Journal of Surveying Engineering

To develop a universal-outliers processing algorithm under the conditions with equality constraints, the equality-constrained nonlinear Gauss–Helmert (GH) model, which contains the equality-constrained Gauss–Markov (GM) and errors-in-variables (EIV) models as special cases, is selected as the research object in this paper. The least squares solution for the nonlinear GH model with equality constraints is obtained using the Euler–Lagrange approach, and then, it is equivalently formulated as the standard constrained least squares (CLS) problem. To construct the test statistics for the outliers detection, a distinctive sensitivity analysis approach is introduced into this CLS problem. The local sensitivity of the weighted sum of squared residuals to the perturbations of observations in the CLS problem is discussed, and then, the local test statistics are constructed based on these sensitivity indicators. To verify the performance of the sensitivity-based test statistics, the proposed data-snooping algorithm for the equality-constrained nonlinear GH model is applied to a three-dimensional (3D) symmetric similarity transformation. The computational results of the simulated and real examples manifest that the proposed data-snooping algorithm using the sensitivity-based test statistics can effectually decrease the negative impact of the outliers and derive reliable parameters. It should be pointed out that the new algorithm is applicable in various kinds of equality-constrained least squares and total least squares problems.

To further improve the quality of estimated values based on variance component estimation of the partial errors-in-variables (EIV) model, the jackknife resampling method is introduced in this paper. Focusing on the bias of variance component estimation and combining with the jackknife method, bias calculation and bias correction are performed. Two schemes for parameter estimation are identified, and detailed calculation steps and the whole procedure are given. The jackknife method for variance component estimation of the partial EIV model is evaluated. Meanwhile, these two new algorithms are applied to the straight-line fitting model, space-line fitting model, and plane coordinate transformation model. As shown in the experimental estimation results, both methods proposed can obtain more accurate estimated values than the traditional variance component estimation method, and the method with bias correction can obtain the optimal parameter estimates. The case studies demonstrate the effectiveness and reliability of the proposed procedure, which extends the theory of the jackknife method in parameter estimation and provides resampling insight to further investigate variance component estimation.

Structure-from-motion (SfM) is currently used for geological-geomorphological purposes under the condition that the modeling is based either on several ground control points (GCPs) well distributed in the scene or on direct georeferencing (DG). In emergency conditions and in presence of active morphodynamic processes, it could be unfeasible to use GCPs or DG. A study aimed at evaluating the quality of the results achievable by means of completely free SfM modeling of images taken from a distance of some hundred meters is shown in this paper. It is based on an experiment with an artificial target and some surveys of a bedrock scarp, where resolution and precision are evaluated as empirical functions of distance and focal length, taking into account the issues related to the scale factor. The problems related to the recognition of localized surface changes by means of multitemporal surveys are also studied. The primary result is that the free approach can really be used in geomorphological and seismotectonical surveying carried out in emergency conditions.

Instabilities in observed environments at sea, such as the sudden appearance of fog, varying temperature or pressure, changes in wind speed and direction, and differential gravitational and magnetic fields in each observed station, will lead to fluctuating observations and difficult data processing. These factors must be considered to achieve high-precision leveling. Therefore, a sea-crossing trigonometric leveling method that satisfies the national second-order leveling accuracy is proposed according to the sources and variable characteristics of errors under the condition of lacking corresponding specifications. The procedure is described and a mathematical model is deduced in this paper. A sea-crossing leveling engineering work is used to evaluate the performance of the proposed method. The result of engineering work indicates that the proposed method can lighten the surveyor workload and improve the precision of final values to some extent compared with common trigonometric leveling methods. The method can be used in applications in which two benchmarks are far and mounting a level in the middle place is impossible, such as leveling across rivers, canyons, seas, or dense woods.

Traditionally, GPS real-time kinematic (RTK) technique can provide precise positioning for short baselines, and network-based RTK (NRTK) is much more effective than the single-baseline RTK (SRTK) stemming from its capability of precise positioning over the whole area covered by reference stations. However, NRTK needs to build and maintain the infrastructure of reference stations. With multi–global navigation satellite system (GNSS) multifrequency signals becoming available, one can extend the service radius of SRTK to a few tens to even a hundred kilometers. This paper investigates whether one can realize the precise positioning with SRTK instead of NRTK in a medium-sized city located in the midlatitude region, like Shanghai. Different positioning models of SRTK are introduced and compared to those of NRTK. The variety of experiments were designed with triple-frequency BeiDou data and dual-frequency GPS data. The results show that the positioning results of SRTK and NRTK are comparable, with both having centimeter-level accuracy except for the relatively longer convergence time of ambiguity fixing for SRTK when the baseline is longer than 50 km. It is therefore expected that long-range SRTK is able to provide positioning service comparable to NRTK in most cities like Shanghai, of medium size and at midlatitude. Such a research finding is promising for reducing the cost of establishing and maintaining infrastructure used in NRTK.

The design of surveying networks inside tunnels is of crucial importance because an optimal design enables the network to fulfill its required quality parameters, for instance, precision and reliability. The precision of a geodetic network in a tunnel usually decreases drastically as the network expands inside the tunnel and as the distance to known control points increases. The reliability of tunnel geodetic networks is fairly low due to weak network geometry (limited space in the lateral section of the tunnels). This paper studied the uncertainty of tunnel surveying networks using different observation methodologies in the West Link project, in which an 8-km railway tunnel is to be constructed underneath Gothenburg, Sweden. Adding more free station setups and involving observations from the tunnel wall-bracket points can improve the network uncertainty and reliability. Moreover, including orientation measurements (gyro-observations) has a significant effect on preventing a quick decrease of the precision of networks in long corridors.