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. 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.
Many types of remote sensing images are routinely recorded in digital form and then processed by computers to produce images for interpreters to study. An image recorded initially on photographic film may be converted into digital format by a process known as digitization.