Journal of Surveying Engineering

In the study of map projections, it is relatively simple to obtain meaningful estimators of distortion for a small area. The definition and especially the evaluation of global distortion measures (i.e., estimators representing the distortion worldwide or in a continent-like area) are undoubtedly more troublesome. Therefore, it is relatively common to find that recommendations for the parameters to use in a particular map projection, be it devised for a continent or a country, are based on simple rules (like the one-sixth rule of thumb for conic projections), with no possibility of further improvement in terms of resulting distortions and sometimes even with no knowledge at all of the sizes of these distortions. Although the choice of map defining parameters is normally made for reasons other than distortion minimization, such as ease of use (e.g., integer or half-integer numbers may be preferable), preservation of conventional or traditional definitions, and uniformity of parameters between neighboring regions, it is always worthwhile to know the optimal set of parameters in terms of minimal distortion. Then, the cartographer may mindfully deviate from this optimal set, documenting the differences in defining parameters and in the resulting distortions. The present research provides a means to do this by extending a related work presented in a previous contribution, where the evaluation and optimization of distortions were studied for a single map projection and only two areas of interest. To this end, a new tool has been developed and presented in this paper. This tool allows users to evaluate several measures of distortion for the most common conformal and equal-area projections within user-defined geographic boundaries of interest. Also embedded in the tool and transparent to users are global optimization techniques operating on Fibonacci grids, which permit the optimization of parameters for the particular map projection and area of interest under two possible criteria: minimization of typical distortion or minimization of extreme distortions. This tool and the associated techniques are applied to several official projections to analyze their original performance and to propose new parameters that significantly improve the resulting distortions while leaving room for users to easily evaluate and optimize the tool for the lowest distortions of these projections within their regions of interest.

This paper deals with the detection of small movements of a global navigation satellite system (GNSS) station by means of high-precision estimates of the instantaneous station velocity. The aim is to detect hazardous displacements in real time in order to quickly provide movement information to facilitate a GNSS monitoring system. The station velocities are deduced from time derivatives of GNSS carrier phase measurements. On the basis of epochwise estimates of station velocity, potential movements can be detected by using a statistical hypothesis test. The results from an experimental data set show that the obtainable accuracies of the estimated velocity components can reach a level of a few tenths of a mm/s (1 sigma) and also that it is possible to detect movements on the sub-mm/s level with a high statistical significance. In order to enhance the reliability of the movement detection and to support decision making, two decision criteria extending over several epochs are proposed. The algorithm works in a standalone mode, which means that no data from a reference station or any other external link is needed.

The variants of M_split estimation proposed so far are based on the objective function related to the least-squares method and hence they are known as the squared M_split estimation. We propose to design a new variant whose objective function is based on the L₁ norm condition. The main objective is to investigate the properties of the new solution, especially in the context of robustness. Thus, the respective influence functions and the weight functions are also derived. The influence functions provide us with information about general properties of the method, whereas the weight functions are the basis for the modified iterative process of M_split estimation proposed in this paper. Theoretical considerations are supplemented with empirical analyses that concern M_split estimation of the competitive parameters of the univariate functional model or the linear regression, and finally the multivariate functional model (the latter case is related to the deformation analysis). The results show that the new variant is generally less sensitive to outliers or wrong assignment of a particular observation to its functional model than the squared M_split estimates. The solution proposed in this paper is also more stable, that is, it is independent of the chosen starting point of the iterative process or local fluctuations of random observation errors. That seems to be an interesting practical feature. Both variants have similar accuracy; however, the accuracy of the new variant is higher in many presented cases.

