General Lee's Bird's Eye View

14 Jan, 2008 By: Kenneth Wong

GIS to shed new light on the Battle of Gettysburg.

When Anne Knowles, an associate professor of geography at Middlebury College in Vermont, announced she could see the battle of Gettysburg from Robert E. Lee's point of view, she meant it literally.

"I'd often wondered what Lee could actually see at Gettysburg," she said. "Historians have commented that, when Lee's Confederate troops and Meade's Union forces arrived at this little-known town in Pennsylvania, they didn't know the ground very well and didn't have good maps of the area. This suggests to a geographer like me that the field commanders would have to make deployment decisions based on a quick visual survey of the site and scout reports."

She did travel to the site of the famous three-day battle to experience the landscape firsthand. She had walked across fields in the steps of Pickett’s Charge, where more than 4,500 soldiers perished in two hours. She had climbed Little Round Top, a rugged slope where the Union side undertook a downhill bayonet charge to repel a Confederate assault.

But it had been nearly a century and a half since the rifles from both sides fell silent. Though the primary geographic characteristics -- the contours and elevations -- remain relatively unaltered, new features, like a highway and a quarry, have sprung up, making it difficult for Knowles to see the battlefield as Lee would have.

As it turns out, she could take a peek into the past, back to Gettysburg as it was around Lee's time. She did it through an ArcGIS window.

Digitizing Civil War Cartography
Knowles' guide for her trip to present-day Gettysburg was Col. Kavin Coughenour, a member of The Association of Licensed Battlefield Guides and a contributor to The Journal of Military History. But her guide into the past was a map made in 1874 by the U.S. Army Corps of Engineers.

"This part of Pennsylvania has pretty gentle slopes. The available digital data for the site was in 10-foot contours. So it wasn't that detailed," she explained. To Knowles' delight, the 1874 memorial map created by the U.S. Army Corps had four-foot contours, giving her the data she needed to digitally recreate the geography at the time.

She initially tried to extract the contour lines by importing the map into ArcScan for ArcGIS, a raster-to-vector conversion application. But the nineteenth-century cartographers' rich details overwhelmed the software, preventing it from detecting the contour lines.

"So we extracted the contours by tracing them by hand, then we built a digital terrain model of the historical site in ArcGIS from ESRI," Knowles explained.

Armed with this model, Knowles and her research assistants performed a viewshed analysis to identify the areas of the battlefield that would have been visible to Lee from various vantage points, such as the Lutheran Seminary cupola.

According to ESRI's online literature, "Viewshed is useful when you want to know how visible objects might be -- for example: From which locations on the landscape will the water towers be visible if they are placed in this location? Or what will the view be from this road?"

"So you can place a point on the elevation model, then run the algorithm to reveal which areas of the map would be visible to someone at that point," explained Knowles.

In ArcGIS 9.2, viewshed calculations can be done from the ArcGIS Spatial Analyst toolbar. For more, see ESRI's article "Performing a Viewshed Analysis."

Click for larger image Click on image to enlarge.
What could have General Robert E. Lee seen of the battlefield of Gettysburg from the various vantage points accessible to him? The map here shows viewshed analysis from Lee's eye level standing in the cupola of the Lutheran Seminary. The buff-colored areas are what the analysis says he could see (note the 1874 topographic base map, georectified to be able to layer the viewshed on top). The indigo areas show what the analysis says Lee could not see, based on the digital elevation model derived from the 1874 map's elevation contours. The pale white areas are approximately the extent of areas historians have previously mentioned as being visible to Lee (maps courtesy of Anne Kelly Knowles, Caitrin Abshere, and Charlie Wirene).

Historic Reenactment
The Gettysburg campaign tickles Knowles' geographic mind because of the importance of site conditions in the outcome of the battles.

"For example," she said, "on day two, Lee sent his second in command, Lt. General James Longstreet, to march down the valley, loop around, then attack Little Round Top [a strategic advantage in the Union's line, according to Harry W. Pfanz, the author of Gettysburg: The Second Day, University of North Carolina Press, 1987].

"Longstreet famously came up over a ridge, looked towards Little Round Top, saw signalmen from the other side, and decided that if he could see them, they could see his troops too. So he marched further."

His decision, some argue, was a crippling misjudgment that allowed the defenders to get into position for an effective repulse. (For more, read "Longstreet's Lesson," Time, July 14, 1941.)

"So we placed ourselves digitally where Longstreet had stood, then performed a viewshed analysis to understand how exposed he might have felt," Knowles said. From her perspective, Longstreet has been criticized a bit too harshly for what seems like a rational tactical move.

Placing History
Placing History: How Maps, Spatial Data, and GIS Are Changing Historical Scholarship (ESRI Press, 2008) contains case studies showing how GIS helps modern scholars verify the myths and theories surrounding famous historic episodes, such as the 1930s Dust Bowl and the Battle of Gettysburg.
Further Studies
Knowles' research forms the basis for one of the ten chapters in Placing History: How Maps, Spatial Data, and GIS Are Changing Historical Scholarship, edited by Knowles and recently published by ESRI Press.

According to the publisher's announcement of the book, "Five scholars describe in case studies how they're using GIS to study different aspects of history such as the 1930s Dust Bowl in the American Great Plains, the Civil War battle of Gettysburg in 1863, China from 222 B.C. to 1911 A.D., and even colonial New England husbandry. The book comes with a CD-ROM to help introduce students to GIS in historical scholarship and includes PowerPoint presentations, videos, GIS projects, and map layers. The book also includes GIS software, ArcExplorer--Java Edition for Education, which can be used in conjunction with the GIS projects."

When not teaching or writing, Knowles likes to ski. She admits the ability to perform viewshed analysis of the slopes gives her an unfair advantage.

About the Author: Kenneth Wong

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