Journal of Surveying Engineering

The observations of geodetic very long baseline interferometry (VLBI) measurements refer to an invariant point of the radio telescope. This point is often called the reference point. For the interpretation of VLBI results it is necessary to know the position of this invariant point and its possible variation with time. Only the knowledge of the reference point makes it possible to combine VLBI results with results from other colocated techniques and in the framework of the global geodetic observing system (GGOS). Within the GGOS an automated monitoring of the reference points and therefore of the local-tie vectors at colocation stations is discussed earlier in other studies. In this paper a new mathematical model is described to allow such an automated monitoring of the reference point of an azimuth-elevation type radio telescope.

Laser scanning surveys were performed on the primary mirror of the very long baseline interferometry (VLBI) telescopes situated at Medicina and Noto observatories, with the specific purpose of investigating (1) gravity deformation patterns of the radio telescopes’ primary reflector and (2) the magnitude and relative variations of focal length as the antennas are steered in elevation. Both instruments have Azimuth-Elevation mounts and have 32 m parabolic mirrors which were surveyed in steps of 15° spanning the 90–15° elevation range. The scanning sessions were performed from two standpoints using a GS200 Trimble-Mensi; the sampling interval was set to 2 cm at a distance of 15 m. The complete surface of the main reflector at every elevation position was obtained by merging the two separate point clouds acquired from the two standpoints; each elevation is represented by at least 1.3 millions points. The merged clouds were compared for determining relative deformation patterns and magnitude. As the elevation decreases from 90−15°, the edges of the primary mirror of both telescopes fold in by a couple of cm. A least-squares adjustment was applied to point clouds corresponding to different elevations aimed at estimating the parameters of the rotational paraboloids that better fit the experimental data. This led to estimate the focal length variations induced by the structure deformative behavior. The focal lengths of the best-fit surfaces were compared. Their largest variation is found to be 2.5 cm at Medicina, between the 90° and the 15° positions. The clouds were also used to attempt a direct computation of the incoming radio signal’s path length variation due to primary reflectors’ deformations. Finally, two Leica total stations, a TDA5005 and a TC2003, were used to perform a survey of the local ground control network and of some selected targets placed on the edge of the dish. The comparison of the distances determined with the two terrestrial surveying methods (laser scanner versus triangulation and trilateration) highlights a statistically significant scale factor of about $1.0005\pm 0.0002$ , being the laser estimates smaller than those obtained with total stations. This study proves that laser scanners can be efficiently used to determine gravitational influences on large VLBI telescopes’ primary reflectors: deformation patterns are clearly and reliably depicted, focal length and incoming radio signal path variations are precisely quantified.

This contribution can be seen as an attempt to apply a rigorous method for variance components in a straightforward manner directly to GPS observables. Least-squares variance component estimation is adopted to assess the noise characteristics of GPS observables using the geometry-free observation model. The method can be applied to GPS observables or GNSS observables in general, even when the navigation message is not available. A realistic stochastic model of GPS observables takes into account the individual variances of different observation types, the satellite elevation dependence of GPS observables precision, the correlation between different observation types, and the time correlation of the observables. The mathematical formulation of all such issues is presented. The numerical evidence, obtained from real GPS data, consequently concludes that these are important issues in order to properly construct the covariance matrix of the GPS observables. Satellite elevation dependence of variance is found to be significant, for which a comparison is made with the existing elevation-dependent models. The results also indicate that the correlation between observation types is significant. A positive correlation of 0.8 is still observed between the phase observations on L1 and L2.

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.

The resection problem consists in finding the location of an observer by measuring the angles subtended by lines of sight from this observer to three known stations. Many researchers and practitioners recognize that Tienstra’s formula provides the most compact and elegant solution to this problem. Unfortunately, all available proofs for this remarkable formula are intricate. This paper shows how, by using barycentric coordinates for the observer in terms of the locations of the stations, a neat and short proof is straightforwardly derived.

Surveying technology as regards total stations (TS) has changed in the last fifty years. The original idea of measuring and storing polar coordinates has been progressively revolutionized by means of reflectorless distance measuring capabilities, as well as steer measurements, robotic, camera, and scanning functions. This enables the so-called intelligent tacheometry which is explained in detail herein, relying on the instruments capability to react autonomously on the object due to the reflectorless measuring device and eventually with image assistance and in interaction with the robotic drive. This paper revises thoroughly the evolution of conventional TS as figured out in the sixties until the promising image assisted scanning total station that are starting to come out with many surveying capabilities, but half way of what it might be an image assisted photogrammetric scanning TS (IAPSTS) with full photogrammetric functionalities. Following the writers’ opinions, the IAPSTS might surprisingly change the way both field surveying and processing are carried out in scenarios such as documentation of cultural heritage and industrial factory measurements.