perpendicular to the surface of the reference spheroid. 4. The average, regular, or expected value of a quantity.

normal-angle lens-A lens having an angle of coverage from 60° to 75°. A lens whose focal length is equal approximately to the diagonal of the format.

normal contour-See accurate contour.

normal equation-One of a set of simultaneous equations derived from observation, condition, or correlate equations. In a least-squares adjustment, values obtained from the solution of normal equations (either directly or through the correlate equations) are applied to the observation or condition equations to obtain the desired

corrections.

normal gravity-The gravity caused by the attraction of the ideal geoid (ellipsoid) combined with the centrifugal force due to

its rotation.

normal orbit-The orbit of a spherical satellite about a spherical primary during which there are no disturbing effects present due to other celestial bodies, or to some physical phenomena. Also called unperturbed orbit.

normal section azimuth-The angle between the geodetic meridian of the observer and the plane containing the ellipsoidal normal of the observer and the projection of the observed point on the reference ellipsoid,

measured in a plane perpendicular to the ellipsoidal normal of the observer, preferably clockwise from the north.

normal section line-A line on the surface of the reference spheroid, connecting two points on that surface, and traced by a plane containing the normal at one point and passing through the other point.

normal tension-(taping) The tension to be applied to a tape to compensate for the shortening effect of sag in order to bring the tape to standard length. That pull at which the tension correction and sag exactly balance each other.

oblique pole-One of the two points 90° from an oblique equator.

oblique rhumb line-1. A line making the same oblique angle with all fictitious meridians of an oblique Mercator projection. Oblique parallels and meridians may be considered special cases of the oblique rhumb line. 2. Any rhumb line, real or fictitious, making an oblique angle with its meridians. In this sense the expression is used to distinguish such rhumb line from parallels and meridians, real or fictitious, which may be included in the expression rhumb line. See also fictitious rhumb line.

oblique sketchmaster-A type of sketchmaster

in which oblique photographs are utilized.

oblique sphere-The celestial sphere as it appears to an observer between the equator and the pole, where celestial bodies appear to rise obliquely to the horizon.

obliquity of the ecliptic-The acute angle between the plane of the ecliptic (the plane of the earth's orbit) and the plane of the celestial equator.

obliterated corner-(USPLS) A corner at which there are no remaining traces of the monument or its accessories, but whose location has been perpetuated or may be recovered beyond reasonable doubt, by the acts and testimony of the interested landowners, competent surveyors, other qualified local authorities, witnesses, or by some acceptable record evidence.

observation equation-A condition equation which connects interrelated unknowns by means of an observed function, or a condition equation connecting the function observed and the unknown quantity whose value is sought.

observed altitude-Corrected sextant altitude; angular distance to the center of a celestial. body above the horizon, corrected for instrumental errors, personal error, dip, refraction, and semidiameter and parallax if necessary. Also called true altitude.

observed angle-An angle obtained by direct instrumental observation. A measured angle which has been corrected for local conditions only at the point of observation, is considered an observed angle.

observed gravity-Uncorrected value of gravity at a station as determined from a gravity meter.

observed gravity anomaly-See gravity anomaly.

observed value-A value of a quantity that is obtained by instrumental measurement of the quantity. The term observed value is often applied to the value of a quantity derived from instrumental measurement after corrections have been applied for systematic errors, but before accidental errors have been taken out by some method of adjustment.

obsolete chart-A chart which is not considered safe to use for navigation because it does not contain the latest important navigational information.

obsolete grid-See secondary grid.

occultation-1. (astronomy) The disappearance of a celestial body behind another body of larger apparent size. When the moon passes between the observer and a star, the star is said to be occulted. 2. (surveying) Name applied to a geodetic survey technique which employs the principle of occultation where repeated observations are made on an unknown position, accurately timed with similar observations at another unknown station, and mathematically reducing this data to determine the exact geodetic position of the unknown stations. See also star occultation method.