Head up display for navigation purposes in a vehicle

A control unit regulates a display device to show an image at a position according to the position and azimuth of the vehicle and the measured head position, on the front windshield portion (131). A vehicle measurement unit (120) measures a position and azimuth of the vehicle on a world coordinate system, while a head measurement unit (140) measures the position of the head of a passenger (132) on the vehicle on a coordinate system defined in the vehicle. Independent claims are also included for the following: (a) an image display method; (b) a measurement method; (c) a computer program; (d) a computer readable storage medium; (e) an information processing method; (f) an information processing apparatus; and (g) an index identification method.