Terms and Definitions Used in Remote Sensing |
Algorithm - A mathematical relation between an observed quantity and a variable used in a stepby- step mathematical process to calculate a quantity. Altitude - Height above the Earth's surface. Band - In radio, a continuous sequence of broadcasting frequencies within given limits. - In radiometry, a relatively narrow region of the electromagnetic spectrum to which a remote sensor responds; a multi-spectral sensor makes measurements in a number of spectral bands. - In spectroscopy, spectral regions where atmospheric gases absorb (and emit) radiation, e.g., the 15 µm carbon dioxide absorption band, the 6.3 µm water vapor absorption band, and the 9.6 µm ozone absorption band. Bandwidth - The total range of frequency required to pass a specific modulated signal without distortion or loss of data. The ideal bandwidth allows the signal to pass under conditions of maximum AM or FM adjustment. (Too narrow a bandwidth will result in loss of data during modulation peaks. Too wide a bandwidth will pass excessive noise along with the signal.) In FM, radio frequency signal bandwidth is determined by the frequency deviation of the signal Calibration - Act of comparing an instrument's measuring accuracy to a known standard. Data* - Any collection of related facts, from raw numbers and measurements to analyzed and organized sets of information. In a GIS, data is stored in attribute tables and provides all the information about the geographical features on the GIS map. Data Directory* - A folder or area on the hard drive Mcimage.exe of your computer or your network’s computer that is available for storage of GIS projects and data that you create. Decimal Degrees* - Degrees of latitude and longitude expressed in decimals instead of in degrees. Latitude and longitude is most typically displayed in degrees, minutes, and seconds. Decimal degrees converts the degrees, minutes, and seconds into a decimal number using the mathematical formula of: decimal degrees = degrees + minutes / 60 + seconds / 3,600 73° 59 15 longitude is equal to 73.9875 decimal degrees. Digital Image - An analog image converted to numerical form so that it can be stored and used in a computer. The image is divided into a matrix of small regions called picture elements or pixels. At sub-satellite point each pixel represents a specific amount of area. For example, in APT each pixel represents 4.1 kilometers. Each pixel has a numerical value or data number value, quantifying the radiance of the image at that spot. The data number value of each pixel usually represents a value between black and white, i.e., shades of gray. Geographic Information System (GIS) - A system for archiving, retrieving, and manipulating data that has been stored and indexed according to the geographic coordinates of its elements. The system generally can utilize a variety of data types, such as imagery, maps, table, etc. Georeference* - To assign coordinates from a reference system, such as latitude/longitude, to the page coordinates of an image or map. Global Positioning System (GPS) ** - A satellite-based global navigation system that consists of (a) a constellation of 24 satellites in orbit 11,000 nmi above the Earth, (b) several on-station (i.e., in-orbit) spares, and (c) a ground-based control segment. The satellites transmit signals that are used for extremely accurate three-dimensional (latitude, longitude, and elevation) global navigation (position determination), and for the dissemination of precise time. GPS-derived position determination is based on the arrival times, at an appropriate receiver, of precisely timed signals from the satellites that are above the user's radio horizon. Hyperspectral Scanner (HSS) *** - The use of many narrow sections of the electromagnetic spectrum in remote sensing. Image - Pictorial representation of data acquired by satellite systems, such as direct readout images from environmental satellites. An image is not a photograph. An image is composed of two-dimensional grids of individual picture elements (pixels). Each pixel has a numeric value that corresponds to the radiance or temperature of the specific ground area it depicts. See gray scale. Image Resolution - The area represented by each pixel of a satellite image. The smaller the area represented by a pixel, the more accurate and detailed the image. For example, if a U.S. map and a world map are printed on identically sized sheets of paper, one square inch on the U.S. map will represent far less area and provide for more detail than one square inch on the world map. In this example the U.S. map has higher resolution. APT has a resolution of 4 km, HRPT has a resolution of 1.1 km and WEFAX resolution is 8 km. Imager - A satellite instrument that measures and maps the Earth and its atmosphere. Imager data are converted by computer into pictures. Landsat - Land Remote-Sensing Satellite, operated by the U.S. Earth Observation Satellite Company (EOSAT). Commercialized under the Land Remote-Sensing Commercialization Act of 1984, Landsat is a series of satellites (formerly called ERTS) designed to gather data on the Earth's resources in a regular and systematic manner. Objectives of the mission are: land use inventory, geological/mineralogical exploration, crop and forestry assessment, and cartography. Landsat has a spatial resolution of 28.5 meters. Restructured Federal agency