Comparison of the materialisation of the tide gauges in use in Italy as support for the correct implementation of the national seagrass mapping project

The definition of a reference system is the first moment of a survey operation but is in some ways perhaps the most critical because on its correct definition depends on the reliability and significance of all subsequent operations. From the simplest local survey to the most complex surveys of large extensions that often require the joint use of different technologies, it is in fact of paramount importance to define a planimetric or planoaltimetric datum that allows all subsequent survey operations to be correctly referenced and make the various observations consistent. In this paper we illustrate the first considerations that have been made for the correct framing of the high-resolution bathymetric surveys of a future national project to survey all the coastal areas and the relative seafloor for the entire Italian coastline; peninsula, major islands and minor islands. The different geomatic techniques that will necessarily be used require the study and definition of certain methodologies to refer all observations to a common reference to ensure compatibility and comparability. Particularly critical is the altimetry aspect for which it is necessary to consider the shift between ellipsoid heights, orthometric heights and heights referring to the local mean sea level. It is precisely the last transition that is perhaps the most critical because it requires the local study of the framing and materialisation of the tide gauges. The main tide gauges of the national system have been studied here, confirming the complexity of this certain definition and suggesting possible developments in the definition of conversion methodologies.