Georeferencing of aerial images from small unmanned aerial systems (sUASs) is often achieved using (1) an abundant number of ground control points (GCPs), or (2) global navigation satellite system (GNSS) postprocessed kinematic (PPK) georeferencing, which utilizes accurate positioning for captured images and waives the requirement of having GCPs. In addition, GCPs allow for camera self-calibration when accurate camera calibration is not available in advance. This study assesses the impact of GCP number (and their separation distance) on elevation accuracy of sUAS surveys and examines their impact on georeferencing and camera self-calibration. In addition, this study assesses how GNSS-kinematic positioning can enhance georeferencing and camera self-calibration, and reduce the number of required GCPs. sUAS data were collected at the Pennsylvania State University Wilkes-Barre campus and were compared with reference elevation information derived from terrestrial laser scanning and checkpoints collected using total station observations. Two flights with GNSS-kinematic capability at different altitudes were used, namely, flying altitudes of 90 and 50 m (3.8- and 2.2-cm average point spacing, respectively). Scenarios with varying number of GCPs were tested to identify the cases that yield poor elevation accuracy. Based on the scenarios and data of this study, in the GCP-only case, at least 12 GCPs (65-m distance separation) are needed to achieve reliable georeferencing and camera self-calibration for this study area and data. This leads to an elevation accuracy at the centimeter level (1–2 cm). However, in the GNSS-assisted case, the required number of GCPs drops to six (105-m separation distance) in order to achieve the same accuracy level as the GCP-only case. Results and conclusions of this study can aid practitioners in sUAS survey planning.

The registration and calibration of data captured with terrestrial laser scanner (TLS) instruments can be effectively achieved using signalized targets comprising components of both high and low reflectivity, so-called contrast targets. For projects requiring tens or even hundreds of such targets, the cost of manufacturer-constructed targets can be prohibitive. Moreover, the details of proprietary target center coordinate measurement algorithms are often not available to users. This paper reports on the design of a low-cost contrast target using readily available materials and an accompanying center measurement algorithm. Their compatibility with real terrestrial laser scanner data was extensively tested on six different instruments: two FARO Focus three-dimensional (3D) scanners, a Leica HDS6100, a Leica P40, a RIEGL VZ-400, and a Zoller+Fröhlich Imager 5010. Repeatability was examined as a function of range, incidence angle, sampling resolution, and target contrast. Performance in system self-calibration and from independent accuracy assessment is also reported. The results demonstrate compatibility for all five scanners. However, all data sets except the FARO Focus 3D require exclusion of observations made at high incidence angles in order to prevent range biases. Results also demonstrate that the spectral reflectivity of the target components is critical to ensure high contrast between target components, and therefore high-quality target center coordinate measurements.

This paper points to the limited possibilities of improving the existing situation of transition curves used in geometric layouts of the railway track. Difficulties in the practical implementation and maintenance of very small horizontal ordinates of the transition curve and the ordinates of the gradient due to cant in the initial section, appearing on smooth transition curves, were indicated. The main reason for this was the excessive smoothing of the curvature in their initial section. Employing the method of curvature identification by differential equations, a new form of the curve was obtained, referred to as the smoothed transition curve. A definite advantage of this curve was shown, from the implementation point of view, over representing the smooth transition curves of the Bloss curve and the parametric curve. It seems that it could successfully compete with the commonly used clothoid, to which it is similar in the initial section, while it differs significantly along its further length, especially in the final section, where it provides a gentle entry from the transition curve into a circular arc.

The National Geodetic Survey has developed four versions of the online positioning user service (OPUS) for postprocessing dual-frequency, static global positioning system (GPS) data. Each version was designed for a different application and outputs coordinates at a mark to varying accuracy. To compare the accuracy of these four versions, 88 10-h-duration GPS data files collected on 18 marks in Oregon were processed in OPUS-Projects, producing horizontal coordinates and ellipsoid heights at these marks with an estimated network accuracy of less than 0.5 and 1.3 cm at 95% confidence, respectively. Then, these data files were windowed into sessions ranging from 20 min to 10 h in duration. The windowed files were processed in OPUS-static (OPUS-S), OPUS-Net, and OPUS-rapid static (OPUS-RS), and the resulting coordinates at each mark were compared with the coordinates from OPUS-Projects. At a session duration of 2 h, OPUS-RS was found to be more accurate than both OPUS-S and OPUS-Net. For sessions less than 2 h, OPUS-RS frequently produced poor solutions with error messages. At a session duration of 4 h, OPUS-S and OPUS-Net produced coordinates with both horizontal and ellipsoid height errors less than 3 cm at 95% confidence.