pigs can measure the roundness of the pipeline to determine areas of crushing or other deformations. Some smart pigs use a combination of technologies, such as providing MFL and caliper functions in a single tool. Trials of pigs using acoustic resonance technology have been reported. During the pigging run the pig is unable to directly communicate with the outside world due to the distance underground or underwater and/or the materials that the pipe is made of. For example, steel pipelines effectively prevent any significant radio communications outside the pipe. It is therefore necessary that the pig use internal means to record its own movement during the trip. This may be done by odometers, gyroscope-assisted tilt sensors and other technologies. The pig records this positional data so that the distance it moves along with any bends can be interpreted later to determine the exact path taken. Location verification Location verification is often accomplished by surface instruments that record the pig's passage by either audible, magnetic, radio-transmission or other means. The sensors record when they detect the passage of the pig (time-of-arrival); this is then compared to the internal record for verification or adjustment. The external sensors may have Global Positioning System capability to assist in their location. A few pig passage indicators transmit the pig's passage, time and location, via satellite uplink. The pig itself cannot use GPS as the metal pipe blocks satellite signals. After the pigging run has been completed, the positional data from the external sensors is combined with the pipeline evaluation data (corrosion, cracks, etc.) from the pig to provide a location-specific defect map and characterization. In other words, the combined data reveals to the operator the location, type and size of each pipe defect. This information is used to judge the severity of the defect and help repair crews locate and repair the defect quickly without having to dig up excessive amounts of pipeline. By evaluating the rate of change of a particular defect over several years, proactive plans can be made to repair the pipeline before any leakage or environmental damage occurs. The inspection results are typically archived (perhaps in Pipeline Open Data Standard format) for comparison with the results of later pigging runs on the same pipeline. In popular culture A pig has been used as a plot device in three James Bond films: Diamonds Are Forever, where Bond disabled a pig to escape