Why is height in Spain measured with respect to the sea level in Alicante? When the altimetric survey of Spain was projected at the end of the 19th century, the measurements confirmed that the difference in sea level between low tide and high tide was the smallest on the Alicante coast.

For this reason, the altimetric survey of the Madrid-Alicante itinerary began in 1871, establishing the first signal on the first rung of the Alicante Town Hall staircase. In Madrid, the Astronomical Observatory was taken as the altimeter point. The signs were identified with the nomenclature N.P.1 for Alicante and N.P.26 for Madrid.

After years of observing the sea level in Alicante, the zero level was set. Thus, signal N.P.1 was assigned a height of 11,177 (3,407 meters), which allowed obtaining that of signal N.P.26, which turned out to be 625,562 meters. This serves as an origin to calculate all the other levels of the country.