Cascais tide gauge

The Cascais tide gauge (Marégrafo de Cascais) was the first tide gauge installed in Portugal, and its story goes back to 1882. It stands in Cascais Municipality, Lisbon District. It was one of the first sea-level recording systems on the European coast and it is still in use today. The site appeals to anyone curious about science, surveying and the history of measuring instruments.

What does a tide gauge do?

A tide gauge records how the sea level rises and falls with the tides. The instrument at Cascais has been logging these changes for well over a century. Its readings make it possible to follow mean sea level and compare it with results from stations around the world. Data from Cascais have gone to the international Permanent Service for Mean Sea Level for more than 120 years.

Why was it built in Cascais?

The device was made in Paris in 1877 by the clockmakers Amédée-Philippe Borrel and Jean Wagner. The reason for installing it was practical: a sand bar in the Tagus estuary made access to the port of Lisbon difficult, so reliable tide data were needed. In 1881 an analogue gauge was planned for Cascais. That year served as a testing period, and full operation began in 1882. Sources differ slightly on the start date, with some saying 1881 and others 1882.

The gauge was linked to the oceanographic laboratory of King Carlos I, a keen student of the sea. It was first placed on the east side of the rock fortress above the bay. On 23 August 1900 it was moved to its present spot, about 30 metres away.

Why does it matter to Portugal?

Readings from Cascais were used to define the Portuguese altimetric datum, the cartographic zero. It was calculated from mean sea levels between 1882 and 1938. This height reference, mean sea level, became the basis for all Portuguese cartography. Every elevation on the country's maps therefore traces back to this modest installation.

Who runs it today?

The original gauge is still working and is operated by the Portuguese Geographic Institute. The Cascais municipal website says it has been in continuous operation since 1882.

Can you visit it?

Visits are possible by prior appointment. The municipality of Cascais publishes details of such visits, including some held for National Sea Day. Check the municipal pages for current dates and rules before you go.

Frequently asked questions

Where is the Cascais tide gauge? In Cascais Municipality, Lisbon District, Portugal, at roughly 38.694 N, 9.418 W.

How long has it been operating? Measurements began in 1882, after a testing period in 1881.

Is it still in use? Yes, it still records sea level and sends data to the international sea-level service.

Can I visit? Yes, by arranging a visit in advance.