Journal of Surveying Engineering

The geoid is the equipotential surface that coincides with the sea level in the absence of such dynamic effects as tides, wind, ocean currents, and other disturbing forces. On land, geoid models are determined by using precise leveling, Global Navigation Satellite System (GNSS) measurements, and gravimetry measurements. However, in coastal areas, where performing precise leveling is not possible, geoid modeling relies solely on gravity measurements. Spaceborne gravimetry missions provide gravimetric measurements with global coverage. These measurements have low spatial resolution, which can reach distances of tens and hundreds of kilometers between tracks. Airborne and shipborne gravity measurements can be used to fill the gaps between tracks. In the absence of airborne and shipborne gravity measurements to densify the measuring data, shipborne GNSS measurements may provide a low-cost method for overcoming this problem in coastal areas. After applying several corrections to the raw GNSS measurements (accounting for systematic effects induced by ship dynamics and other static and dynamic impacts from tides, atmospheric pressure, or wind stress), approximate geoid height with fair accuracy could be extracted. An experiment was conducted along the Israeli coast, during which several GNSS surveys were carried out between the years 2011 and 2015. This paper illustrates the steps needed for the extraction of such heights and provides a comparison of the results between the Israeli official geoid model and the expanded one, complete with its quality assessment.

In the last few years, satellite-based positioning techniques, such as the Global Positioning System (GPS), proved their abilities in various geodetic fields. Height determination, as one of such applications, requires a more likely correct geoid model to provide reliable geoid heights for transformation of the ellipsoidal heights to orthometric heights. An important step is then to establish such a model by optimal combination of the available geoid models. This can be achieved via variance component estimation (VCE) methods, which provide appropriate weights to GPS and leveling observations as well as the geoid models. The authors demonstrate the efficacy of the least-squares VCE (LS-VCE) to this problem. The algorithm is applied to real data sets in Shahin-Shahr, Isfahan, Iran, to evaluate the EGM2008 and GGMplus Earth geopotential models and a regional geoid model (named IRGeoid10) in terms of agreement to the GPS/leveling observations and introduce the more likely correct model over the case-study area. The results indicate that the EGM2008 model shows a good agreement (2-mm precision on the fitted surface) with the results of the GPS/leveling observations in this very small area. It is notable, however, that the present contribution is mainly of interest from an algorithmic point of view because concrete conclusions cannot be made when comparing the results due to the small case-study area used. By optimal combination of these data sets, the weights of which are estimated by LS-VCE, a geometric surface is presented to approximate the local vertical datum in the case-study region. This surface can convert the ellipsoidal heights, which can be obtained from GPS, to the orthometric heights with high precision.

Video theodolites were used in the 1980s for highly accurate, automated measurements. However, they disappeared from the market and research on these instruments was done by only a few institutions using self-made prototypes. Because of the release of new-generation image-assisted total stations (IATS) by different manufacturers since 2004, these instruments have become relevant again for broader user groups. In this article, different error sources that occur when working with an IATS are assessed. The theoretical origins of these errors are discussed, their dependence on the measurement geometry is worked out, and strategies for avoidance and modeling are provided. In experimental geodetic network measurements, the impact of the different error sources on the results are evaluated. With standard deviations of a few 0.01  mm for the estimated three-dimensional (3D) coordinates, it is demonstrated that the telescope camera of a modern IATS can be used as a highly accurate measurement sensor.

This paper explores the influence of flight altitude, terrain morphology, and the number of ground control points (GCPs) on digital surface model (DSM) and orthoimage accuracies obtained with unmanned aerial vehicle (UAV) photogrammetry. For this study, 60 photogrammetric projects were carried out considering five terrain morphologies, four flight altitudes (i.e., 50, 80, 100, and 120 m), and three different numbers of GCPs (i.e., 3, 5, and 10). The UAV was a rotatory wing platform with eight motors, and the sensor was a nonmetric mirrorless reflex camera. The root-mean-square error (RMSE) was used to assess the accuracy of the DSM (Z component) and orthophotos (X, Y, and XY components RMSE_X, RMSE_Y, and RMSE_XY, respectively). The results show that RMSE_X, RMSE_Y, and RMSE_XY were not influenced by flight altitude or terrain morphology. For horizontal accuracy, differences between terrain morphologies were observed only when 5 or 10 GCPs were used, which were the best accuracies for the flattest morphologies. Nevertheless, the number of GCPs influenced the horizontal accuracy; as the number of GCPs increased, the accuracy improved. Vertical accuracy was not influenced by terrain morphology, but both flight altitude and the number of GCPs had significant influences on RMSE_Z; as the number of GCPs increased, the accuracy improved. Regarding flight altitude, vertical accuracy decreased as flight altitude increased. The most accurate combination of flight altitude and number of GCPs was 50 m and 10 GCPs, respectively, which yielded RMSE_X, RMSE_Y, RMSE_XY, and RMSE_Z values equal to 0.038, 0.035, 0.053, and 0.049 m, respectively. In view of these results, the map scale according to the legacy American Society for Photogrammetry and Remote Sensing map standard of 1990 will be approximately 1:150, and an equivalent contour interval of 0.150 m is sufficient for most civil engineering projects.

