GIS (geographic information system)

            Generally speaking, Remote Sensing is the acquisition and analysis of information about objects or phenomena from a distance.   In regards to the discipline of geography, it is the acquisition and analysis of information about the Earth (or other planetary bodies)  through the use of computer and sensor systems via electromagnetic radiation.  It could be argued that Remote Sensing originated with any human gaining a high perspective of an area, but in a more sophisticated sense it began in the 1830s with the invention of the camera.  Major advancements were made during World War II with the development of RADAR (Radio Amplified Detection and Ranging) and SONAR (SOund NAvigation and Ranging).  Side Looking Airborne Radar (SLAR) was also invented during World War II and its product is a high resolution image. 

            In the Late 1950's, fixed wing aerial photography was extensively developed. In the early 1960's, the space race had begun between Russia and the United States.  Image based satellite systems were developed, especially with regard to military spy satellites and civilian weather observation satellites. Radar and was vastly improved upon.  Synthetic Aperture Radar (SAR) systems were developed was also developed during this time.

            Remote sensing came of age in the 1970's with the refinement of satellite imaging. In 1972 the Earth Resources Technology Satellite (ERTS) was renamed to LANDSAT (NASA).  The sensor had an 80 meter/pixel spatial resolution. In 1975, constant image download was available from LANDSAT, with an 18 day temporal resolution (passing over the same geographical area every 18 days). So much data became available, that Earth Resources Observation Systems (EROS) data center was established in South Dakota. Initial cost for four band (Red, Blue, Green and Infrared) LANDSAT scenes (approximately 100km x 100km area) averaged $200.

            The electromagnetic (EM) spectrum is the complete range of wavelengths of electromagnetic radiation emitted from the sun, ranging from the extremely short Gamma rays to the longer radio waves. The incident energy emitted from the sun is never destroyed; it is absorbed, reflected, or transmitted by an object.  Of the total EM spectrum, visible light comprises a tiny sliver.  The visible portion of the EM (.4 - .7 micrometers) is especially important in assessing the biomass (health of pigmentation) of vegetation.  Healthy plants tend to have high chlorophyll content.  In the visible spectrum, chlorophyll absorbs red and blue light and reflects green light.  Greater chlorophyll content will result in an increased reflectance in the green portion of the EM, and producing a visual green appearance of a healthy plant.

            Multispectral scanners can image from UV through the thermal IR band.  The Near Infrared portion of the EM (.7 1.3 micrometers) is sensitive to leaf structure.  The greatest amount of EM energy reflected the plants is in the Near IR.  The variability in reflectance is associated with the mesophyll layer of plant leaves.  Younger plants tend to have a well defined mesophyll layer resulting in higher Near IR reflectance.  As leaves mature or is stressed by environmental influences (drought, disease), the mesophyll layer structure deteriorates.  This results in a lower reflectance in of Near IR.  Using this knowledge enables scientists to track the state of vegetation coverage without having to do field analysis.  Most often in remote sensing images, healthy vegetation appears red.   

Microwave wavelengths, from 1mm to approximately1 meter, have been imaged using RADAR, SLAR (Side Looking Airborne Radar), SAR (Synthetic Aperture Radar), SIR (Shuttle Imaging Radar), and SRTM (Shuttle Radar Topography Mission).  Advantages of RADAR include the ability to penetrate cloud coverage and image surfaces in total darkness.  Because of its ability to penetrate through clouds, RADAR has been used to map the surface of the planet Venus.