Hydrographic data a crucial factor in the success of D-Day and beyond

From Hydro by Andrew Leitch

Accurate and up-to-date intelligence

In 2025, the world will mark 80 years since the end of World War II – a conflict defined by pivotal moments such as the Normandy invasion.
Known as D-Day, this extraordinary operation saw 156,000 Allied troops land on the beaches of Normandy, France, in the largest seaborne invasion in history.
What many may not realize is that hydrographic survey data played a critical role in its success, ensuring precise planning and execution of the landings.

Launched on 6 June 1944 under the code name Operation Overlord, D-Day set in motion the Allied campaign to liberate Western Europe, defeat Nazi Germany and bring the war to an end.
Insights from the UK Hydrographic Office (UKHO) Archives reveal how meticulous hydrographic work underpinned the Normandy landings and many other operations during World War II.
This story not only underscores the immense contribution of hydrography to the Allied victory but also highlights the enduring importance of preserving our hydrographic heritage for future generations.
Collecting hydrographic data to support D-Day

The use of hydrographic data was pivotal during the D-Day landings.
While we often focus on the immediate planning and execution of Operation Overlord, the UKHO’s archives reveal a deeper layer of detail and intricacies.
These documents, preserved in one of the world’s largest collections of navigational data, stretch back over 400 years and continue to unveil hidden details, many of which reshape our understanding of important moments in history.
There can be few of more significance than D-Day, which is particularly fresh in many of our memories following the 80th anniversary commemorations that took place this summer.

The Allied Forces required a variety of sea charts and coastal diagrams during the war and the Admiralty of the British Government tasked one of its departments, the UKHO, with gathering the appropriate information, drawing the charts and maps, printing them in great numbers – as well as great secrecy – and issuing them to the fleet for operational use.
A series of ‘Special Charts’, including top secret maps and charts of the Allied minefields, wrecks and enemy minefields were drawn by the cartographers at the UKHO to ensure that the British and Allied warships and merchant ships could navigate safely.

The hydrographic data used in the D-Day landings was a combination of long-established charts and ‘borrowed’ German and French records, based on both existing survey data and intelligence, and highly detailed, newly collected information.
Accurate and up-to-date intelligence was essential to the success of the planned assault on the Normandy beaches, and the information shown on these maps had to be gathered without betraying a hint of interest in the area.

D-Day Naval ship movement and navigation channel chart showing routes to the beaches from the south coast of the UK.
The large circle south of the Isle of Wight was a mustering area known as ‘Picadilly Circus’ and represented a gathering point for most of the landing craft on their way to the beaches of Normandy.

In the years leading up to D-Day, reconnaissance missions and covert operations were carried out to gather fresh data on the English Channel and the waters off the Normandy coast.
A variety of methods were used, including x-craft submarines that surveyed the shoreline depths and evaluated defensive and shoreline features, and aerial recon from Mosquitos and Spitfires for photographic records.
Perhaps most daring of all, hydrographic surveyors were sent by the Admiralty to make rapid reconnaissance surveys of the coast under the cover of darkness, though at the time they were kept blissfully unaware of the significance of their work.

In August 1943, a special covert unit was set up to gather this information, code-named Operation Neptune.
Based at Cowes on the Isle of Wight, two hydrographic surveyors, Lieutenant Commander Berncastle and Lieutenant Glen, were issued with two 32-foot landing craft in which to operate.
The low profile of these vessels made them difficult to detect by German radar and canoes were used when daring landings on the beaches were required.

The surveying could only take place under certain conditions, such as when there was no moon and a high tide during the small hours.
On dark nights, the craft were towed halfway across the Channel, then motored quietly the rest of the way using silenced engines and underwater exhausts.
The surveyors would begin their work just before midnight and were under strict instructions to leave the French coast by 04:00 hours to meet the gun boat and be towed back to safety.
The top secret information they gathered was added to the charts and landing maps by draughtsmen at the UKHO.

The missions to the French coast were carried out without major injury or attack, although they were spotted one night and flares were sent up by the Germans.
The only injury suffered was when someone burnt himself on a tin of the self-heating soup supplied to sustain the men on the long dark nights at sea.

When combined with other sources of intelligence, the information that made its way onto these charts went far beyond what would normally feature.
It is remarkable to consider the intricacies that were involved in calculating every aspect of the operation, reflecting the breathtaking complexities required if D-Day was to succeed.
This included every gun emplacement along the Normandy coast, the direction and distances covered by artillery shells, the size of the shells and the overlapping arcs of impact, all of which were meticulously mapped.
This was crucial in informing the choice of the landing sites, given the belief of the German forces that it would be impossible to successfully execute a landing and bring ashore the volume of material required to sustain operations anywhere other than a major port.

 

Chart of Arromanches-les-Bains, Calvados, Normandy, France 

(Gold Beach, D-Day landing site), 1944.

Supporting the success of D-Day

This data was critical in charting the best possible navigation routes for the enormous fleet, which comprised over 4,000 vessels of various sizes.
The UKHO’s navigational charts detailed everything from the south coast of England, through to rendezvous points such as the mid-channel mustering point south of the Isle of Wight, named Piccadilly Circus, to the precise landing points on the Normandy coast.

The incredible coordination on the day of the landings involved not only mapping and planning but also the precision of naval bombardments.
Shelling began from the battleships, many of which had been uniquely prepared to adjust the angle of their guns to hit their targets accurately.

Each vessel involved in the landings carried custom-made charts, produced by the UKHO at its printing facilities in Taunton, Somerset.
By the time D-Day arrived, over 1.5 million charts had been printed; a reminder of the critical role played in the war effort by so many away from the front line of the conflict.
These charts ensured that commanders could lead their forces and fulfil their role in the operation, navigating minefields and hazards while guiding troops to their designated landing beaches.
Even the troops themselves carried charts of the shores and inland areas to facilitate post-landing navigation.