responsibilities for the Landsat program are effective for the acquisition and operation of Landsat 7. New operating policy specifies that NOAA will be responsible for satellites after they are placed in orbit, NASA will be responsible for the development and launch of Landsat 7, and that the U.S. government will provide unenhanced data to users at no cost beyond the cost of fulfilling their data request. Multi-Spectral Scanner (MSS) - A line-scanning instrument flown on Landsat satellites that continually scans the Earth in a 185 km. (100 nautical miles) swath. On Landsats 1, 2, 4, and 5, the MSS had four spectral bands in the visible and near infrared with an IFOV of 80 meters. Landsat-3 had a fifth band in the thermal infrared with an IFOV of 240 meters. Radiation - Energy transfer in the form of electromagnetic waves or particles that release energy when absorbed by an object. Remote Sensing - The technology of acquiring data and information about an object or phenomena by a device that is not in physical contact with it. In other words, remote sensing refers to gathering information about the Earth and its environment from a distance, a critical capability of the Earth Observing System. For example, spacecraft in low-Earth orbit pass through the outer thermosphere, enabling direct sampling of chemical species there. These samples have been used extensively to develop an understanding of thermospheric properties. Explorer-17, launched in 1963, was the first satellite to return quantitative measurements of gaseous stratification in the thermosphere. However, the mesosphere and lower layers cannot be probed directly in this way--global observations from space require remote sensing from a spacecraft at an altitude well above the mesopause. The formidable technological challenges of atmospheric remote sensing, many of which are now being overcome, have delayed detailed study of the stratosphere and mesosphere by comparison with thermospheric research advances. Some remote-sensing systems encountered in everyday life include the human eye and brain, and photographic and video cameras. Resolution - A measure of the ability to separate observable quantities. In the case of imagery, it describes the area represented by each pixel of an image. The smaller the area represented by a pixel, the more accurate and detailed the image. APT has a resolution of 4 km, i.e., each pixel represents a square, 4 km on each side. HRPT has a resolution of 1.1 km at nadir (4 km at edge of scan), and WEFAX of 8 km. See resolution cell. Resolution Cell - The smallest unit of area in an image of discrete elements. The area represented by a pixel. Sensor - Device that produces an output (usually electrical) in response to stimulus such as incident radiation. Sensors aboard satellites obtain information about features and objects on Earth by detecting radiation reflected or emitted in different bands of the electromagnetic spectrum. Analyzing the transmitted data provides valuable scientific information about Earth. Weather satellites commonly carry radiometers, which measure radiation from snow, ice, clouds, and bodies of water. Spaceborne radars are used for Earth observations, bouncing radar waves off land and ocean surfaces to study sea-surface conditions, ice thickness, and land surface features. A wind scatterometer is a special type of radar designed to measure ocean surface winds indirectly by bouncing signals off the water and measuring them from various angles. Infrared (IR) detectors measure heat generated by Earth features in the IR band of the spectrum. Photographic reconnaissance sensors in their simplest form are large telescope-camera systems used to view objects on Earth's surface. The bigger the lens, the smaller the object that can be detected. Camera-telescope systems now incorporate all sorts of sophisticated electronics to produce better images, but even these systems need cloudless skies, excellent lighting, and good color contrast between objects and their surroundings to detect objects the size of a basketball. Some of the satellites produce film images that must be returned to Earth, but a more convenient method is to record the image as a series of digital code numbers, then reconstruct the image from the electronic code using a computer at a ground station. Spectral Band - A finite segment of wavelengths in the electromagnetic spectrum. Spectrum - The series of colored bands diffracted and arranged in the order of their respective wave lengths by the passage of white light through a prism or other diffracting medium and shading continuously from red (produced by the longest visible wave) to violet (produced by the shortest visible wave). - Any of various arrangements of colored bands or lines, together with invisible components at both ends of the spectrum, similarly formed by light from incandescent gases or other sources of radiant energy, which can be studied by a spectrograph. - In radio, the range of wavelengths of radio waves, from 3 centimeters to 30,000 meters, or of frequencies of radio waves, from 10 to 10,000,000 kilocycles. Also radio spectrum. - The entire range of radiant energies. See electromagnetic spectrum.
SOURCES:http://www.casde.unl.edu/vn/glossary/intro.htm* http://www.forsyth.k12.ga.us/schools/liberty/dmashburn/define.htm** http://www.atis.org/tg2k/_gps.html *** http://en.mimi.hu/gis/hyperspectral.html |