On May 6, Lt. Albert Lanker of the 31st Photographic Reconnaissance Squadron made the first dicing flight. Flying 30 feet (9 meters) above the Channel to slip under German radar, he turned sharply over a large dune on the Normandy coast and, at 360 mph (580 km/h), buzzed German soldiers working on defenses. Photo interpreters examined his photos and discovered that the dune was a gun emplacement, which was added to the maps.

Lt. Allen Keith, while zooming along at about 360 mph(580 km/h), hit a gull. The bird smashed through the windscreen but was stopped by newly installed bulletproof glass on brackets in front of the pilot. Spattered by blood and feathers, Keith could not see for a few seconds but wiped off his goggles and never lost control. Photos from another of the dicing missions, this one flown by Lt. Garland A. York, revealed log posts jammed in the sand, angled seaward and topped by mines. Other photos showed that the tidal flats of the beaches were studded with "hedgehogs"—steel rails welded together and resembling giant versions of children's jacks. The obstacles were designed to impale or rip open the hulls of landing craft approaching the beach at high tide.

Some air reconnaissance photographs were processed so fast that fingerprints still appear on the negatives, showing that they were snatched from the developer for circulation. "Recent wave-top aerial photo reconnaissance," says a May 29 intelligence bulletin, "reveals that practically all types of underwater obstacles may be armed with . . . mines." The dicing photos convinced planners that landing craft had to come in at low tide and discharge troops before hitting the obstacles. So mapmakers had to figure ways to display tides and beach slopes.

Head-on aerial photos of the Normandy shoreline made by the dicing flights produced eye-level views for Allied coxswains to use as they aimed their landing craft toward D-Day beaches. BIGOT artists turned the photos into paintings that showed landmarks, such as church steeples and seaside houses. One of these artists was Navy Lt. Frederick S. Wight, who later would be renowned in civilian life as a curator and historian of modern art in the U.S.

Another BIGOT artist was Navy Lt. William A. Bostick, who worked in a commandeered London apartment. He and other artists used the pilots' panoramic photos "to make watercolors of the beaches as landing craft skippers would see them as they approached." Bostick's watercolors, emphasizing terrain features and landmarks, formed a narrow band under the maps. On the back of the maps was an invasion almanac with information about sun, moon, tide, and currents from May 25 to June 21. (The precise date of the invasion was not set until after the maps were finished.)

Bostick was especially proud of an ingenious transparent overlay that showed profiles of large and small landing craft. By adjusting the sheet over a graph of the beach slope, navigators could see where their craft would run aground and what the water depth would be. "The Army called them maps and the Navy called them charts," Bostick said. "So we called them chart/maps—pieces of paper that showed the Navy where to land the Army."

While the maps were evolving, a group of intelligence officers was busy crafting the greatest hoax of the war—a spy-running operation that was not fully revealed until the 1980s. Earlier in the war, British counterintelligence officers gave captured German spies a simple choice: Be hanged or work for us. Most chose to live. Directed by their handlers, the turncoats used seemingly clandestine radios to transmit to German spymasters a mix of real and counterfeit information. The operation, run by the wryly named XX (double cross) Committee, was meant to convince the German high command that the invasion would strike at either Nazi-held Norway or at Calais, across the English Channel from Dover.

Adding to the deception were two huge but imaginary military units. One, supposedly preparing to invade Norway, provided German radio interceptors with the busy radio traffic of a simulated 350,000-man army whose needs included "ski training" and "handbooks on engine functioning in low temperatures." A second phantom army appeared poised to strike at Calais under the command of Lt. Gen. George S. Patton. Spies tipped the Germans that Patton had arrived in England to lead the Calais invasion. Luftwaffe reconnaissance flights photographed the evidence of Patton's army: rows of tanks and barracks, fleets of landing craft in nearby ports, even an oil dock. All were illusions, made of wood, rubber, and papier-mache by fakers who included movie stagehands.

An XX committee operative added her contribution by reporting to the Germans that she was dating a staff officer of the nonexistent U.S. Fourteenth Army, which had moved its headquarters to the Dover area, opposite Calais, to prepare the invasion. (So complete was the deception that Fourteenth Army shoulder patches appeared alongside real ones in a 1944 National Geographic booklet on U.S. armed forces insignia.)

But what about reports the Nazis were receiving of a Normandy invasion? That, said the Germans' most trusted spy (and XX's star performer) would only be a diversion. A message sent on June 9—and read by Hitler himself—warned that D-Day was a trap designed to draw off German reserves so that the Allies could launch a decisive attack, "probably . . . in the Pas de Calais area." The ruse would keep German forces in Calais for weeks after D-Day, awaiting the "real" invasion.

While the Germans built up their forces around Calais, Field Marshal Rommel placed the underwater obstacles at Normandy that the dicers had spotted. Rommel had asked in vain for more cement and mines for Normandy. Denied, he made do with obstacles of timber and steel. "Our only possible chance will be at the beaches," he said. If the invaders came to Normandy, he would stop them there. He expected them to come in on a high tide and impale their craft on his barriers.