Beneath the seas: the adventure of marine cartography

 

 The Brest harbor and its surroundings in 1820, with numerous measurements taken one by one to determine the depth at each point. | SHOM

 

From Ouest France by Angellina Thieblemont

“Some Maps Remain ‘Classified’”: 

The History of Nautical Charts Told at the Brest Naval Museum

The exhibition at the Brest Naval Museum, “Beneath the Seas: The Adventure of Marine Cartography,” opens on June 26.

It traces the evolution of this practice over the past 500 years, as well as its military, archaeological, and economic uses—aspects largely unknown to the general public.

Stories of shipwrecks lost in Brest Harbor (Finistère), British blockades of the port, and maps classified as “defense secrets”… 

All these stories have one thing in common: they chronicle the development of underwater cartography in the new major exhibition at the Brest Naval Museum (Finistère), which opens on June 26, 2026.

 

Brest, home to the world’s oldest underwater mapping service

To mark the 400th anniversary of the French Navy, an exhibition on “The Call of the Deep” is being held in Paris, Toulon, Port-Louis, Rochefort, and Brest.

Each location of the National Maritime Museum explores a different aspect of this exploration of the ocean depths.

In Brest, the focus is on the history of marine cartography, from the 16th century to the present day. 

It is no coincidence that this theme was assigned to the “City of the Ponant”: since 1971, Brest’s Bergot district has been home to the Navy’s Hydrographic and Oceanographic Service (SHOM).

Founded in 1720, it is the oldest marine mapping service in the world.

 

A rare document depicting the Crozon Peninsula, Pointe Saint-Mathieu, and the Brest harbor, known as “Baye de Breft.” | NATIONAL MARITIME MUSEUM

In two rooms, shielded from the heat wave by the fort’s thick walls, the exhibition explains why and how nautical charts have been made since the 16th century. Among the hundred or so works on display are period maps featuring drawings of the monsters believed to lurk beneath the sea’s surface, as well as parchments, paintings, models, and a full-scale replica of a chart room complete with period tools…

Visitors also learn about the story of Charles-François Beautemps-Beaupré, the father of modern hydrography. In the 19th century, this pioneer set out to survey the coast of Brest. It was one of the first seabeds in the world to be mapped. On one of his maps, thousands of numbers are scattered across a drawing of the harbor. They give a sense of the titanic undertaking: for each point, a rope with a lead weight was lowered from their small boat and used as a sounding line.

 

A map of the Mediterranean and the eastern Atlantic, 1537. This map was painted on parchment by Vesconte de Maggiolo (1457–1530). | NATIONAL MARITIME MUSEUM

Deliberately Falsifying Maps

The exhibition features several thematic sections, highlighting the many fields in which hydrography is useful, particularly in archaeology and the installation of internet cables.

Brest. Me, the mayor...

But since the 16th century, this science has been most widely used in the field of defense: mapping sandbars to avoid running aground during battles, creating false maps that omit hazards to trap opponents… 

Such was the case with the Rocher de la Rose, an obstacle that loomed at the entrance to the Penfeld channel until the 19th century and was blown up by divers. 

It was deliberately omitted from maps in order to cause enemy ships to run aground. 

Visitors can also view one of the maps of the Normandy beaches that was used to prepare for the 1944 D-Day landings.

 

 

The 1842 painting *Combat du Grand Port* depicts a battle in which a British ship is defeated and runs aground on sandbars, while the French ships—which benefit from highly accurate nautical charts—escape unscathed. | NATIONAL MARITIME MUSEUM/A. FUX

 

Only 26% of the ocean floor will be mapped by 2026

Today, some maps are classified as defense secrets because of their quality and accuracy. 

“Accuracy is a weapon,” explains Jean-Yves Besselièvre, director of the National Maritime Museum. Since the 16th century, only 26% of the planet’s ocean floor has been mapped. 

“We’ve mapped the surface of the moon better than we’ve mapped the ocean depths,” says Jean-Yves Besselièvre.
Titled “Beneath the Seas: The Adventure of Underwater Cartography,” the exhibition runs through March 2027. 

A family guidebook has been created for children.

It allows them to explore the exhibition in a fun way by inviting them to conduct a mini-investigation.