In Italy, there are three definitions of the vertical datum: - for the continental part, the Genoa tide gauge, mean sea level of 1942; - for Sicily, the Catania tide gauge, mean sea level of 1965; - for Sardinia, the Cagliari tide gauge, mean sea level of 1956. The Conventional Genoa mean sea level corresponds to the reference benchmark of the altimetric network of the Italian peninsula and derives from the average of the observations carried out by the Genoa tide gauge over a period of 9 years (1937-1946) also known as Genoa 1942. The elevations of all the benchmarks of the peninsular network are therefore derived by altimetric transport from those of the Genoa tide gauge with levelling measurements. All the tide gauges of the network of the Military Hydrographic Institute (IIM) and ISPRA ( www.mareografico.it ), located in the main ports, are all connected altimetrically to the same levelling network and therefore to each other. This allows, in principle, comparisons of local mean sea levels along national coastlines. The Genoa 1942 altimetric datum (hydrographic zero) corresponds to the IGM (Istituto Geografico Militare) zero, also present in monographs and calculated using leveling. A geoid model is used for elevation measurements performed via GPS. For Italy, the ITALGEO2005 model is adapted to the national leveling network to minimize deviations between the model surface and the zero-level surface to which the leveling is referred. Deviations of a certain size are, however, inevitable, both due to measurement errors that accumulate in the leveling network, the approximation of the model and the interpolators used for it, and finally, to altimetric deformations of the Earth's crust due to seismic, subsidence, and bradyseism phenomena. The monographs of the tide gauges often report both vertical references, Genoa1942 and ITALGEO2005.

The semidiurnal tide , which has a high and low point spaced 12 hours and 25 minutes apart, can be divided into direct and opposite tide. The former is created by the gravitational forces of the Moon, which act on the water masses, causing them to slide and accumulate in its direction, while the opposite tide is caused by the centrifugal force of the Earth-Moon binary system, which rotates around a common center of mass.
A similar action is exerted by the Sun, and the amplitude of its tides is approximately half that of the Moon. Sometimes the two phenomena combine, and other times they contrast; it all depends on whether the Sun, Earth, and Moon are aligned (with a full or new Moon) or form a 90° angle (first and last quarter Moon). The former is called a spring tide , and the latter a neap tide . The evolution of these theoretical tides is further complicated by various factors, including local ones, and phenomena such as the lunar orbit, the oscillations of water masses between continents, or the rotation of tidal waves. Based on observations made by individual tide gauges in the national network, local tide levels are extracted that vary from area to area and depending on the observation period.

On Italian nautical charts, the hydrographic zero (in English chart datum), also called the reference level of the soundings ( LRS ), is the representation of the sea level (or the depth of the seabed) obtained from the average of the lowest level of the low spring tides ( MLWS ) measured locally over a long period of time, or from the lowest possible level that the tide can theoretically reach in that area.
In IIM nautical charts, the information in the margins of the charts provides the value of the Z0 elevation , referred to the LRS, with respect to the local mean sea level ( MSL ).
Other countries use the lowest astronomical tide ( LAT ), i.e. the height of the water at the lowest theoretical possible level of the tides that can be predicted under average meteorological conditions. The value of the LAT is obtained using a rigorous calculation using harmonic constants and considering not only gravitational effects but also other factors such as, for example, the meteorological effects of high or low pressure systems. These forecasts can also be provided by the IIM.
The depth value reported on Italian nautical charts ( Pc ) refers to the LRS and its value referred to local sea level (MSL) via the Z 0 value which is a constant for each nautical chart.

These tide levels will therefore vary from area to area with respect to the Genoa 1942 reference, which is a fixed and verifiable level with respect to any IGM reference point or with respect to the zero of any tide gauge on the national territory.

Since there has been a rise in sea level since 1942, the IGM zero (Genoa 1942) is generally found several centimetres below (10-20) the local MSL, as is obviously the reference level for echo sounders used in nautical charts.
The depth value for hydrographic surveys ( Pb ) is generally referred to the IGM zero, connecting to the tide gauges and using the relative tide. This represents the best choice both for having a unique reference and for connecting the bathymetric surveys with the topographic surveys of the emerged part.

A good approximation of the IGM "O" can also be obtained locally with GPS measurements and conversion of the inorthometric ellipsoidal elevation using IGM grids.
The case of the reference zero chosen for the design of maritime works is different, which should be the local mean sea level calculated by averaging data from a national network tide gauge or even specifically placed in an area of interest for a few months or years or, as seen, for the safety of vessels where, as in nautical charts, a precautionarily low reference level is preferred.