Remote sensing is the science of obtaining information about something such as an object or a process without direct physical contact, generally because such contact is impossible or very costly to obtain. Information obtained through remote sensory techniques may simply involve observation, such as a picture taken from an aerial camera, or it may involve more nuanced measurements, such as sonar, which is used to collect data for both distance and size measurements. Remote sensory techniques are used in many different fields, including biology, chemistry, and physics. In most cases, however, "remote sensing" refers to techniques used to collect images and data about objects on Earth from a distance; satellite mapping of Earth is an example of this.
Some forms of remote sensing, such as photography, are passive, in that they simply collect information that is already there. A camera, for instance, does not emit anything; it instead produces images based on reflected light already present in the environment. Other forms of remote sensing, on the other hand, use active techniques to gather information. Active sonar systems, for instance, work by emitting sound bursts and using the echoes to remotely gather information about distant objects. It is an active method because pulses of sound must be emitted before information of any form can be gathered.
Applications for remote sensing are numerous, as there are many objects and phenomena about which it is difficult to gain information directly. Radar, for example, is used to remotely monitor weather conditions; measure the speed of drivers; and oversee air traffic, particularly around airports. Additionally, aerial photography methods are used to develop aerial maps. Some techniques in aerial photography can even be used to make topographic maps, which provide detailed information about terrain and elevation. Still other forms of remote sensing are used to measure earthquakes, range weapons, measure atmospheric chemical concentrations, and measure emitted radiation.
Much of the data obtained through remote sensing methods is analyzed through computers because computers can analyze the information much more efficiently and accurately than humans can. Computer algorithms, for instance, are often used to distinguish valuable data from the noise of the instrumentation. Computers can be used to clean up images and to make rapid calculations about the data obtained. Computer programs can even be used to accurately scale images. This is particularly important when remote sensing methods are used to develop images of very large objects or areas, as in aerial photography.