Boundary line retracement surveying is for when you own an existing parcel of land, or are interested in purchasing a parcel of land, and don't know where the true boundary lines are, you would require the assistance of a licensed surveyor to locate and monument all corners or angle points that define that particular parcel of land.
Typically, property corners and angle points are marked with a 30" long, 5/8" rebar that has a plastic cap on top of it designating the name of the surveyor that placed the rod, and the surveyor's license number, such as "J.D.Barnes P.S. 8411". This cap would identyify that Barnes Surveying set this property corner.
Other types of markers that have been used would include rocks with chiseled "X" in the top, rocks with drill doles in them, iron pipe, railroad spikes, Parker Kaylon nails (P.K. nails), even old axles from cars and trucks have been used to mark corners. Many old surveys even call out "witness trees" that mark the corner of the particular parcel being surveyed.