Meridian Calibration and Spatial-Temporal Reference Frames for the “Aula della Meridiana” of University of Genoa (Italy)

The meridian located in the “Aula della Meridiana” of the historic Rector’s Palace of the University of Genoa, realized in 1771 by François Rodolphe Corréard, represents a rare historical gnomonic hole meridian in Liguria region (Italy). Recent observations revealed an anticipation of approximately 35 seconds in the Sun’s transit compared to the expected time, motivating a comprehensive geomatic survey. This study combines GNSS, total station, and terrestrial laser scanner surveys to accurately reconstruct the room and the meridian line, linking internal features with the external reference system. Data processing included reference frame transformation and computation of the theoretical gnomonic hole position based on solstice and equinox points. The present study approaches the effectiveness of integrated surveying techniques for investigating historical meridians. Indeed, results highlighted discrepancies between the actual and theoretical setups: the gnomonic hole is shifted +7 mm eastward and -34 mm downward, the meridian line exhibits an azimuthal deviation of +31 mm eastward between its extremes, separated by a distance of 5.526 m, and floor non-planarity produces a -17 mm height difference between solstice points. While reference frame transformations proved negligible effect, the construction, restoration, and architectural factors mainly contribute to observed temporal shifts in Sun transit.