Abstract
The datum problem is a fundamental issue in the network adjustment when connecting a local measurement network to an external reference frame. Datum elements in 3D networks are scale, translation, and orientation. We consider here the local tie network at geodetic core stations, where the external reference frame is the latest ITRF realization, ITRF2014, in the mean epoch of terrestrial observations.
Accurate distance measurements are used for the determination of the network scale. Thus the improvement of its accuracy and the inclusion of weather measurements to account for refraction errors are essential. For rotation and translation of the network, we need external information. Angle observations are related to the coordinate system of the instrument (e.g. a tachymeter) which is usually aligned to the plumb line. Instruments have different vertical orientation at every station point and the direction of the plumb line does not coincide with the normal vector of the reference ellipsoid. Horizontally the observed set of angles are oriented in arbitrary or approximately oriented directions.
External information which is needed for solving the absolute orientation are datum points, providing the link to the global coordinate system, and correction terms for the vertical orientation (deflection of the vertical), which can be derived from combined terrestrial/GNSS observations, from a gravity based geoid model, or from astronomical observations.
In this article, we present the solutions/options for the datum problem in the framework of the EMPIR GeoMetre project using the example of the ITRF core stations Metsähovi and Wettzell using transformation-free approaches. The inclusion of distant targets is promising, since in small networks even a millimeter change in the coordinates of a datum point can significantly affect a local tie vector. It is shown that at both stations the determination of the deflection of the vertical using different techniques yield the same results within the measurement error.
The 18SIB01 GeoMetre project has received funding from the EMPIR programme co-financed by the Participating States and from the European Union’s Horizon 2020 research and innovation programme.
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Acknowledgements
The very productive support of Cornelia Eschelbach and Michael Lösler is gratefully acknowledged. The FGI team thanks Jyri Näränen, Joona Eskelinen and Arttu Raja-Halli for their help at Metsähovi station during preparations and measurements.
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Kallio, U. et al. (2022). Datum Problem Handling in Local Tie Surveys at Wettzell and Metsähovi. In: Freymueller, J.T., Sánchez, L. (eds) Geodesy for a Sustainable Earth. International Association of Geodesy Symposia, vol 154. Springer, Cham. https://doi.org/10.1007/1345_2022_155
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DOI: https://doi.org/10.1007/1345_2022_155
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