I'm a geographer（地理学者） at Middlebury College, and I use digital technologies to re-imagine the past. I want to take you to Gettysburg, Pennsylvania（宾夕法尼亚州（美国州名））. July 1, 1863, we're right in the middle of the Civil War. From the northwest, the Confederate（同盟的） forces under Robert E. Lee, and from the southeast, the Union forces under George Meade converge（聚合） at this place more or less by chance. They didn't plan to fight here, but the Battle of Gettysburg turns out to be the turning point of the Civil War.

  Now Robert E. Lee is probably the most famous American general, widely（大大地） respected, but at Gettysburg he made some crucial（至关重要的） mistakes. Probably the most important was in ordering Pickett's Charge. I'm going to show you how I took a new look at Pickett's Charge with historical maps and GIS. My key map was this extraordinary（非凡的） thing, 12 feet by 13 feet in the vault（拱顶） of treasures（财富） at the National（国民） Archives. Here are some of my students at Middlebury to give you a sense of that scale（刻度）.

  It was recompiled into a finished map the size of a large poster. You can see the layout（布局） of the town of Gettysburg. You see the undulating shape of the terrain（地形）. If you look at other details you can see forests, and orchards（ 果园）, and streams（溪流）, and roads. I want you to look at those very fine black lines. Those are called contour（轮廓） lines, and they show the elevation（高地） at four-foot intervals（间隔）, the most detailed（细节的） elevation（高地） I have ever seen.

  Now before I explain this image I need to tell you a little about GIS. It stands（站立） for Geographic（地理的） Information Systems. It's a kind of software that allows you to map（映射） almost anything. You can also use it to do terrain analysis. For example, if you're building a ski resort（求助） and you want people to get off the lift and have the most spectacular（引人入胜的） view possible, you use viewshed analysis that shows you what you can see from a certain point on the terrain. I used that to place myself digitally in the footsteps（脚步） of Robert E.

  Lee to ask what could he see and what could he not see that might have influenced his command（ 命令） decisions. Now back to these contour lines. This is the best elevation data that I could find. I traced（跟踪） all of the lines you see in the black and white drawing some of those lines, stitched（缝） them together, gave them elevation（高地） values, and then transformed（改变） it within the GIS program into a continuous terrain（地形）. This is a simulation（ 模拟） of the ground of the battlefield（战场）. Now I'm ready to place myself in Lee's boots and ask what he could see.

  The particular moment I want to look at is that battle that I mentioned, Pickett's Charge. Lee makes a crucial decision on the morning of the third day. This is July 3rd, 1863.