1. A portable unit comprising: a background image of a stationary map at a given scale; a second location in the background image displaced from a first location in the background image by a known vector; an image displayed at the given scale on a screen of the portable unit and the background image at the given scale are matched and remains superimposed over one another; a diagonal of the screen less than a distance of the known vector; and only the first location in the background image displayed on the screen at the known scale, wherein the portable unit is moved along the known vector until the image containing the second location is displayed on the screen.