When you embark on a major venture, you need to understand the risks faced in as much detail as possible in order to be able to manage them adequately.
In late November and through December 1864, General William Tecumseh Sherman marched an army of 64,000 deep into the Confederate heartland, with a mission to destroy the Confederacy’s strategic, economic and psychological capability to wage war, and so hasten the end of the American Civil War, then in its fourth year.
To do this, the army would not use supply lines but live off the land, as they went. That gave them speed, and prevented the mission being compromised by supplies being attacked. This was a new tactic that Sherman’s mentor, Grant (later President Grant), had implemented in the mid-west. Abraham Lincoln felt it was too risky, but deferred to the opinion of his general.
So, Sherman’s army blazed a trail through Georgia, destroying strategic assets and living off the land. They besieged Savannah, and the Confederate defenders fled, leaving the citizens to surrender to the Union army. It was a heavy blow to the Confederacy’s war effort.
Before setting out, Sherman was given a detailed, data-rich map of Georgia based on the 1860 census. It showed, county by county, the population of whites, slaves and men of military age, heads of livestock and acres of crops. Consequently he knew exactly where to get provisions for his army, which fields to destroy, and where he would meet the most resistance. He later said that without the map, the success or failure of his operation would have depended on “blind chance”.
Sources: “A History of America in 100 Maps” – Susan Schulten, the Economist magazine, Wikipedia