Journal of Surveying Engineering

In December 2001, Natural Resources Canada and the U.S. National Geodetic Survey (NGS) jointly adopted values for a set of 14 parameters for transforming positional coordinates and velocities between the International Terrestrial Reference Frame of 2000 (ITRF00) and the North American Datum of 1983 (NAD 83). Seven of these parameters characterize the variation with respect to time of the standard seven parameters (three shifts, three rotations, and a differential scale). In March 2002, the NGS updated the NAD 83 positional coordinates for all continuously operating reference stations to be consistent with their corresponding ITRF00 coordinates at an epoch date of 2002.00. Also, the NGS has incorporated these adopted values for the 14 transformation parameters into software packages, such as HTDP (horizontal time dependent positioning) for transforming positional coordinates across time and between reference frames, and OPUS (On-line Positioning User Service) for processing global positioning system data. Both HTDP and OPUS are available through the NGS Tool Kit (http://www.ngs.noaa.gov/TOOLS/).

Structural engineers may utilize geomatics techniques for precise and accurate measurements of discrete points, detailed façade recording, and the production of engineering drawings and plans. Techniques commonly used include direct measurement using a tape or gauge or, more recently, observations made from a reflectorless total station. Photogrammetric methods are also suitable for structural surveying. Terrestrial laser scanners have recently taken large steps in development and have the potential to become a useful survey tool. An overview of current recording techniques along with an introduction to laser scanning is given, followed by details of a test involving terrestrial survey, photogrammetry, and laser scanning at a site in the United Kingdom. Analysis of the results showed that measurement to targeted points using the laser scanner was comparable to measurement using traditional stereo photogrammetric methods, although care needs to be taken to reduce the impact of mixed pixels and multipath occurring within the scanned scene.

During the design phase of a bridge, certain characteristics are assumed, such as traffic and wind loading. In reality, however, these characteristics can vary, therefore it is very important that the health of such a structure is monitored to evaluate whether it is holding up to the true loading. Due to the ever-increasing size and cost of these structures, it is vital that any anomalies from the expected deflections are detected as soon as possible, allowing remedial action to be taken, and hence trying to prevent disastrous consequences. The use of a kinematic global positioning system (GPS) now allows subcentimeter accuracy to be achieved at a rate of up to 20 Hz, and at maximum distances from the reference GPS receivers to the bridge receivers of up to 20 km. This makes such a system capable of detecting the deflections of long bridges. In addition, accelerometers have been proven useful for such monitoring, allowing precise readings at rates of up to 1,000 Hz. However, both systems have their limitations. A GPS is limited partly by multipath and cycle slips, relatively low frequency of data, as well as the need to have good satellite coverage, while accelerometers are limited due to the fact that the derivations (velocities, displacements) from the original uncompensated acceleration readings will drift over time. The integration of the two systems, however, results in a hybrid arrangement that helps to eliminate the disadvantages of the two separate units. This paper presents an integrated monitoring system, consisting of Leica CRS1000 series and SR530 dual frequency code/carrier phase GPS receivers and a Kistler triaxial accelerometer. One of the SR530 GPS receivers and the accelerometer are physically integrated, and their data are synchronized. Using spectrum analysis, main natural frequencies of a monitored suspension footbridge are identified from the hybrid system. A simple data processing algorithm is presented, as well as the results from field trials to show the potential applications of such a system.

Fitting plane curves to the three-dimensional (3D) points defining those curves can be performed by ordinary general least squares adjustment. To this, a conformal transformation of the initial 3D system to a two-dimensional system in the best-estimated plane of the curve is used. The kind and the number of the unknown transformation parameters are selected according to one’s needs. In this adjustment, the coordinates of the defining points are considered observed parameters while the parameters of the aforementioned transformation and the ones defining the curve unobserved parameters. Finally, the curve—with respect to the initial system—is fully determined. Coordinate determination, together with least squares fitting of certain curves to these coordinates, is usually performed by advanced theodolite intersection systems. The straight line is a special case of curve lying in infinite number of planes, therefore, the suitable selection of the plane in which the adjustment is held is indicated, each time, by current convenience. Fitting a straight line to 3D points, a process hardly noticed in least squares textbooks, is extremely useful in high-precision deformation check and a lot of industrial survey applications.

