This textbook presents an intermediate survey of a changing technology. Its aim is for the reader to understand what has gone before and for the material to stimulate information sharing among everyone involved. The author distinguishes between what we know and what we assume. He looks at the key questions and unknowns, the possibilities, and the improvements which should be worked on. Four-dimensional monitoring is like the news business - we collect information on what is happening, try to make sense of it, think about what its impacts might be, and then present it so that people will take notice.