Data consist of measurements. Here we consider the nature of the phenomenon that many, though not all, remote sensing systems measure: electromagnetic energy. Many of the objects that make up the Earth's surface reflect and emit electromagnetic energy in unique ways. The appeal of multispectral remote sensing is that objects that are indistinguishable at one energy wavelength may be easy to tell apart at other wavelengths. You will see that digital remote sensing is a little like scanning a paper document with a desktop scanner, only a lot more complicated.
(Just for fun: Can you think of a remote sensing technology that does not measure electromagnetic energy? We'll name one in the last page of this chapter.)