The documents that were created, particularly the charts used by the invasion forces, demonstrate how multi-layered the operation was.
The charts were not simply guides for navigating the waters; they combined geographical, geological and military intelligence into a single resource.
Using these charts, the landing craft were directed to precise sections of the beach, and every soldier knew their role based on these detailed instructions.

These historical records, now housed in the UKHO’s collection, preserve not only the physical artefacts of that time but also the stories of the bravery and coordination that made D-Day possible.
Every detail, from the depth of the water to the positions of the troops, was considered and mapped with an astonishing level of precision and detail, which therefore led and contributed to the success of the landings and the eventual victory.

Juno Beach Landing Craft designation chart showing the sections of beach and area for navigation on D-Day.
Ships were given designated locations to come ashore (e.g. Mike red), and in this way the landings and troops could be directed to specific points.

 

The role of hydrography following D-Day

The days after D-Day placed continued reliance on the UKHO’s hydrographic data.
The surveyors continued their work along the coast to ensure the ports and beaches were well surveyed for potential dangers to the following fleet.
After the beachheads were secured, the Allies had to establish supply lines and reinforce their positions.
To do this, they constructed two massive artificial harbours, known as the Mulberry Harbours, on the Normandy coast.
Hydrographic data was crucial to positioning these harbours and other offshore breakwaters, ensuring that supplies could flow in while the Allies pushed further inland.

The planning, management and execution of these ongoing operations, including the use of sunken ships as breakwaters and wave-deflecting systems, known as bombardons, was another triumph of hydrographic intelligence.
These were charted meticulously by the UKHO and new charts were issued showing the positions of the Mulberry Harbours and breakwaters, as well as the new wrecks of landing craft where six weeks before the area had been littered with German sea defences.
These historic charts stand as a testament to the ongoing importance of hydrography in the crucial days and weeks after D-Day.
Hydrography in conflict and humanitarian operations

While D-Day was a prominent example, hydrographic data played a broader role in the wider Allied war effort.
Throughout World War II, knowledge of shorelines, underwater features and ocean conditions was essential for military operations across all aspects of the war.
The UKHO’s charts facilitated safe navigation, mission planning and coordination of naval forces, and continue to do so to this day.

The UKHO’s data remains crucial for humanitarian operations.
Whether responding to natural disasters or supporting peacekeeping efforts, hydrographic data enables the safe navigation of humanitarian aid vessels, ensuring timely relief to disaster-stricken areas.
This data is used globally to protect oceans, support global commerce and aid in defence operations.

Throughout the past 80 years since D-Day, there have been many examples of the vital role of hydrography in disaster relief.
In recent times, this has been most evident in the support that the UKHO has been able to provide to relief and recovery efforts following extreme weather events.
In the aftermath of the devastating impact of Cyclone Pam on the South Pacific island chain of Vanuatu, the UKHO rapidly produced and freely distributed two special purpose charts of Vanuatu to assist with the humanitarian relief effort.

Similarly, when the British Virgin Islands were struck by Hurricane Irma, the UKHO was able to provide emergency assistance.
This was initially through the team of surveyors who coincidentally were already located in the area and took immediate action by conducting a lead-line survey that supported the reopening of ports, jetties and coastal facilities for the relief efforts, with the support of the Royal Navy.
The UKHO’s team subsequently returned to gather data on bathymetric profiles, tidal information and navigational aids that enabled the reopening of Road Harbour, one of the islands’ main hubs.

Hydrography continues to play an important role in today’s conflicts.
Last year, the UKHO donated £1.6 million worth of equipment, including two full singlebeam echosounder systems and two multibeam echosounder systems, to the State Hydrographic Service of Ukraine, to assist with efforts to keep the seas around Ukraine safe and to protect Ukraine’s ports and shipping lanes, including attempts to establish a humanitarian grain corridor.

Secret June 1944 mustering chart of the Solent and surrounding areas showing the locations where D-Day landing vessels and support craft were to gather in advance of being given the ‘go signal’ to move off on their routes to the beaches at Normandy.

Learning for the future by preserving the past

The UKHO’s extensive archival collection, one of the largest in the world, has continued to inform and support our understanding of the importance of hydrography to the world around us.
Though initially compiled as part of an imperial mandate to chart the globe for British interests, these records now serve a more global purpose.
Modern hydrographic work involves collaboration with international partners, using historical data as a foundation for modern safety standards, innovative navigation tools and new partnerships.
For example, hydrographic charts from the eighteenth century once used during military engagements are now being shared with our counterparts in the United Arab Emirates, ensuring safer navigation, a shared understanding of our history and improved cooperation.

Our archives continue to educate us, not just about historical operations such as D-Day, but also about the value of historic records in shaping our understanding of the past and our approach to meeting today’s hydrographic needs.
The work of cataloguing these archives is far from complete, but every new discovery adds another chapter to the story of hydrography’s importance in our history.

 

Aerial photo of Arromanches-les-Bains, Calvados, Normandy, France (Gold Beach, D-Day landing site), 13 October 1944.
The surviving artificial Mulberry Harbour at Gold Beach allowed the continuous supply of equipment, supplies and troops to service the Allied invasion of Europe until early 1945.
The Hydrographic Office has all the planning, designs and logistical charts showing its inception, delivery and operations.

Links :

• GeoGarage blog :'Top Secret' maps reveal the massive Allied effort behind D ... / Turning the tide: D-Day from an oceanographer's perspective / D-Day: the largest seaborne invasion in History / Historian uncovers covert D-Day chart maker's tale / Mapping out a vital part of D-Day history / The dominance of chaos : Weather prediction will always have a ...