Dead Reckoning - DR
Dead reckoning is the process of determining a current location by using position change information that is in reference to a previously known location. The calculations may be based on speed, time elapsed and direction. Because the measurements used for dead reckoning have limited accuracy and the errors in position accumulate over time, precise navigation data is more likely to be obtained by other position location systems such as GPS or LORAN. Dead reckoning is more commonly used to update position location information during times and in areas where other position location systems are unavailable (such as when a GPS receiver is located in a tunnel).