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#### Journal of Surveying Engineering

Archives Papers: 112
The American Society of Civil Engineers
Error Models and Propagation in Directly Georeferenced Terrestrial Laser Scanner Networks
Abstracts:Cultural heritage recording and engineering surveying are prime applications for terrestrial laser scanners (TLSs) because of the high spatial resolution, high accuracy, and fast data capture rates this technology offers. To date, insufficient attention has been given to the many error sources contributing to the uncertainty of scanner datasets. A full error budget for directly georeferenced terrestrial laser scanner networks that considers both relevant error sources fundamental to surveying and those unique to sampled laser scanner systems is derived. In the case of the latter, new probabilistic models are proposed for angular positional uncertainty due to laser beamwidth and centroid-based target pointing. Analysis of a cultural heritage-recording project in Ayutthaya, Thailand, highlights the disparity between “expected” precision and the more realistic precision indicated by the error budget and demonstrates that the beamwidth error can be a significant factor. The causes and effects of several systematic errors inherent to TLS datasets are also discussed.
Weighted Total Least Squares with Singular Covariance Matrices Subject to Weighted and Hard Constraints
Abstracts:Weighted total least squares (WTLS) has been widely used as a standard method to optimally adjust an errors-in-variables (EIV) model containing random errors both in the observation vector and in the coefficient matrix. An earlier work provided a simple and flexible formulation for WTLS based on the standard least-squares (SLS) theory. The formulation allows one to directly apply the available SLS theory to the EIV models. Among such applications, this contribution formulates the WTLS problem subject to weighted or hard linear(ized) equality constraints on unknown parameters. The constraints are to be properly incorporated into the system of equations in an EIV model of which a general structure for the (singular) covariance matrix $QA$ of the coefficient matrix is used. The formulation can easily take into consideration any number of weighted linear and nonlinear constraints. Hard constraints turn out to be a special case of the general formulation of the weighted constraints. Because the formulation is based on the SLS theory, the method automatically approximates the covariance matrix of the estimates from which the precision of the constrained estimates can be obtained. Three numerical examples with different scenarios are used to demonstrate the efficacy of the proposed algorithm for geodetic applications.
Hybrid Survey Networks: Combining Real-Time and Static GNSS Observations for Optimizing Height Modernization
Abstracts:To derive ellipsoid heights on passive marks with centimeter-level accuracy, many current specifications require the collection and adjustment of long-duration, static, postprocessed global navigation satellite system (GNSS) sessions. To increase efficiency, a campaign-style survey procedure that includes real-time kinematic (RTK) vectors from a real-time GNSS network was evaluated. Thirty different hybrid networks involving three to nine network RTK (NRTK) vectors per mark and some static GNSS vectors were developed from surveys completed in Oregon and South Carolina. The variance-covariance matrices of the static and kinematic vectors were scaled by variance-component estimation procedures to produce realistic error estimates for stochastic modeling. After least-squares adjustment and formal random-error propagation of the networks, the resulting ellipsoid heights on the passive marks had network accuracies ranging from 0.6 to 3.6 cm (95% confidence). These network accuracies reduced to < 2 cm when using six or more NRTK observations per mark. Further, the use of NRTK vectors obtained from observables of both the U.S. global positioning system (GPS) and Russia’s GNSS (GLONASS) were, on average, 19.2% more accurate vertically than vectors obtained solely from GPS observables.
Arctic High-Resolution Elevation Models: Accuracy in Sloped and Vegetated Terrain
Abstracts:New high-resolution elevation models for Alaska have recently been released; they were created using interferometric synthetic aperture radar (IFSAR) and automated matching of high-resolution optical satellite stereo imagery (OSSI). These products promise to fill a void in available digital elevation models (DEMs) for the Arctic. However, the effective use of these models requires knowledge of their expected accuracy, and to date, a detailed analysis of these models in remote Arctic locations has not been undertaken. Expected accuracy is necessary to gauge the uncertainty of any scientific conclusions based upon analysis of these DEM sources. To that end, both aforementioned DEM techniques were compared to airborne LiDAR (light detection and ranging) in the area surrounding Sitka, Alaska. It was found that both the IFSAR and OSSI DEMs provide vertical accuracy at the 2–4-m level (1 σ) in flat and open terrain but perform significantly worse in areas of vegetation cover with standard deviations increasing to ∼7–12 m. The DEM errors were found to have a strong positive correlation with vegetation height, and the overall error pattern suggests that neither OSSI nor IFSAR accurately model either the ground or top of the tree canopy, instead representing a surface between the canopy and topographic elevation.
