The Gettysburg Battlefield is the area of the July 1–3, 1863, military engagements of the Battle of Gettysburg within and around the borough of Gettysburg, Pennsylvania. Locations of military engagements extend from the 4 acre (0.02 km2) site of the first shot[G 1] at Knoxlyn Ridge on the west of the borough, to East Cavalry Field on the east. A military engagement prior to the battle was conducted at the Gettysburg Railroad trestle over Rock Creek, which was burned on June 27.
Within 10 miles (16 km) of the Maryland/Pennsylvania state line, the Gettysburg Battlefield is situated in the Gettysburg-Newark Basin of the Pennsylvania Piedmont entirely within the Potomac River Watershed near the Marsh and Rock creeks' triple point with the Susquehanna River Watershed (near Oak Hill). Military engagements occurred within and around the borough of Gettysburg (1863 pop. 2,400), which remains the population center for the battlefield area at the intersections of roads that connect the borough with 10 nearby Pennsylvania and Maryland towns (e.g., antebellum turnpikes to Chambersburg, York, and Baltimore.)
The battle began on the west at Lohr's, Whistler's, School-House, and Knoxlyn ridges between Cashtown and Gettysburg. Nearer to Gettysburg, dismounted Union cavalry defended McPherson's Ridge and Herr's Ridge, and eventually infantry support arrived to defend Seminary Ridge at the borough's west side. Oak Ridge, a northward extension of both McPherson Ridge and Seminary Ridge, is capped by Oak Hill, a site for artillery that commanded a good area north of the town. Prior to Pickett's Charge, "159 guns stretching in a long line from the Peach Orchard to Oak Hill were to open simultaneously".