The location of an object may come from a number of sources. Digitization involves transferring the location of a point in space on a paper map or a photograph to the computer in a systematic way. For paper maps, this is done by using the same map projection on the computer as on the paper map, an verifying the location of certain control points. Remote sensing is the technique of obtaining digital data from a distance, for example, from an aircraft or satellite. Remotely sensed data is obtained by defining objects by their spectral signature, or the amount and type of electromagnetic energy the object reflects.
Electromagnetic energy is the energy transferred by radiation.
Every object reflects this electromagnetic energy in a unique way. The electromagnetic energy has both an electric and magnetic components and can be viewed as a series of sinusoidal waves of varying wavelength. There are many forms of electromagnetic energy including gamma rays, x-rays, ultraviolet radiation, visible light, infrared radiation, microwaves and radio waves. The range and distribution of these wavelengths is called the electromagnetic spectrum. Electromagnetic energy is normally classified into groups of wavelengths termed spectral bands.