Retired Colonel Jeremy Green said: "The map is actually quite interesting because it only has coastal towns, there's nothing interior.
"It demonstrates the route the allies are going to take through this very large German minefield."
Across the map are a number of lines, showing the routes ships were to take from their port to the Normandy beaches.
The map also shows individual ships and the designation for convoys.
But despite all the thorough planning, Southwick House managed to keep the invasion secret through a variety of techniques.
"The Germans knew we were coming - they didn't know where or when," Colonel Green said.
"We had a massive deception plan of double agents and also a ghost army on the east coast of England which posed a threat from Dover to Calais which is what the Germans thought we would do - the shortest route."