The two-prism surveying probe reaches points on which it is impossible to preserve the verticality of a prism pole or the line of sight between the total station and this prism. Developed at the Centre for Research in Geomatics at Laval University, the probe consists of two miniprisms located along a surveying pole. The two prisms and the probe tip being collinear, three-dimensional coordinates of the point in physical contact with the probe tip can be obtained by taking measurements on each prism and by knowing the distance between these two prisms and the distance between one of these prisms and the probe tip. Moreover, constraints on the distance between the two prisms and on the zenith angle of the probe can be included into the mathematical model. Field tests presented in this paper show that the surveying probe positioning can achieve an accuracy of 0.7 cm (root sum square value) for the three components of the surveyed points.

Theories and computer programs are developed for single- and multiepoch analyses of a global positioning system (GPS) baseline network using the datum-free and weighted constraint solutions. Single-epoch analysis is for evaluating data quality, removing outliers, examining network precision, and estimating coordinates. Multiepoch analysis is for simultaneously determining epoch coordinates and station velocities. The datum-free solution yields estimable data residuals and relative coordinates and is suitable for adjusting data only and for detecting changes of relative coordinates. The weighted constraint solution allows a lower-order network to be attached to an existing, higher-order network. The analysis methods are applied to a multiepoch GPS baseline network in central Taiwan, which was hit by the 1999 Chi-Chi earthquake measuring 7.3 on the Richter scale. The velocities computed by the methods developed in this study are consistent with those from the Bernese GPS software. The result of the multiepoch analysis indicates that, in the first year after the Chi-Chi earthquake, the crust east of the Chelungpu Fault moved northwestwards at a mean rate of about 10 cm/year, while the crust west of the Chelungpu Fault moved in the opposite direction at a much lower speed.

This paper deals with the issue of incorporating pseudolite measurements into an integrated Global Positioning System/Inertial Navigation System (GPS/INS) positioning and attitude system with a view to improving signal availability, solution reliability, and accuracy in a localized area. Existing GPS/INS systems can overcome inherent shortcomings of each of the navigation technologies (line-of-sight signal requirement for GPS and INS errors that grow with time); therefore, such systems are now used for a wide variety of land, sea, and airborne applications where accurate positioning and/or attitude information is required with high output rate. However, their performance can still be degraded under certain conditions, such as when the duration of satellite signal blockage exceeds a certain time period (related to the quality of the INS), resulting in large accumulated INS errors. Such a scenario is a common occurrence for many kinematic applications. In an integrated GPS/Pseudolite/INS scheme, in order to gain the maximum benefit from additional pseudolite measurements, it is necessary to investigate how pseudolites can best be deployed to complement an existing GPS/INS system. A series of simulations, as well as field experiments with a GPS/Pseudolite/INS system comprising a NovAtel Millennium GPS receiver, an IntegriNautics IN200 pseudolite, and a MIGITS strapdown INS, were carried out, and the impact on performance of integrating pseudolite(s) has been assessed for a variety of operational conditions and different system configurations. The results indicate that the overall performance of the system can indeed be significantly improved using additional pseudolite measurements.

When global positioning system (GPS) signals propagate through the ionosphere, they experience an extra time delay. The ionospheric delay can be estimated using $\mathrm{L1}/\mathrm{L2}$ GPS measurement. In order to mitigate the pseudorange multipath effect on real-time ionospheric delay estimation, the multiday multipath template technique has been developed. The algorithm permits the pseudorange multipath error for a specific day to be corrected using the generated multipath template. Data from National Chengchi University, in Taiwan was used to test the application of the multipath template to real-time ionospheric delay estimation. Test results indicate that: (1) The performance of the various 5 day multipath templates is better than that of other multiday multipath templates and (2) the multipath template technique is an effective method to mitigate the GPS pseudorange multipath and should be implemented at reference stations within local-area differential GPS, wide-area differential GPS, and wide-area augmentation system networks, GPS deformation monitoring networks, etc.