Remote sensing is a tool for gathering information about the Earth's surface and monitoring how it changes. A large number of remote sensing systems have been developed that exploit different parts of the electromagnetic spectrum and that produce data in different formats at varying scales and costs. These data may be acquired for virtually any place on Earth. A body of manual and automated techniques have evolved for processing and interpreting remote sensing data. The selections of a remote sensing system and of the techniques by which data are processed are driven by information requirements of varying detail and constrained by cost.