The astrogeodetic or Helmert deflection of the vertical (DoV) data, as opposed to data obtained through other geodetic instruments (e.g., GNSS (geodetic coordinates), levelling (normal or orthometric heights) and gravimeters (gravity data)), cannot be readily obtained by astrogeodetic instruments. In order to obtain these data, geodetic and astronomical latitude and longitude information is required. Currently, geodetic latitude and longitude (φ, λ) are measured with GNSS receivers, while astronomical latitude and longitude (Φ, Λ) are observed with Digital Zenith Camera Systems (DZCSs) or the QDaedalus system. The result of these observations are the astrogeodetic North-South (ξ=Φ-φ) and East-West (η=(Λ-λ)cosφ) DoV components. This study investigates the precision and accuracy of the newly installed tachymeter-based (multi station (MS) 60) QDaedalus astrogeodetic measurement system’s DoV data with the aid of promising DoV data from the Compact Digital Astronomical Camera (CODIAC) DZCS. For this study, we used the CODIAC (accuracy of ∼0.05″) and QDaedalus (accuracy of ~0.20″) systems to conduct parallel measurements over 2 nights (measurement duration of more than 20 hours) at the Zimmerwald Observatory in Switzerland. Both the CODIAC and QDaedalus systems were developed at ETH Zurich. CODIAC has the standard DZCS components seen in other DZCSs in Germany, Latvia, Turkey and China: telescope, charge-coupled device (CCD) camera, tiltmeters, focuser, single frequency GNSS receiver and antenna, laptop, and substructure. While each DZCS has a unique design, CODIAC has upgraded hardware components (such as the use of four tiltmeters-two Wyler and two Lipmann High-Resolution Tiltmeters) as well as advanced software and processing technology. The key components of the QDaedalus which differ from the DZCS are tachymeter, interface box, and mountable meniscus lens. Also, the measurement principle and data evaluation processes of these two systems differ; therefore, we also describe the data evaluation methods for both the CODIAC and QDaedalus systems. Finally, we discuss the results of the parallel measurements and their implications for further astrogeodetic work.

State-of-the-art astrogeodetic measurement systems—Digital Zenith Camera Systems (DZCSs) and the tachymeter-based QDaedalus systems—are currently used to obtain astrogeodetic deflection of the vertical (DoV) components. In this study, we use the CODIAC DZCS (accuracy ~0.05″) and two QDaedalus systems (accuracy ~0.20″) developed by ETH Zurich to collect DoV data in the Surses Region (Canton of Grisons), Switzerland. Based on these data, we assess the quality of the DoV predicted by the Global Gravity Model GGMplus and the Swiss Geoid model CHGeo2004. DoV data previously observed with the QDaedalus system in coastal (Istanbul) and smooth (Munich region) terrain areas were compared with GGMplus to determine the accuracy of the model for both types of terrain. The Surses Region was chosen for these observations because of its mountainous nature to examine the quality of the GGMplus model in mountainous terrain, as well as the GGMplus’ quality with the CHGeo2004. Astrogeodetic observations were conducted at 15 benchmarks in the astrogeodetic profile, at elevations ranging from 1185-1800 m and a station spacing of ~1.5 km. At one benchmark (BM12), observations were done with all three systems (eccentric BMs), while at 13 benchmarks, observations were only completed with the QDaedalus systems. Observations at the final benchmark were completed only with the CODIAC (0⁰ zenith angle) due to line of sight obstacles. The standard deviations of the QDaedalus data for each session are 0.04″-0.22″ and 0.01″-0.20″ for the N-S and E-W components, respectively. These high-quality data were compared to DoV data derived from GGMplus and CHGeo2004. The residuals from GGMplus are: The N-S component exhibit large residuals ranging from -2.31″ to 1.75″, while the E-W component residuals are -0.27″ to 1.80″. The residuals from CHGeo2004 range from -0.60 to 1.21 for the N-S components, and -1.01 to 0.32 for the E-W components. The results show that the predicted DoV data from CHGeo2004 are closer to the observed DoV, and more accurate than GGMplus. We conclude from these results that GGMplus is of significantly lower quality in mountainous terrain than in the smooth terrain of the Munich region (~0.2″, and reaching a maximum of 0.3″ and 0.4″ for the N-S and E-W components).