Abstract: In an electro-optical measuring system which utilizes a sin/cos signal pair to determine the extent and direction of physical displacement, measurement errors often occurring as a result of extraneous DC level shifts in generated signals are corrected in a procedure which regularly samples the maxima and minima of the signals, derives from those values the median DC levels of the signals, compares the median levels to a reference DC level in the system to determine any differences, and applies any detected differences as corrective factors to position measurement signal values.

Abstract: An AC coupled amplifier system is protected from overloading input signals by means of a "bootstrap" circuit which provides a compensation signal substantially matching the overscale portion of such input signal and applies that compensation signal to the coupling capacitor as an offset in order to prevent excessive charge accumulation on that capacitor. A voltage controlled current source serves as a signal range limiter in the circuit and responds to overscale input signals by causing a circuit loop opening and forcing loop closure in a secondary feedback circuit from which the extent of input signal overscale is determined. The matching compensation signal is then derived from such determination. The protection circuit further provides a reset function to account for extended overscale input signal levels.

Abstract: An electro-optical measuring system in a photogrammetric comparator or stereocompiler comprises a visible light transparent phase grating which is carried upon a surface of the photo support plate of the device. The grating is formed as a composite layer of .lambda./4 films of at least two dielectric materials of different refractive index which are deposited on the support in such a manner as to yield parallel bands in which the two film materials are in alternating sequence from band to band. As a result of this structure which ensures a constant physical and optical thickness with respect to transmitted imaging light over the whole granting pattern, image-degrading diffraction of the transmitted visible light is avoided, yet the grating pattern provides interfering diffraction in reflection which may be utilized in a precise displacement measuring system.

Abstract: Means for conjointly mounting center members of dissimilar metals for stable concentric rotation about a theodolite spindle comprises a plurality of radially-extending guide grooves on one of the members disposed regularly about the axis of the spindle, and a like plurality of support elements affixed to another of the center members and similarly disposed about the spindle axis to seat respectively in the guide grooves. The support elements being thus limited by the guide grooves to precise radial movement, any dissimilar thermal expansions of the center members result in relative movement between them which is concentric with the spindle axis.

Abstract: A low-profile theodolite leveling means comprises a tribrach assembly including three separate platforms of which the second is mounted upon the first by means of a pair of pivot elements which establish a first fixed axis of tilt of the second platform, and the third is similarly mounted on the second by means of a pair of pivots which likewise establish a second fixed axis of tilt, orthogonal to the first axis, for the third platform. Located in use between a tripod head and the base of a theodolite geodetic instrument, the tribrach assembly provides means for tilting the theodolite instrument with respect to independent planes defined by the orthogonally disposed axes of tilt and respective adjustment screw members.

Abstract: Means for centering a reticle on the optical axis of a telescope comprises, within the telescope body, a split collar tapped to accommodate two pairs of diametrically opposed adjustment screws which bear upon rectilinear surfaces of a centrally located reticle mount. The arrangement provides a dust-proof seal for the telescope interior and ensures against stresses tending toward reticle misalignment.

Abstract: The surveying system comprises a theodolite which provides digital data from electro-optical horizontal and vertical circle reading components and includes a level sensor which generates digital data relative to the attitude of the vertical axis of the instrument. Additional means are included for presetting instrument correction and scaling factors which together with angle and level data are processed by an on-board microcomputer to yield accurate digital displays of horizontal and vertical angles. Provision is also made for the input of digital slope range data from an electronic distance measuring instrument to be displayed directly or processed with horizontal and vertical angle data by the microcomputer to provide vector coordinates of the observed target. In response to operator selection from parameter and vector functions the control program cycles the processing of relevant data in order to provide a repeatedly updated display of any desired function.

Abstract: Azimuth and elevation adjustments in a geodetic instrument are provided by systems which include coaxial locking and two-speed adjustment control knobs. In each such system, the control knobs are accessible at both the front and rear of the instrument to facilitate operation in either of the forward- or back-sighting mode.