100 Years of Geodetic Measurements in the Piazza del Duomo (Pisa, Italy): Reference Systems, Data Comparability and Geotechnical Monitoring

The entire Pisa plains is subjected to subsidence movements, including the world-renowned Piazza dei Miracoli. This phenomenon, as a whole, has long been investigated, but over time the value of the site has commanded specific and steady focus. Early geodetic observations, dating back to 1908-1912, included the monitoring of the inclination of the Torre Pendente (Leaning Tower). Only in the1960s, geodetic and topographical measures were extended to the entire site, using as altimetric reference a benchmark, named φ', placed at the Baptistery. In 1989, a specific committee for the consolidation and restoration of the Leaning Tower was established, and in 1990 the Tower was closed to public access. The ensuing reconnaissance and executive planning provided the restoration and redesign of the levelling network for monitoring vertical movements of the site, also revealing that reference benchmark φ' was in fact subjected to subsidence. This issue was overcome by instating benchmark #999, which was anchored to the deep sands layer and acted as altimetric reference for any successive survey. Since 2012, monitoring of the vertical movements in the Piazza has been entrusted to ASTRO Laboratory of Pisa University. Lately, some interest has developed for investigating the evolution over time of subsidence in the site, thus requiring the use of a shared reference for different data sets. These uncertainties have had relevant consequences on monitoring of monuments, in particular about the tower, which has been subjected to important stabilization measures during the last decade of XX century. Since the observational method has been applied to the tower, levelling surveys became mandatory; a thorough discussion about reference points has also been carried out. The present paper hasprimarily historic connotations and describes the procedures followed to attune the diverse data sources in order to ensure their usability after roughly 100 years.