Geometrical instruments – Straight-line light ray type – Rod or target
Patent
1997-09-24
1999-04-13
Bennett, G. Bradley
Geometrical instruments
Straight-line light ray type
Rod or target
G01C 1506
Patent
active
058932143
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a measuring sphere reflector for direction measurements and/or distance measurements.
Measuring spheres are frequently used as a physical realization of a target point for geodetic and industrial coordinate measurements. They are generally metallic spheres having a specularly reflecting surface which are mounted on a pin. The pin is mounted at a coordinate point to be measured in the field or on the measurement object.
The sighting telescope of a theodolite is trained on the sphere, and the latter is sharply imaged in an image plane by focussing. The sighting direction of the theodolite is determined in terms of coordinates by aligning a collimating mark or the cross hairs with the measuring sphere. A special illuminating device on the theodolite is frequently used to increase the display accuracy, and in this case the light reflection on the specular surface of the sphere is taken as the target point.
By aiming at the same measuring sphere from a different sighting direction, for example using a further theodolite, the distance to the center of the measuring sphere can be calculated with the aid of the known separation of the two theodolites. Since the distance of the center of the sphere from the base of the retaining pin is known, the coordinates of the measuring point can be determined in this way. Determining the theodolite distance is superfluous in the standard method with simultaneous measurement of a calibration distance (calibration standard) in object space. The dimensioning of the measuring sphere reflectors and the length of the retaining pins are normalized in order to simplify the computer programs employed. The diameter of the measuring sphere is therefore 12.7 mm.
In addition to coordinate measurement via two independent measurements of direction and base distance (triangulation), it is also possible to make polar measurements, that is to say to combine a direction measurement with a distance measurement, that is to say to connect a theodolite to a telemeter. Optical absolute telemeters evaluate the transit time of light pulses or modulated light beams which are reflected at the measurement object. It is necessary, in turn, to realize the target point physically for the purpose of accurately determining the coordinates of a measuring point. Since it is mostly a light beam of parallel alignment which is employed in distance measurement, triple prisms have proved themselves as target reflectors. The latter have the property that a parallel beam entering the base face is reflected parallel to itself, irrespective of the inclination of the base face with respect to the light beam direction.
The distance is altered by a fixed addition constant in the case of vertical incidence through the glass body. The target point for the distance measurement is the vertex of the triple prism. Since this cannot be sighted optically from larger distances, for the purpose of angular measurement in geodesy, the triple prism is connected to a target board which is sighted using the theodolite. The coordinates of the measuring point can be determined from a position by using the geometrical assignment between the triple prism and target board, as well as the holder thereof relative to the measuring point.
As already mentioned, it is an essential property of the triple prism that the beam reflection is parallel to the incident beam irrespective of inclination of the base of the triple prism relative to the beam direction. The optical path length is, however, dependent on the inclination of the base face. Given an inclination about the vertex of the prism, the optical path increases in proportion to the angle of inclination in a nonlinear fashion. An identical dependence results in the case of an inclination about the base of the altitude of the triple prism, but in this case the entire optical path is reduced by the changing of the location of the vertex. This change in the optical path naturally influences the accuracy of the distance measurement. It is known that the dependence on inclin
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Zeitschrift Fur Vermessungswesen, Jun. 1988, West Germany, Bd. 113, Nr. 6,ISSN 0340-4560, pp. 249-258.
Meier Dietrich
Schertenleib Werner
Bennett G. Bradley
Leica Geosystems AG
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