Jerzy Janusz, Wojciech Janusz, Mieczysław Kołodziejczyk - Monographic Series No 7
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Length calibration system for bar code staffs prepared at the Institute of Geodesy and Cartography in Warsaw
Jerzy Janusz, Wojciech Janusz, Mieczysław Kołodziejczyk
Length calibration system for bar code staffs prepared at the Institute of Geodesy and Cartography in Warsaw
Verification of graduation of bar-code staffs cannot be done using technologies for calibrating traditional staffs which have regular graduation in metric system. This results from the method of reading of the code graduation and its conversion to digital output, which is done by a digital leveling instrument itself. Functional relation between the digital leveling instrument and the bar code staff is so complex that it becomes necessary to verify not only staff graduation, but also to check simultaneously the correct functioning of inner systems for staff reading and conversion of these readings to metric system.
Limiting staff length verification to checking the spacing of bar graduation through comparison with a coding algorithm must be recognized as incomplete, because it does not take into consideration possible systematic errors of "automatic" readings i.e. systematic errors of the leveling instrument's replica code and errors of opto-electronic image processing.
In 2002 scientists from the Finnish Institute of Geodesy constructed a comparator for simultaneous verification of graduation of bar code staffs, as well as for checking automated systems of reading. According to [14] this was the third comparator in Europe of that type (following Graz and Munich comparators). In this system the constructors applied the principle of measurements by laser interferometer of vertical shifts of a moving bar code staff, while simultaneously readings were performed using a fixed digital
leveling instrument. Such a comparator can be installed in the laboratory with minimal height of 7 meters due to:
- length of staff
- indispensable range of its movement
- space under staff for installation of interferometer.
In this publication the construction of bar code staff comparator designed and built in March 2003 at the Institute of Geodesy and Cartography (IGiK) in Warsaw is described. A different set-up of surveying and supporting furniture was developed to that described in [14] - readings are performed on a fixed bar code staff by a moving digital leveling instrument whose vertical shifts are measured simultaneously by an interferometer.
Specific capabilities of IGiK comparator are as follows:
- to make calibration in the laboratory with height of 3.05 m applying inverse readings or in a room with height of 4 m without inverse reading,
- to determine values of random and systematic errors introduced by a particular set of bar code staff and automated reading system (leveling instrument) to measurements of height differences (including verification of leveling instrument compensator efficiency).
Preliminary studies concerning use of comparator installed at IGiK are described. Results of calibration thus obtained reveal that it is possible to determine deviation of the mean value of the effective meter length carried by a set of NA3003 leveling instrument and Leica GPCL staffs from normal meter with mean error of ±3 μm/m.
The authors anticipate to conduct wider studies, concerning both practical features of the designed comparator and statistical studies on quality of sets of digital leveling instruments and bar code staffs offered by various companies.
Jerzy Janusz, Wojciech Janusz, Mieczysław Kołodziejczyk
Field Comparator of the Length of Precise Levelling Staffs
Analyses of long-term measurements of vertical displacements of some high dams revealed appearance of great systematic errors. Deviations of mean meter and changes of length of leveling staffs are among the main reasons for these errors. Authors came to a conclusion that apart from calibrating staffs at laboratories it is also necessary to carry out frequent scale checks in the field. For that reason a field comparator for checking staffs with traditional, regular graduation has been designed.
Comparator design objectives were as follows:
- to ensure sufficient accuracy of calibration, similar to that obtained for calibration performed at the laboratories,
- enable calibration to be made independently by a basically trained resident surveyor,
- relatively cheap construction.
The new field comparator exploits the idea of staff length checking by comparison with a staff serving as working standard, calibrated periodically at the laboratory with the use of laser interferometer.
Field comparators can be installed close to high hydro technical constructions, enabling on-site check of staffs. This is especially useful when a person conducting measurements has doubts after possible hit or fall of staff if the scale of staff was not damaged, which in consequence can render it inadequate for further usage. Calibration of the staff which serves as working standard and which is kept with the field comparator with proper caution, can be done in larger time intervals with the use of laser interferometer, at frequency set according to changes of its length determined in the course of previous calibrations.
Field comparator can be constructed as horizontal or vertical instrument, adjusted to calibrate staffs in their working positions.
Differences (K-W) of position of corresponding marks on the graduation i.e. on the checked staff (K) and on the working standard (W) are determined with the use of comparator. On the basis of differences (K-W) mean errors m of graduation, value of systematic deviations of mean meter A, and error of systematic deviation mA are calculated.
This publication comprises a description of the prototype of field comparator which was installed and checked in detail at the Institute of Geodesy and Cartography. Using this comparator a dozen or so staffs were verified; mean errors of graduation m of ±15 μm and errors of systematic deviations of graduation mA of ±3 μm/m were obtained.
Institute of Geodesy and Cartography can undertake works concerning construction and installation of field comparators and can make trainings for persons making leveling measurements, to enable them to use the apparatus independently to check staffs in the field. However, it must be emphasized that frequent verification of staffs using the field comparator cannot free them from periodic calibration of those staffs at the specialized laboratories.