Accurately determining the location of indications is essential because operators must physically locate a defect to report it. Location accuracy is also critical for imperfections in areas where additional inspections or preventative maintenance work are difficult or expensive, for example in subsea lines or at river crossings.

Overall location requirements are often based on the need to prevent unnecessary excavations. Typically, a bellhole is used to inspect a possible defect. Bellholes range in size but usually expose 3 to 15 feet of pipe. So, an inspection tool must locate an indication to within several feet to reduce unnecessary excavations. Location requirements in critical areas are even more restrictive. In subsea applications, it may be necessary to uncover a buried line and to remove concrete coatings. Here, it may be necessary to locate an indication to within one foot or less.

In general, MFL pigs locate defects to an accuracy of +/- 0.1 percent from the nearest reference point such as a mainline valve, branch connection, or road crossing. Often, to attain the necessary location accuracy, pigging vendors provide above ground marker (AGM) systems that detect the passage of the in-line tool, usually by detecting the a rapid change in magnetic field. The AGM notes the time of passage, allowing the location to be entered as a fixed reference on the data log. Global Poison Satellite (GPS) or survey data can be used, and the location should be permanently marked where practical.