Journal of Surveying Engineering

The main problem in conventional deformation analysis (CDA) is that if there is more than one displaced point in a given monitoring network, the least-squares estimation (LSE) smears the disturbing effects of the displaced points over all other points. Therefore, only one displaced point can be detected successfully using CDA. If the number of displaced points increases, the success of detecting these points decreases significantly. This weakness of CDA originates mostly from the configuration of the monitoring network. To detect the weakness of any monitoring network, a new approach that is based on applying the median of differences between the measurements of Epoch I and Epoch II related to the same point has been developed. In this way, the displaced points that are detected by CDA can be checked whether these points are actually displaced from the point of view of the configuration weakness of the network. To design a capable deformation network that has at least a minimum configuration isolating smearing effect of LSE makes it possible to detect displaced points with highest success.

Strain tensor analysis plays an essential role in geophysics and engineering applications, because it provides a numerical measure on the relative deformation behavior of the object under investigation. A strain tensor can be estimated by observing the positional coordinates of the deforming network at different epochs. Consequently, the quality of a strain analysis is highly dependent on the accuracy of position measurement and network configuration. In this study, a strain sensitivity index, namely the minimum detectable principal strain parameter, has been developed based on the concept of the invariant function and statistical theory. A simulation test and a real case study have both been carried out to demonstrate the feasibility of the proposed approach. Based on the numerical results from the case study on the first-order global positioning system network in Taiwan, it has been revealed that the coastal areas in Taiwan have a lower capability in detecting surface strains because of poor network configuration. On the other hand, it has also been illustrated that, by incorporating the network optimization guidelines developed in this study, the strain sensitivity index can be substantially improved in these areas.

Many modern terrestrial laser scanners (TLSs) are equipped with inclination/tilt sensors—also referred to as level compensators—that can correct out-of-level imperfections in an instrument setup. Some users elect to disable the inclination sensor when their scanner is equipped with one. Those who use an inclination sensor may override its leveling data by linking to external control that defines the level plane. Our case study shows that inclination sensor data can be a valuable quality check for the control data and overall point cloud alignment. Quality inclination sensor data requires careful scan planning. Specifically, laboratory tests indicate that scanner rotation speed influences the reliability of inclination sensor readings recorded during rotation. These inertial effects can be canceled to a limited extent by measuring during both clockwise and counterclockwise rotations. A scan duration within a “stability threshold” can also minimize the inertial effects and variability in inclination sensor readings for scanners continuously measuring inclination data during rotation. Finally, inclination sensor readings from a full 360° rotation are recommended to reduce systematic bias.

Due to gravitation, the main reflector of a radio telescope underlies a deformation that causes a change in focal length depending on the variations of the elevation angle of the telescope. To estimate these gravity dependent deformations of the main reflector of the 100-m radio telescope at Effelsberg, Germany, this study proposes a measurement concept based on a laser scanner being mounted upside down on the subreflector. The measurements that have been performed at seven different elevations between 90 and 7.5° are used to estimate the focal length variation of the main reflector parameterized by a rotational paraboloid. To guarantee reliability of the adjustment, this study uses an orthogonal distance regression (ODR) rather than a classical least squares adjustment in a Gauss-Helmert model and formulates the independence of the focal length estimation from the absolute position and orientation of the main reflector in space as a requirement for a reliable adjustment approach. This investigation attests that the ODR has superior reliability with regard to this criterion. A three-step adjustment procedure based on an alteration of the ODR and several outlier eliminations is used to determine the variations of the focal length due to gravitation. The estimated focal length decreases by a maximum of 12.6 mm when tilting the reflector from 90 to 7.5° elevation angle. The postfit discrepancies between the best-fit paraboloid and the reflector’s surface are Gaussian distributed within the accuracy of the measurements. This face supports the assumption of a homologous deformation of the main reflector.

The number of global positioning system (GPS) tracking stations is increasing, primarily because the stations are multifunctional. In civil engineering, they can be used for precision positioning; in the earth sciences, they can be used to monitor faults and earthquakes; and in the atmospheric sciences, they can be applied to predict perceptible water vapor. Currently, there are more than 400 GPS stations in Taiwan; however, the data obtained through such stations are not being assessed carefully. Experienced scientists and engineers examine the data in advance to see if they qualify for research purposes, but inexperienced users can adopt poor quality data that eventually lead to inaccurate research results. Of the observation stations with receivers that were renewed between 2006 and 2008 in the International GNSS Service Network, four stations (ZIMM, BOR1, NRC1, and NICO) were selected to be the subjects of this research. Six indexes of data quality were observed to calculate the quality of data obtained before and after receiver renewal. Then, analyses were conducted to understand the relationship between the quality indexes and positioning precision. The results showed that after receiver renewal, the positioning precision of the four stations was improved by 1–19%. Therefore, positioning precision is positively affected by the six data quality indexes. It was also discovered that receiver clock error was the most critical factor among the six indexes. In conclusion, if data quality control can be applied to GPS tracking stations, the data obtained will be more reliable for research purposes, and the accuracy of subsequent engineering and science measurements will be improved.

This study demonstrates the usefulness of an approach based on the Online Positioning User Service (OPUS) provided by the National Geodetic Survey (NGS) of the National Oceanic and Atmospheric Administration (NOAA) to process Global Positioning System (GPS) data and conduct long-term landslide monitoring in the Puerto Rico and Virgin Islands region. Continuous GPS data collected at a creeping landslide site during 2 years were used to evaluate different scenarios for landslide surveying: continuous or campaign, long duration or short duration, morning or afternoon (during different weather conditions). OPUS uses the Continuously Operating Reference Station (CORS) network managed by the NGS as control points and user-collected data to solve for the position of the occupied station (rover). In July 2011, there were 19 NGS CORS sites in the Puerto Rico and Virgin Islands region. This dense GPS network provided a precise and reliable reference frame for subcentimeter-accuracy landslide monitoring in this region. OPUS static solutions (OPUS-S) for sessions as short as 4 h, and OPUS rapid static solutions (OPUS-RS) for sessions as short as 15 min, can achieve subcentimeter horizontal accuracy if the collection of data during extreme weather conditions is avoided. The uncertainty (peak-to-peak error) reported by a single OPUS-S solution differs from the “true” accuracy by a factor of 1.7 for the horizontal components and 1.3 for the vertical component. The uncertainty reported by a single OPUS-RS solution differs from the accuracy by a factor of 1.4 for horizontal components, while the uncertainty of vertical component statistically agrees with the vertical accuracy. This study also indicates that rainfall events can seriously degrade the performance of high-accuracy GPS. Field GPS landslide surveying should avoid rainfall episodes when accompanied by thunderstorms and the passage of detrimental weather fronts. Once appropriate precautions are taken, the results of this investigation show that OPUS-S and OPUS-RS are ideal alternative tools for subcentimeter-accuracy landslide monitoring.

The subtle curves of the masterpiece of ancient Greece, the Athenian Parthenon, have been the object of many investigations of scientists of various disciplines. A lot of discussion has centered on the shape of the monument’s curved stylobate surface and the kind of mathematical curve that best describes it. This paper presents a new approximation of the stylobate’s curves using the Parthenon’s transverse and longitudinal inclinations estimated from the measured uneven heights of the lowermost drums of its peristasis columns. This study uses least squares techniques and their appropriateness checked through their correlation coefficients to estimate two kinds of conic sections: a circle and a parabolic curve. Finally, the minimum detectable constructional inaccuracy is estimated.