1. A method of moving a portable unit to present a new portion of a stationary background image on a screen of the portable unit comprising the steps of: displaying a first image on the screen of the portable unit matched and superimposed to a corresponding portion of the stationary background image; mapping a first point of the first image located in a center of the screen of the portable unit to a corresponding reference point in the stationary background image; determining a first vector between the center of the screen of the portable unit and the corresponding reference point in the stationary background image; moving the portable unit along at least one of the orientations of the first vector; and presenting a new location in the new portion of the stationary background image on the screen of the portable unit.