Alicante, and more specifically its port, has the privilege of being the "zero level" of the National Altitude Reference, still in force today.
The Mediterranean Sea, due to its smaller size, its geographical location and the fact that it is a relatively confined body of water, means that its coast suffers less fluctuations in level than other coastal areas of Spain. Alicante has mean oscillations in sea level that are far from other cities and, in addition, the existence of one of the first railway lines that connected the city with the rest of Spain facilitated operations to transfer altitude to other places, so that its location as a reference point was considered suitable.
The National Geographic Institute, founded during the regency of General Francisco Serrano on September 12, 1870, was entrusted with carrying out a national topographical survey that would establish the exact altitudes, both of each locality and of the different geographical accidents. Therefore, it was an essential reference for obtaining the altimetry of the future National Topographic Map.
To obtain this altimetric reference, a technician from the Institute observed and recorded, for 4 years (1870-1874) and at different times of the day (9, 12, 15 and 18 hours), the level of the tide measured on a metal ruler (called a ruler). of tides) located in the Levante wharf of the Port of Alicante and, finally, obtained an average sea value of 0.430 m with respect to the reference point of the tide gauge.
For practical reasons, this reference was moved to another place with better guarantees of durability, specifically to the first step of the main entrance of the Alicante Town Hall, where a bronze disk marks the fundamental signal NP1 and whose altimetric value is 3.4095 m. above mean sea level in the port.
This was the first altimetric reference obtained on the mainland and has served since then as the origin of the precision leveling of the entire Spanish peninsular territory.
On March 1, 1874, the observations on the tide rule were interrupted and the measurements on the first tide gauge that worked in Spain and that was operational until 1924 began. In August 1926 it was installed in the fundamental tide gauge station (Alicante I) , built in 1925 on the outer breakwater of the Levante boardwalk, the Thomson tide gauge that was operational until 1969.
More recently (1954) a new tide gauge station (Alicante II) was put into operation, located at the mouth of the port, in which modern equipment has been installed which, in addition to providing tide gauge data, has made it possible to carry out other important works (geodesy, altimetry, gravimetry, etc.) for the maintenance of the current geodetic reference system (ETRS89).
The superimposed images of the door of the Fundamental Tide Station of the Mediterranean and the plate that recognizes Alicante as the national altimetric reference appear on the stamp.
