From John Hennessy:
I have always found this to be one of the most affecting of all images taken during the Civil War–the remnants of a Union line of battle at Gettysburg. Its location has remained a stubborn mystery for 148 years (eluding Frassanito and dozens of other sleuths), but now Scott Hartwig, the fine historian at Gettysburg, seems to have it figured out. He presents his conclusions over at that park’s new blog. You can find background on the image in Part 1 of his post and his conclusions in Part 2.