This study proposes a method for detecting track static regularities based on a laser tracker. Given the laser tracker’s advantages of high tracking-measuring accuracy and high sampling rate, the proposed method integrates static and dynamic measurements and is simultaneously applicable to both fine-adjustment measurement in the construction stage and detection in the operation-maintenance stage. In this paper, the design and implementation of the hardware-integration scheme, software development, and data processing are discussed. To verify the detection and data-processing methods for track static regularity, a case study was conducted on part of the Shanghai Metro Line 13 at Nature Museum Station. An SGJ-T-CEC-I passenger-dedicated line track geometric state measurement device of China Railway Engineering Consulting Group Co., Ltd., was used for comparative analysis. The feasibility and high accuracy of the proposed method were verified by using this device.

The authors present a new geoid modeling procedure that can greatly improve relative geoid accuracy in mountainous areas, leading to improved applications for modern geodetic techniques, such as light detection and ranging (LIDAR), in mapping orthometric heights over steep terrain on which precise slopes are needed to assess the risk of landslides and the suitability of industrial development. The new procedure (1) measures gravity gradients or computing modeled gravity gradients from a regular grid of gravity anomalies, (2) uses these gradients to refine gravity anomalies, and (3) uses the gravity anomalies to compute geoidal undulations. This new procedure was tested in Taiwan. In situ gravity gradients were measured at approximately 4,000 gravity sites to compare the modeled gravity gradients. In the test, ground gravity observations are reduced to gravity anomalies at mean sea surface using three types of gravity gradients: normal, modeled, and observed. The researchers’ geoid modeling uses the method of least-squares collocation (LSC) with the remove-compute-restore (RCR) procedure. Free-air gravity anomalies, as reduced using the observed and modeled gravity gradients, deviate from those using normal gradients by up to 100 mgal in high mountains. Using free-air gravity anomalies derived from observed and modeled gravity gradients, the authors can improve the relative geoid accuracies by up to 17 and 18 cm, respectively, for Route 3 (an area with high mountains) in Taiwan.

The national/regional unified height datum is of great importance for the establishment of geospatial information infrastructure and the corresponding surveying applications. The geopotential-difference method is proposed for long-distance height-datum connection across seas based on global positioning system (GPS)/leveling, global geopotential model (GGM), and satellite altimetry data sets. For a case study, the Chinese height datum 1985 (CNHD85) in the mainland was connected to the local vertical datum (LVD) in Hainan across Qiongzhou Strait up to a distance over 30 km. The results show that this method is independent of the selection of virtual leveling lines. Furthermore, evaluation with an indirect approach shows that the choice of GGM affects the quality of the normal height connection, where the relative accuracies of the solutions based on the Earth Gravity Model 2008 (EGM2008), European Improved Gravity Model of the Earth by New Techniques 6C (EIGEN-6C), EIGEN-6C3STAT, and EIGEN-6C4 models are 0.012, 0.0132, 0.022, and 0.009 m, respectively. The final CNHD85 normal heights of GPS/leveling benchmarks in Hainan were computed based on the EIGEN-6C4 model, and the closure of the height difference for the computed normal heights meets the tolerance requirement of the second-order spirit leveling. Moreover, the corresponding results indicate that the vertical datum difference between CNHD85 and LVD over Hainan is approximately at the level of 0.186 m.

At least two control points (CPs) must be available to determine the position and orientation of a total station (TS). This paper analyzes the optimum horizontal location of the TS with respect to the CPs and answers the question: What is the best location of the TS with respect to the CPs if the goal is to determine the coordinates of the TS and detail points (DPs) as precisely as possible? The optimality is based on the uncertainty of the horizontal coordinates of the TS and of the points measured from it, as well as on the uncertainty of the TS orientation. The investigation of this optimality problem was performed both analytically and by the trial-and-error method. It was found that the optimum location of TS is in the center of gravity of all CPs. For a given configuration of the control and DPs, the location of the TS does not influence significantly the positional uncertainty of the surveyed DPs.

A new ray-tracing method for deriving the effect of instrumental systematic errors in terrestrial laser scanner (TLS) instruments is proposed. Three cases of mirror inclination error for the panoramic scanner architecture are used as examples to demonstrate the applicability of the method. The results show that all three errors can be effectively compensated for by model variables already inherent to the augmented TLS positioning equations. Specifically, the effects can be compensated for by the collimation axis and trunnion axis error terms and the tertiary rotation angle of the exterior orientation parameters.

Digital levels are high-precision instruments that use bar-code rods and sensitive charged-coupled device (CCD) linear sensors. To determine the height difference, it is necessary to bring the line of sight to the horizontal position. Horizontality of the line of sight is achieved by the compensator. Technical characteristics of the level are checked within the calibration. Level calibration was performed in an accredited laboratory of the University of Belgrade, Faculty of Civil Engineering. However, between two calibrations, the level was used in a variety of conditions and was exposed to different environmental conditions. To check the performance of the entire system (i.e., level, tripod, and rods), an experiment was conducted to calculate calibration curves of the line of sight of three sets of equipment that include three digital levels. The experiment was carried out in cold storage with the temperature ranging from +10°C to –15°C. The results of the conducted experiment are part of this paper and point to the magnitude of the variation of the inclination of the line of sight, effects of relative change in the tripod height, and effects of relative change in the rod length. The obtained results imply that the collimation angle appears as a rod-reading error and can be eliminated by entering the corrections. Values of the accessory corrections (tripod and rod) for the effect of the temperature change indicate that they should be taken into account (corrections must be calculated and entered into the measurement results) in works with high accuracy.