Mapping the Coast for D-Day

The 70th anniversary of the Normandy Landings has just passed. To mark the occasion, GW looks back into archives of Geomatics World and its predecessor tile Surveying World, to discover the role played by surveyors ion that momentous day and in the days following.

In 1995, Surveying World published articles on the topographic and hydrographic surveying work that was carried out in preparation for the invasion. This article is compiled from four articles which are available from the GW archives. References are provided at the end of the article.

The D-Day landings of 6th June 1944 marked the beginning of the end of World War II in Europe, but preparation started over a year earlier and was a monumental task.

Topographic mapping

The main topographic task was to map an area of northern France extending 200km in the east-west direction and 100km north-south. The source data was aerial photography and taking it was made all the more hazardous, because the Spitfire and Mosquito aircraft used for the purpose had to be disarmed, so that they could fly faster through enemy airspace. The photography was produced at 1:30,000 scale from a height of 30,000 ft.

The mapping task was code-named ‘Benson’. It was split between United States, Canadian and British mapping units and began in winter 1942-43, well over a year before the invasion. The plan ground control came from an official list of coordinates for French triangulation stations which was brought to Britain in 1940. The trigs were identified on the photography from the point descriptions. At first, minor control points were established and coordinated by the British using entirely graphical methods, until the Americans and Canadians arrived and brought with them Multiplex stereo plotters and the slotted template technique. Spot heights were taken from existing 1:80,000 scale mapping and contours plotted using a stereoplotter – no doubt a difficult task given the lack of ground height control. The base mapping scale was 1:25,000 with 1:12,500 for the immediate invasion area.

Beach gradient observations

There was one major concern about the landing craft – that if the gradients of the beaches were too shallow, landing craft would be grounded far from the shore. Two methods were used to measure gradient. One involved photographing the beach at six stages of the tide. A mosaic was made from low tide photography and then the intermediate water lines were superimposed upon it. The second method was based upon the fact that waves are slowed down by a shelving beach. Photographs were taken at the right tide level and in the first and last two hours of photographic daylight to ensure long shadows. Despite all the efforts to measure beach gradient, the results were not entirely accurate and had to be supplemented with ground survey.

Hydrography under cover of darkness

The hydrographic survey effort concentrated around gathering detailed information close to the beach that would be used for installation of the Mulberry harbours as well as for the landings themselves, an operation not for the feint hearted as the entire coast was heavily guarded, mined and watched by the Germans.

The operation was described in detail in an article by Commander Nisbet Glen (SW September 1995). The only available charts were made in 1875 by lead-line soundings, so it was considered essential to carry out check surveys by boat. Two craft were provided which, when loaded with equipment, could proceed at 9 knots. Each boat was equipped with a primitive echo sounder that did not take kindly to damp and salty surroundings; often demanding a sharp blow with a fist to get it going. They were also equipped with a ‘taut wire machine’, consisting of a drum with nine miles of single strand piano wire, passing through a measuring device. The end of the wire was attached to an anchor which was dropped from the stern of the boat. The echo-sounder trace was tagged with distances along the wire.

One of the boats was also equipped with a QH receiver, an early form of Decca Navigator, which was used to navigate to the right general area. To fix the position of the sounding lines, on reaching the beach the surveyors took compass observations to features on the shore. They were in identifying shoreline features by low level oblique aerial photography. The team made a total of six missions. Glen’s article gives full details and is a fascinating read.

Buoys, ports and. . . mines

During the assault itself, buoys were laid to mark mine-swept corridors across the Channel then, on arrival, rapid surveys were made of the beaches to plot any uncharted dangers. Setting-out buoys were placed and detailed surveys carried out before positioning the Mulberry harbours. Then, as each channel port was liberated, survey vessels would survey them, so they could be used by allied shipping. This was no mean task, given the obstructions that had to be contended with – Boulogne alone was blocked by 26 sunken ships. On land, surveyors were given specific tasks during the landings. Alan Gordon (GW July/August 2004) describes one, in which the surveyors had to establish control for the Royal Artillery to position and orientate their guns. This could only do done with any safety with tripod legs spread so that the theodolite was barely off the ground.

Secrecy was of course paramount and various devices were employed to ensure that Operation Overlord was not compromised. These included the production of the ‘bogus’ map series, based upon the ‘Benson’ maps but with place names replaced, for example, Ouistreham was replaced by Oslo. These allowed the assault troops to become familiar with the terrain whilst not knowing the real operational location.

Overall, the task was massive. The data was used for a multiplicity of charts for the assault and a total of 30,720 map packs were distributed prior to D-Day. As Alan Gordon records, “The statistics of D-Day are all superlatives and those relating to maps and charts are no exception. In just under two years 170 million maps were produced.”

This article was published in Geomatics World July/August 2014