This invention relates to a method and apparatus for qualifying and quantifying the quality of navigational data emanating from any GPS receiver using its NMEA 0183 output messages for use in a dead reckoning system. It does this by deriving a first distance measurement travelled for a predetermined time period from a dead reckoning system or a GPS system. It then also calculates a second distance being the distance between two global positioning system (GPS) position measurements in the predetermined time. Finally, it calculates the difference between the first and second distances to derive a distance error. The statistical variance of the distance errors derived by repeating the above steps a predetermined number of times is representative of a figure of merit of a prior GPS position of the vehicle.