One can think of any image as consisting of tiny, equal areas, or picture elements, arranged in regular rows and columns. The position of any picture element, or pixel, is determined on an xy coordinate system; in the case of Landsat images, the origin is at the upper left corner of the image. Each pixel also has a numerical value, called a digital number (DN), that records the intensity of electromagnetic energy measured for the ground resolution cell represented by that pixel. Digital numbers range from zero to some higher number on a gray scale. The image may be described in strictly numerical terms on a three-coordinate system with x and y locating each pixel and z giving the DN, which is displayed as a gray-scale intensity value. Images may be originally recorded in this digital format, as in the case of Landsat. An image recorded initially on photographic film may be converted into digital format by a process known as digitization.