The Kronstadt staff-level gauge is an instrument designed to measure the zero level of depths and heights both of the Baltic Sea and other absolute heights within the territory of the Russian Federation. It is one of the oldest leveling posts in the world. The staff is installed in the form of a porcelain scale with graduations on the abutment of the Blue Bridge over the Obvodny (Wire) Canal and is intended for measuring the height of the Baltic Sea level.
The Kronstadt staff is the first in Russia. A staff (from the German Fuss-stock) is a rod or beam with graduations installed at a water gauge post for observing water levels in the sea, river, or lake. The staff service monitoring sea level fluctuations in Kronstadt has been operating since 1707, although observations of changes in the Baltic water area began in 1703 by order of Peter I.

Strong fluctuations in water level necessitated regular monitoring of this indicator. This was important not only for timely warning of the population and prompt response in case of floods but also for planning navigation through the shallow waters of the Gulf of Finland and the mouth of the Neva River, as well as for constructing defensive structures on Kotlin Island.

Over time, the practice of regular observations and the staff service continuously improved. Since 1731, sea level observations were conducted in the Middle Harbor of Kronstadt. The staff was set according to the "ordinary water," that is, the average sea level at Kronstadt, which was 21 feet (6.39 meters) from the bottom of the canal. The value of the "ordinary water" was fixed as horizontal lines on the walls of locks, canals, and basins.

In 1752, the first marine hydrographic station in Russia was opened in Kronstadt, where constant sea level observations were conducted. In 1777, with the organization of the Baltic Sea staff service in Kronstadt, observations began to be made using the staff installed in the Merchant Harbor. At that time, the measuring rod was located on the western wall of the Merchant Harbor. Only in 1800 was the staff moved to the abutment of the Blue Bridge over the Obvodny Canal.

Regular sea level observations by the staff in Kronstadt began in 1806. The zero of the staff, as in the previous period, coincided with the "ordinary water" mark, i.e., 21 feet (6.39 meters) above the canal bottom. However, the "service" as an ordinary level also carried its dangers, as vividly reminded by the November events of 1824.

During the famous flood from November 6 to 7 (old style) (November 19 new style), the most severe flood in the entire history of observations struck St. Petersburg, when the water level rose 3 meters 67 centimeters above the ordinary level. The city forever remembered this disaster, immortalized by Pushkin in the poem "The Bronze Horseman," and on the houses along the Neva embankments, commemorative marks can still be found at the level of the middle of the second floor, where the water rose.

The memory of the flood is also marked on the Blue Bridge in Kronstadt, where a bronze commemorative plaque indicates the water level during this devastating St. Petersburg flood. The Kronstadt staff itself was demolished and later restored.

In 1840, hydrographer Mikhail Reinecke, who had been studying sea levels for 15 years, proposed installing a mark on the abutment of the Blue Bridge (the zero of the Kronstadt staff). It reflected the average water level in the Gulf of Finland for the period from 1825 to 1839.

The first mareograph — a self-recording sea level gauge — appeared here in 1870.

The zero mark of the Kronstadt staff serves in Russia to determine the height of geodetic points necessary for topographic mapping of the terrain.

Control over the zero position of the staff is maintained using special benchmarks (marks on a stable surface). The benchmark of the Kronstadt staff is engraved on the monument to Petr Pakhtusov, installed near the Italian Palace. In the word "Польза" ("Benefit"), carved on the pedestal, the letter "П," knocked out in a horizontal position, is the benchmark.

The last third of the 19th century was marked by innovations brought by scientific and technological progress. Since 1870, sea level has been observed using a mareograph — a special device for continuous recording of sea level fluctuations. Slightly later, the mareograph was moved to a small pavilion with a deep well, where the mareograph’s self-recorder impartially recorded sea fluctuations, noting both ebbs and floods.

A significant achievement in observing water level fluctuations was the establishment of leveling connections between the zero of the Kronstadt staff and marks on the mainland.

The first leveling network in the territory of the present Leningrad region, which includes the eastern part of the Gulf of Finland, was created at the end of the 19th century. The average level in Kronstadt, calculated over observation periods of various durations, was taken as the reference horizon for these leveling networks. By that time, there were several leveling systems named after their authors (Rylke system, Vitram system, Fuss system, Rudovitz system).

In 1886, astronomer-geodesist Fyodor Fyodorovich Vitram embedded a copper plate with a horizontal line into the stone at the zero mark, which represents the zero of the Kronstadt staff.

In 1898-99, a mareograph was installed on the shore of the Obvodny Canal in a wooden building. The automatic device recorded the water level in an abandoned well relative to the zero mark of the staff.

The Great Patriotic War brought new trials to the Baltic ordinary: the Kronstadt staff was severely damaged — the rod bent, and even the zero-point mark was damaged. A special Interdepartmental Commission for the regulation of the Kronstadt staff found its condition unsatisfactory. The commission noted that the staff rod and the "zero board" of the staff were neglected; in winter, the staff and zero point were not cleared of ice, resulting in level readings being taken "by eye"; the pipe connecting the mareograph well with the canal had not been cleaned for about 10 years; the level gauge rod was bent, etc.

But almost immediately after the war, on April 7, 1946, by decree No. 760 of the USSR Council of Ministers "On the introduction of a unified system of geodetic coordinates and heights on the territory of the USSR," the Baltic Sea — Kronstadt staff — was adopted as the reference level for heights. The Main Directorate of Geodesy and Cartography (GUGK) was tasked with recalculating the leveling network performed before 1946 into a unified height system (to complete the work within five years).

In May 1948, a decision was made to construct a new centennial installation of the Kronstadt staff. It was also decided to enclose the existing zero-point plate with a protective frame and manufacture a new metal staff. The interdepartmental commission thoroughly discussed the project of the new mareograph installation, and in 1949, the engineering construction department of the Navy began building a well and mareograph booth near the Blue Bridge. Simultaneously, other restoration and repair works planned by the commission began at the "national significance" site.

In 1950, an octagonal brick pavilion was built for the mareograph near the Blue Bridge. The pavilion covers a 7-meter deep well with concrete walls. At its lower part, the well connects to the Obvodny Canal by a horizontal pipe, and under the pavilion roof, a special device — a self-recorder — is installed, which records water level parameters and transmits this data to the meteorological station. Measurements of the Baltic Sea depth and absolute heights throughout the former Soviet Union territory are made relative to the zero of the Kronstadt staff (within the Baltic height system). All geographic maps of Russia are aligned to the Kronstadt reference point. In 1983, due to the upcoming construction of a hydraulic complex to protect Leningrad from floods, whose dams would affect the accuracy of readings taken from the Kronstadt staff, a decision was made to create duplicates of the staff in Kronstadt and Lomonosov (west of the dam). The duplicates were built about 40 kilometers west of the city of Lomonosov, south of the Shepelevsky lighthouse near Cape Kamenny of Battery Bay and represent hydrometeorological and level gauge complexes with observatory buildings. Observations at Shepelevsky are synchronized with observations at level gauge posts in Kronstadt and Lomonosov and have been regularly conducted since November 1, 1987.

There is a legend in Kronstadt that the first cosmonaut of the planet — Yuri Gagarin — called the location of the staff and mareograph the "Navel of the Earth." It is quite possible that this is true.

Sources:

https://kronshtadt.spb.ru/catalog/dostoprimechatelnosti/kronshtadtskij-futshtok/

https://love-gorod.ru/kronstadt/obj/kronshtadtskiy-futshtok

https://a-121.ru/kronshtadtskij-futshtok-rossijskaya-tochka-otscheta-ili-pup-zemli/