Definition of Remote Sensing

Historically, it is recognized that the term Remote Sensing was created to designate the development of this new instrument technology capable of obtaining images of the Earth’s surface at remote distances. Therefore, the best known or classic definition of remote sensing is: Remote sensing is a technique for obtaining images of objects on the earth’s surface without physical contact of any kind between the sensor and the object.

Source: CDC / Centers for Disease Control and Prevention

The main condition imposed by this classic definition, which is that the sensor is at a remote distance from the object, establishes the basis for defining remote sensing in a slightly more scientific conception, which is governed by the following precepts: i) requirement: absence of matter in the space between the object and the sensor; ii) consequence: the object’s information is possible to be transported by empty space; iii) process: the communication link between the object and the sensor is electromagnetic radiation, the only form of energy capable of being transported through space.

Based on these precepts, a more scientific definition that can be given to Remote Sensing would be: Remote Sensing is a science that aims to develop the acquisition of images of the earth’s surface through the detection and quantitative measurement of the responses of the interactions of electromagnetic radiation with the terrestrial materials. This definition of remote sensing is explicit in stating that the imaged object is registered by the sensor through measurements of electromagnetic radiation, such as sunlight reflected from the surface of any object.

No other type of sensor that obtains images other than by detecting electromagnetic radiation should be classified as remote sensing. The most common confusion is made with airborne geophysical sensors, such as magnetometers, which generate an image from measurements of the magnetic field strength fields of the earth’s surface, therefore without any relation to electromagnetic energy.

Source: A burst of colour lights the shallow waters of the Gulf of Mexico off the Yucatan Peninsula. The swirls of tan, green, blue, and white are most likely sediment in the water. The sediment scatters light, giving the water its color. The sediment comes from two sources: the land and the sea floor. Some of the color may also come from phytoplankton, tiny plant-like organisms that live in the sun-lit surface waters of the ocean. Near the shore, the water is tan where rivers carry dirt from land to the ocean. As the sediment disperses, the water fades to green and then black. To the north (top), the water is more blue and white than tan and green. In these regions, the sediment has likely come from the sea floor. Made up of chalky white calcium carbonate from shell-building marine life like coral, sea floor sediment gives the water a white or bright blue colour. The sediment was probably brought to the surface in shallow waters by strong waves. A few days before the image was taken, strong winds churned the Gulf. The blue-green cloud in this image roughly matches the extent of the shallow continental shelf west of the peninsula. NASA image courtesy the MODIS Rapid Response Team at NASA GSFC, 2021.

On the other hand, there is a misconception of only considering remote sensing images obtained from satellites, since the term was created when the space age began. Aerial photographs, which for over a century have been used as a way of observing the Earth, are necessarily a class of remote sensors. Photographic film was the first material built by man capable of recording electromagnetic radiation and transforming it into the image of the photographed object. From the definition of remote sensing, it is obvious that the process of image acquisition and analysis can only be understood if we first know what electromagnetic radiation – REM is.

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