a technique for displaying topographical data , for example , on a screen of a portable hand - held device includes : optionally receiving gps signals and generating a digital output indicative of a position location of the display device in response to the received gps signals or inputting location data manually to produce an image on a display screen of the device in response to the digital output ; and storing true digital elevation data as compressed cells and decompressing on - the - fly only data needed to provide elevation data for an image area displayed on the display screen .