map data of a given area in advance and comprises a semiconductor memory cassette tape cd rom ic memory dat or the like the above described display 7 comprises a crt display crystalline liquid display or the like and displays a road map that the vehicle is travelling and a current location of the vehicle the above described navigation controller 5 is constituted by a figure processor an image processing memory and the like and generates instructions to perform a retrieval of the map on the display 7 switching of scale scrolling a display of the vehicle current location and the like the above described keyboard 8 includes a key for generating a trigger to obtain the gyro data before a vehicle is turned next the method of calculating the scale factor of the gyro will hereinafter be described the data of the gyro are sampled every a constant cycle by the a d converter 43a and the data sampled are indicated by vi vi 1 2 3 the gyro data vi can be written by the following equation where u03c9 is an angular velocity of a vehicle a is a scale factor of a gyro b is a gyro bias and ni is a noise component if the gyro data vi obtained as the vehicle turns through 360 u00b0 are integrated the integrated data will be expressed by the following equation where u03c3 b is a heading error and u03c3 ni is the sum of noise components if the gyro is bias corrected u03c3 b will be zero and if a noise is a random one u03c3 ni will be zero.