The technological limits of surveillance during the American Civil War dictated that commanders often decided where to deploy their troops based largely on what they could see. We know that Confederate general Robert E. Lee was virtually blind at Gettysburg, as his formerly brilliant cavalry leader J.E.B. Stuart failed to inform him of Federal positions, and Confederate scouts’ reconnaissance was poor. The Confederates’ field positions, generally on lower ground than Yankee positions, further put Lee at a disadvantage. A striking contrast in visual perception came when Union Gen. Gouvernour K. Warren spotted Confederate troops from Little Round Top and called in reinforcements just in time to save the Federal line.

What more might we learn about this famous battle if we put ourselves in commanders’ shoes, using today’s digital technology to visualize the battlefield and see what they could see? Our team, which includes myself, researcher Dan Miller and cartographer Alex Tait, have done just that. Alex recreated the 1863 terrain based on a superb map of the battlefield from 1874 and present-day digital data. Dan and I captured troop positions from historical maps. Our interactive map shows Union and Confederate troop movements over the course of the battle, July 1 – 3, 1863. Panoramic views from strategic viewpoints show what commanders could – and could not – see at decisive moments, and what Union soldiers faced at the beginning of Pickett’s Charge. You will also find “viewshed” maps created with GIS (Geographic Information Systems). These maps show more fully what was hidden from view at those key moments.

Altogether, our mapping reveals that Lee never had a clear view of enemy forces; the terrain itself hid portions of the Union Army throughout the battle. In addition, Lee did not grasp – or acknowledge – just how advantageous the Union’s position was. In a reversal of the Battle of Fredericksburg, where Lee’s forces held the high ground and won a great victory, Union General George Meade held the high ground at Gettysburg. Lee’s forces were spread over an arc of seven miles, while the Union’s compact position, anchored on several hills, facilitated communication and quick troop deployment. Meade also received much better information, more quickly, from his subordinates. Realizing the limits of what Lee could see makes his decisions appear even bolder, and more likely to fail, than we knew.

Anne Kelly Knowles is Professor of Geography at Middlebury College. Her books employing GIS for historical research include Placing History (EsriPress 2008) and Mastering Iron: The Struggle to Modernize an American Industry, 1800-1868 (University of Chicago Press 2013). In 2012, she received the Smithsonian American Ingenuity Award for Historical Scholarship.

Dan Miller is a recent graduate of Middlebury College. Dan digitized troop positions and performed historical research to interpret the battle. 150 years ago, Dan’s ancestor fought in the 82nd Ohio Volunteer Infantry at Gettysburg, a connection that Dan was fascinated to investigate using GIS technology.

Alex Tait is Vice President of International Mapping in Ellicott City, Md. He works on map projects ranging from Civil Water battlefields to international boundary disputes.

Allen Carroll heads a "story maps" team at Esri, the leading provider of geographic information systems software, services, and content.

Tim Montenyohl is a 3-D Artist and Animator at International Mapping.

Judith Nielsen is a Senior Cartographer at International Mapping.