Theoretical mean sea level and geodetical leveling systems in Finland

Sea level is measured related to a leveling point inside a mareograph. The leveling point is annually leveled to a nearby benchmark, bound to the bedrock. These benchmarks are bound to the geodetical leveling systems. If the ground beneath the mareograph is moving, the height of the leveling point will change related to the geodetical leveling systems.

There are four geodetical leveling systems currently in use in Finland:

• NN: The first high-precision leveling in Finland was carried out in 1892 - 1910. The height system NN was the result of this leveling. However, it did not correspond to the geoid as accurately as N60. The zero level for NN was defined to be the zero point of a water level scale situated at Katajanokka, Helsinki. It was 30.465 meters below the main benchmark of Finland, located near the Astronomical Observatory in Helsinki.
• N43: The second high-precision leveling in Finland was carried out in 1935 - 1975. The height system N43 vas defined based on the results of years 1935 - 1955. The whole leveling network was not used to define this height system, and neither was land uplift during the measurement period taken into account.
• N60: The height system N60 was calculated based on the two earlier high-precision levelings after the second one was completed. The land uplift was also taken into account. Thus, N60 is considered to correspond to the geoid more accurately than the previous systems, NN or N43.
• The N2000: The N2000 height system is based on the Third Leveling of Finland (1978–2006). It is a Finnish realization of the common European height system, and its datum is derived from NAP (Normaal Amsterdam Peil). The heights of N2000 differ 13–43 cm from the heights of the previous Finnish national height system N60. Most of the difference is due to the land uplift during 40 years: the N2000 heights are based on the values of the vertical motion in 2000, and those of N60 on those in 1960. There are also differences in computation.
  
  The mean sea level height referenced to these height systems has been decreasing over a long period of time. Thus, the Finnish Institute of Marine Research has adopted so called theoretical mean water (MW).

The theoretical mean sea level (MW)

The theoretical mean sea level is a forecast for the long-term mean value (more precisely, expectation value) of sea level, made for practical purposes. The land uplift as well as the slow rise in sea level are taken into account. Because of these changes the theoretical mean sea level is not a constant. Yearly the Meteorological Institute confirms the height of the theoretical mean sea level for 5 years onwards. The theoretical mean sea level is commonly used in Finland as a reference level when sea level information is given to the public, for instance on the internet or radio and in the newspapers.

An example of height system calculations:

In 2000 a sea level value +15 cm, referenced to the theoretical mean sea level, was measured in Helsinki. The sea level in N60 system is obtained as follows:

• The height of the theoretical mean sea level 2000 (MW2000) in relation to the N60 system in Helsinki, -66 mm, is obtained from the table. The zero level of MW2000 is thus 66 mm below the zero level of N60.
• The sea level in N60 is thus: +15 cm - 6.6 cm = +8.4 cm.