Surely you’ve seen one at some point while traveling through parts of Spain. A plaque hanging on the facade of a historic buildinga town hall or railway station indicating the altitude of that place sea ​​level In the city of Alicante. Because Compare altitude The question of what a city like Pamplona or Soria is compared to a Levantine city is a question that has a scientific answer and that we will solve today in Xataka.

The first thing to remember is that sea level is not equal or constant because tides vary. It is not even the same all over the world.. Therefore every country reference level a certain point in their territory. That’s what’s known as level zero. And any altitude that you want to calculate in that country will be done by comparing it to that data.

We can’t forget this either the technology we have now Compared to what existed two centuries ago, it is not even remotely the same. There were no altimeters back then, and now we see them everywhere: in cars, on mobile phones, at gas stations… This made calculating the altitude of any place a very laborious task that required infinite patience.

The task of choosing this reference point also took place in Spain. In particular, it fell National Geographic InstituteIt was opened in 1856 and at the time was entirely dedicated to precision manufacturing. topographic maps in a publication of the institution. 10th Hispano-Portuguese Geodesy and Geophysics MeetingExplain that the choice of Alicante was not a coincidence.

In this article, it is stated that in order for the measurement to be as stable as possible, a low point should be chosen to avoid negative heights, so sea level was chosen. That is why a place was sought on the coast of Spain. the differences between low tide and high tide were the smallest possible. So, I had to have small sea oscillationThis has completely ruled out some cities on the Cantabrian Sea, as sea levels in places like Santander or San Sebastián can vary by up to four metres during the day.

Four years of manual measurements

With this in mind, scientists considered the coastline and waters of the Mediterranean. Alicante, It is in a gulfWhere the daily change in sea level is only a few centimeters. It was also determined that this city is the best in terms of meteorological criteria, atmospheric pressure or seasonal variability.

But the most curious thing about IGN’s decision was not the choice of the city, but where they decided to set level zero. To know exactly where it will be located, They took daily measurements by hand from 1870 to 1874Using only an extension cord, at the Escaleras de la Reina, located in front of the Port of Alicante. After about four years of work, it was decided to place this point on the first step of the interior stairs of the Alicante town hall.

Specifically, this definitive step It was 3.41 meters above sea level. A plaque was placed there that said “NP 1”. That is, Precision Level 1. Since then, all altimeters in Spain measure according to the difference at this point. 3.41 meters“From there, the altitudes were transmitted to the rest of Spain through leveling lines and branches that would form the first High Precision Leveling Network (REDNAP).” Explains the institute in the report.

Obviously, over time and with the continuous expansion of the port network in Spain, the locations of the tide gauges have changed, build new ones To ensure continuity of the data series.

In fact, today there are 10 operational stations throughout the country: Alicante 1, Alicante 2, Cartagena, Almería, La Coruña, Alborán (Alborán Island), Santa Cruz de Tenerife (Tenerife Island), Puerto de la Cruz (Tenerife Island), Los Cristianos (Tenerife Island), Puerto del Rosario (Fuerteventura Island).

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*A previous version of this article was published in September 2023.