Active versus Passive Rotations
Abstracts:Active and passive rotations are used in many applied scientific fields, such as engineering, geodesy, and geophysics, just to name a few. However, a source of confusion could arise when both types of rotations are performed sequentially. The author is not aware of any publication where, in a tutorial manner, the relationship between active and passive rotations is coherently described, untangling in the process some of the most typical misconceptions. This technical note is a modest attempt to remedy this void by clarifying, as much as possible, some essential points that may help the understanding of these two varieties of commonly used rotations and how to properly apply them in some practical situations.
Millimeter-Accuracy Structural Deformation Monitoring Using Stand-Alone GPS: Case Study in Beijing, China
Abstracts:Structural monitoring and engineering surveys that use the Global Positioning System (GPS) have traditionally performed using a so-called carrier-phase double-difference method in data processing. The method requires at least one reference GPS station to be continuously operated in the field during the monitoring period. The stability and data quality of the reference station would directly affect the accuracy of displacement measurements at the rover station. Furthermore, it becomes increasingly difficult to maintain “ideal” reference stations in urban environments. This article introduces a method for using stand-alone GPS units to conduct millimeter-accuracy structural deformation monitoring for poststudy or for near real-time structural health monitoring. Two years of continuous GPS observations obtained from two high-rise buildings in Beijing were used to depict the detailed method. The method comprises three steps: (1) solving GPS antenna positions with respect to a global reference frame (IGS08), (2) establishing a stable local reference frame and transforming the global positions into the local reference frame, and (3) deriving a regional seasonal model and correcting GPS-derived positional time series with the regional model. It is concluded that 2- to 3-mm horizontal accuracy and 5- to 7-mm vertical accuracy can be achieved for daily solutions with the proposed method in the Beijing metropolitan area. The key products from this study are the Stable Beijing Reference Frame (SBJRF) and the local seasonal model for GPS-derived (vertical) positional time series. SBJRF will be incrementally improved and periodically updated to synchronize with the update of the IGS reference frame. The general theory and method presented in this article could be applied to other urban areas for conducting structural health monitoring in near real time.
Terrestrial Laser Scanning of Extended Cliff Sections in Dynamic Environments: Parameter Analysis
Abstracts:Substantial erosion of the unstable seacliffs along the economically important coastline of San Diego County, California, threatens existing developments and public safety. Time-series mapping of the seacliffs and beaches provides valuable information about seasonal and rapid-event erosion. With high resolution terrestrial laser scanning (TLS) georeferenced using real-time-kinematic global positioning systems, it is possible to establish reliable comparisons of time-series surveys for quantitative change analysis of seacliff morphology. This paper introduces new field survey methods for georeferencing TLS surveys collected in dynamic environments where conventional control methods cannot be effectively implemented for large-scale mapping. Specifically, the quality control of scan alignment and the identification of optimal surveying parameters of point separation/density, distance from target, setup spacing, and efficiency are discussed for long cliff sections. These TLS surveys, performed several times along an approximately 17-km segment of seacliffs in San Diego County show an average root-mean-square uncertainty of 7.9 cm between adjacent scans approaching the nominal 7.2 cm accuracy of the survey equipment.
Impact Factor Conundrum
Effects of Displaced Reference Points on Deformation Analysis
Abstracts:The geodetic datum of a deformation monitoring network is defined using its reference points. This article discusses how deformation analysis is affected when reference points have been displaced as a result of rigid body displacement (translation and rotation) and deformation (extension or contraction) of the reference block; this article also includes points with single-point displacements. With this aim, the effects of the displacements of the reference points on the estimated displacements of the network points are considered. The well-known similarity (S)-transformation matrix associated with network datum is a convenient projector for explaining these effects. Some horizontal and vertical monitoring network examples are used to discuss the effects of the displaced reference points on deformation analysis.
Improving the Positional Accuracy of Digital Cadastral Maps through Optimal Geometric Transformation
Abstracts:This paper presents an attempt to reduce positional errors in digital cadastral maps produced by digitization (scanning, georeferencing, and vectorization) of existing Bosnian-Herzegovinian analog (paper-based) maps. The geometric content of the digital cadastral maps generated by the aforementioned procedure is encumbered by nonuniform and irregularly distributed positional distortions. Errors created during the original elaboration of the analog maps (based on hand drawings using the drafting instruments) cannot be corrected through the geometric transformation of the cadastral map with the use of common (tie) points on the map coordinate grid. This study focused on the elimination of errors in the original map drafts through better selection of common points, with the aim of obtaining a homogeneous spatial accuracy of cadastral maps. With the use of the flexible thin plate spline (TPS) transformation model, which includes information on systematic distortions caused by errors of the original mapping process, the authors made a significant step toward the enhancement of the positional accuracy of boundary points in the digital cadastral maps. The use of TPS transformation reduces heterogeneity of each cadastral map, and maps are efficiently georeferenced to the state plane coordinate grid. Digital cadastral maps generated by reconstruction of original state survey measurements are typically more accurate than those obtained by digitizing scanned maps, and these types of maps were not considered in this study.