U.S. Coast Survey’s surveyors were pivotal in the Civil War Battle for New Orleans

March 27, 2012

Plan of Fort Jackson.

Plan of Fort Jackson, 1862 Gerdes and his men surveyed and mapped the flotilla’s successful mortar bombardment of Fort Jackson.

Download here. (Credit: NOAA Coast Survey sketch, from the Coast Survey historical collection)

As the Civil War battle to control the lower Mississippi River raged 150 years ago this April, survey data from NOAA’s predecessor agency was instrumental in the novel strategy to capture New Orleans, the largest city in the Confederacy and a key entry port to the critical waterway.

“U.S. Coast Survey teams mapped the terrain and charted rivers and coastlines for military action during the Civil War,” said NOAA historian Albert Theberge, “but their creative and even daring use of science and engineering went beyond what is normally considered to be ‘surveying.’”

Fort Jackson and Fort St. Philip, two Confederate forts essential to New Orleans’ defense, were located on opposite sides of the Mississippi 70 miles south of the city. To capture the city, and open the Mississippi River to Union forces, Commander David Dixon Porter, under the command of Adm. David Farragut, got President Lincoln’s approval for a daring plan to damage the guns at the forts. With assistance from the Coast Survey, Porter would guide his flotilla of mortar schooners into hidden positions, employing what may have been one of the first instances of “blind firing” mortar artillery.

Coast Survey Assistant Ferdinand Gerdes was one of the unknown heroes of the war. By relying on mathematical calculations — using survey coordinate points established by Coast Survey teams, rather than judging distance by sight — Gerdes would give coordinates to Union flotilla gunboats so they could aim their weapons without seeing the target.

Operations began in earnest on April 12, 1862. Gerdes’ goal was to establish survey markers on the shore as control points for indirect artillery fire into the two forts from Union mortar boats. To be successful, Gerdes’ team had to measure a series of small triangles based on two established locations on both sides of the river. They took their measurements while the gunboats fired on the forts, to distract the Confederates from the surveying operations.

The work continued for several days, sometimes while the Coast Survey men were being fired on. While the surveys were being conducted, Gerdes and other members of his party prepared charts and maps in a converted mess of the steamer USS Sachem.

Finally, on April 17, Gerdes delivered the maps and charts to Commander Porter, who moved the mortar boats into place. The next day, using Gerdes’ measurements, the Union boats began their attack on Fort Jackson. Over the next six days, an estimated 4,000 mortar shells rained on the fort. When the Confederates discovered where the shells were coming from and started firing back, the boats would have to move, which meant additional surveying — again, often under fire.

Admiral Farragut was in command of the Union Navy expedition of 17 wooden ships, 20 mortar boats, and 6,000 Union troops. He began moving the fleet past the forts at 2:00 a.m. on the morning of April 24. Because of the damage inflicted by the mortars — with Gerdes’ assistance — Farragut’s vessels were able to get by the forts with only 37 men killed and 146 wounded and no lost ships. All this despite the presence of a Confederate ironclad, chain barriers thrown across the river, and six other Confederate naval ships.

Alexander Dallas Bache, second superintendent of the U.S. Coast Survey.

Alexander Dallas Bache, second superintendent of the U.S. Coast Survey.

Download here. (Credit: NOAA)

Three days after the Union fleet passed the forts and moved upwards toward New Orleans, the Confederates in both forts mutinied and surrendered. Farragut’s expedition continued unimpeded, capturing the Confederacy’s largest city and effectively taking control of the lower Mississippi. Only Vicksburg, Miss. lay ahead as the final major Confederate river outpost, and it fell on July 4, 1863.

After the Battle of New Orleans, Commander Porter wrote to Coast Survey Superintendent Alexander Bache, commending the service of the men of the U.S. Coast Survey:

“The results of our mortar practice here have exceeded anything I ever dreamed of; and for my success I am mainly indebted to the accuracy of positions marked down, under Mr. Gerdes’ direction, by Mr. Harris and Mr. Oltmanns…. I assure you that I shall never undertake a bombardment unless I have them at my side.”

NOAA’s Office of Coast Survey is presenting a special digital collection of 400 charts, maps and sketches from the Civil War. To explore the collection, and for a more detailed explanation of the Fort Jackson bombardment, go to: http://www.nauticalcharts.noaa.gov/history/CivilWar/.

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