Electromagnetic Spectrum
Sensors collect and store data about the spectral reflectance of natural features and objects, both of which reflect radiation. This radiation can be quantified on an electromagnetic spectrum.
The electromagnetic spectrum is a continuum of electromagnetic energy arranged according to its frequency and wavelength. As the electromagnetic waves are radiated through space, their energy interacts with matter and one of three reactions occurs. The radiation will either be:
Of these three types of radiation, remote sensing is primarily concerned with reflected radiation. This is the same radiation that causes our eyes to see colors, causes infrared film to record vegetation, and allows radar images of the earth to be created.
Spectral reflectance is the portion of incident radiation that is reflected by a non-transparent surface. The fraction of energy reflected at a particular wavelength varies for different features. Additionally, the reflectance of features varies at different wavelengths. Thus, two features that are indistinguishable in one spectral range may be very different in another portion of the spectrum. This is an essential property of matter that allows for different features to be identified and separated by their spectral signatures.
A spectral signature is a unique reflectance value in a specific part of the spectrum. Displayed in the graph below are the spectral signatures for healthy green vegetation, stressed vegetation, and severely stressed vegetation. In the visible region on the electromagnetic spectrum, the three spectral signatures look similar. However, in the near-infrared region of the spectrum, the spectral signatures look very different from each other. The healthy vegetation has the highest reflectance value while the severely stressed vegetation has the lowest reflectance value.
Researchers studying terrestrial vegetation most often use sensors that are able to collect data in the near-infrared region of the spectrum. Near-infrared sensors are capable of measuring the chlorophyll contained in plant material. The agricultural community is a frequent user of infrared remote sensing imagery because it can distinguish crop stress before the human eye can detect it. Using infrared imagery to evaluate the stress level of their crops, farmers can make important water and fertilizer application decisions. The illustration below displays an image captured with an infrared sensor.
Most remotely sensed images are not captured with film in a camera (aerial photographs are the exception to this rule). Rather, images are captured with a digital sensor mounted to an aircraft or satellite. The sensor records energy reflected from the earth. This information is then transferred to users, where it can be processed with a variety of computer software applications.
Remote sensing software applications have been developed for users to see a pictorial representation of the image. Images displayed on a computer screen are composed of pixels (picture elements), the smallest unit of a digital image, that contains both spatial and spectral properties.
A pixel's spatial properties provide information about the resolution or area represented on the earth while its spectral properties provide information about the intensity of the spectral response collected from the sensor.
Spatial resolution describes the area of the earth that each pixel represents. For example, an image might have a spatial resolution of 3 meters. This means that each pixel in the image represents an area on the earth 3 meters by 3 meters. Such an image would be considered high-resolution imagery. High-resolution imagery allows details, like houses and cars, to be seen sharply and clearly. This type of imagery is often used for community and urban planning and for agricultural purposes. Generally, the higher the spatial resolution of the imagery, the smaller the region of earth covered in each image. In order to see a large area, such as a county or a state, numerous high-resolution images would be required— an expensive and time-consuming effort. If an organization is working on a regional scale, lower-resolution imagery, which covers a greater area of land, might be a better choice.
Imagery of lower resolution can be used when studying or planning larger
regions on the earth, such as a county, state, or even a country. Do not be
fooled by the term "lower", it does not mean the imagery is of lesser
quality. Rather, the term "lower resolution" means the spatial extent cover by each
pixel in the image is large. Thus, this type of imagery
can be used for identifying large features such as lakes, forests, and
urban areas that cover a substantial amount of the earth's surface.
