This paper describes a new methodology verifying the accuracy of photogrammetric networks by known distances between imaged points on scale bars. The analysis presents three issues of photogrammetric accuracy (point, distance, and scale accuracy) together with the issue of scale-bar length accuracy. The latter, in turn, involves the uncertainty in reference length, measured temperature, coefficient of thermal expansion, and centering of targets. The methodology is based on a one-time determination of photogrammetric point accuracy using statistics generated by discrepancies between points in a photogrammetric network and their counterparts determined very reliably by other means. It focuses on geometrically strong networks, where the coordinate standard error is approximately the same for all three coordinates of a typical point. This methodology replaces much lengthier and more expensive numerical procedures, such as a Monte-Carlo technique, by a straightforward statistical analysis. Under stipulated conditions, it can assist users of digital photogrammetry in verifying the performance of their system(s) after each survey.
Uncompensated thermomechanical errors in laser tracking interferometers are examined by evaluating the difference between tracking interferometer compensations in a controlled laboratory environment versus being compensated in a factory environment. The hypothesis under test was that compensation in a factory environment does not adversely affect, and may actually improve, the uncertainty of laser tracker systems. This hypothesis was confirmed by measuring a standard (i.e., linear interferometer) using laboratory- compensated and certified instruments, and then compensating the instrument in the factory environment and re-measuring the standard. The results showed that in-shop compensation generates less variation in the measurement of the standard when compared to the laboratory-compensated and certified instruments. Certified weather stations are used to compensate for the uncontrolled atmospheric effects on the range measurement.
This report documents system uncertainty, best practices, and process limitations for the real-time videogrammetry system developed by Metronor, ASA. The report describes the system and the tests used to establish its performance. It then documents the test result and draws conclusions. An uncertainty of 23.6 ppm within the 3.2 by 2.8 by 1.5-meter test volume was achieved with the baseline between the cameras parallel to the length of the target field. A slightly lower uncertainty of 18.5 ppm was achieved when the cameras were setup with their baseline normal to the length of the target field.Results suggest that the orientation of the camera baseline relative to the target field does not play a significant role when targets are more than 2 meters apart. Testing shows a significant dependence in system accuracy relative to the apex angle between the cameras and the targets. When the apex angle was between 60 degrees and 95 degrees the performance specification was met. Conclusions drawn from a series of two-sample F-tests of RSS error values establish that sampling a point more than three times does not significantly reduce its uncertainty. This result is in part explained by the redundant fitting of the five-diode points on the light pen: the fitting effects an averaging process even on samples of one.
Testing has shown significant dependence between the length of the scale artifact and the achievable system uncertainty for high accuracy industrial videogrammetry on high aspect ratio objects. Shop practice traditionally allows scale artifacts under 1/5 the object length. This practice can lead to higher than expected uncertainties because the uncertainty of the metric defining the physical scale must be multiplied by the ratio of object length to scale artifact length. This relationship is incorporated into the U95 uncertainty relationship. Test cases validating the uncertainty model are also presented. A network of scale reference points can also be integrated on an object using a laser tracking interferometer. Testing results show a significant reduction in total uncertainties when using this network to define the scale for videogrammetry applications. Measuring a 500-inch (12.7 meters) object and scaling the survey with a 130-inch (3.3 meters), scale bar produced an uncertainty of 22-ppm. When the 500-inch object survey was scaled with laser tracker data, the system yielded an uncertainty of 9-ppm.