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Remote sensing is one of a suite of geospatial technologies that are having a growing impact in a wide variety of areas from commerce to science in public policy. While remote sensing’s origins and technological advance are an outgrowth of military and intelligence applications, commercial and scientiﬁc applications of the technology have been growing. The ﬁeld of remote sensing evolved from the interpretation of aerial photographs to the analysis of satellite imagery, and from local area studies to global analyses, with advances in sensor system technologies and digital computing. Today remote sensor systems can provide data from energy emitted, reﬂected, and/or transmitted from all parts of the electromagnetic spectrum. These data can then be transformed into information products either by manual or machine-assisted interpretation and employed in an analytical or management decision-making context. Examples of applications of these data include population and demography studies, study of archaeological sites, energy studies using hydrological models, urban planning, environmental treaty enforcement, and agricultural yields. This research paper provides a brief overview of the historical development of remote sensing, emphasizing the increasing complexity of platforms, systems, and tasks. The information provided here has been assembled from previously published sources to give an overview of a rapidly evolving ﬁeld that has had, and will in the future have, an increasing impact on our global society.
The deﬁnition of remote sensing used here, provided by the American Society for Photogrammetry and Remote Sensing (ASPRS), is:
In the broadest sense, the measurement or acquisition of information of some property of an object or phenomena, by a recording device that is not in physical or intimate contact with the object or phenomenon under study; e.g., the utilization at a distance (as from aircraft, spacecraft, or ship) of any device and its attendant display for gathering information pertinent to the environment, such as measurements of force ﬁelds, electromagnetic radiation, or acoustic energy. The technique employs such devices as the camera, lasers, and radio frequency receivers, radar systems, sonar, seismographs, magnetometers, and scintillation counters. (Reeves et al. 1975)
What follows is a brief summary of various sensor systems within the context of an historical background of remote sensing, and a short discussion of the future of remote sensing with some reference to social and behavioral sciences.
1. Sensor Systems
Today, a wide variety of systems are used to detect energy patterns, which provide information about the Earth’s atmosphere, oceans, and land surfaces. These systems range from cameras in aircraft to advanced sensor systems in Earth-orbiting satellites. The aerial camera systems typically used for remote sensing research and applications today are highly sophisticated. Most satellite remotely sensed data are acquired by sensor systems such as multispectral scanners, linear array devices, and synthetic aperture radar systems that operate in the visible, near infrared, thermal infrared, and microwave portions of the electromagnetic spectrum.
These remote sensor systems can be broadly categorized into passive systems and active systems. A camera is an example of a passive system as its ﬁlm records light energy reﬂected from an existing source: the sun. Add a ﬂash attachment to the camera and it becomes an active system, since it now provides its own source of illumination. Passive systems record solar radiation reﬂected from the Earth’s surface. Data derived from passive systems such as multispectral scanners provide information on, among other things, land use, vegetation types, distribution and condition of landforms, soils, surface waters, and river networks. Active microwave (radar) systems are used commonly in geological and hydrological applications. Because active systems do not depend on reﬂected energy from the sun they may be able to acquire data during the day or night. In addition, as the atmosphere does not attenuate microwave radiation as much as shorter wavelength energy, radar systems allow data acquisition in cloudy and rainy weather. This ability to operate day or night, and to create images of the Earth’s surface in spite of cloud cover, makes radar sensors particularly attractive tools for improving our understanding of both tropical and polar areas.
Thermal infrared data, which represent a record of emitted energy from surfaces, have been particularly useful in monitoring ﬁres and in improving our understanding of areas of volcanic and geothermal activity. Surface temperature of the ocean is also related to the dynamics of coastal waters and currents. Over land, plant water stress also induces changes in canopy temperatures, which can be detected by thermal sensors. Thermal imagery from satellites has been used to record the nighttime lights of the globe, giving researchers an indication of the spatial distribution of population centers. Other remote sensing systems detect the earth’s magnetic and gravitational ﬁelds; these tools are used extensively in oil and mineral exploration. Employing imagery from more than one portion of the electromagnetic spectrum can increase the information derived from an analysis, and is typically referred to as multispectral image analysis. Analyses of such multispectral data can, if properly designed, increase both the quantity and quality of information for given applications.
2. Image Interpretation
Interpretation of remotely sensed data is part science and part art. Manual interpretation of remotely sensed data is fundamental to all image interpretation. Even with the ever-increasing sophistication of machine assisted image interpretation software, it is still the human that ultimately must label the feature or phenomenon being identiﬁed. Not all individuals have equal interpretive skills with respect to aerial photographs or images acquired by remote sensor systems. It is axiomatic, however, that the more one knows about the elements, techniques and methods of image analysis the better an interpreter one will be. In addition, the more one knows about the systems which acquire the data one is interpreting and the subject or discipline area most closely related to the interpretation, the better an interpreter one will make. It is seldom that an interpreter uses only the information in the imagery being analyzed as the basis for their interpretation. Ancillary data in the form of interpretation keys, maps, diagrams, textual material, and other forms of site-speciﬁc literature often aid in interpretations. Fieldwork is also a key element in image interpretation and the collection of what is termed ‘ground truth’ should be accomplished whenever possible and practical.
Resolution is at the core of the successful use of remote sensing data. Resolution, as typically applied by a user, normally involves the question ‘can I identify what I need to identify on this imagery?’ Often this involves the use of the term ground resolved distance (GRD). GRD is used to describe the smallest object that can be unambiguously identiﬁed on the ground given good object to background contrast. Object to background contrast is important here. While it may be possible to see white lines on a black road on one set of images, it may not be possible to see the same size white lines on a concrete stretch of the same road. Resolution is thus dependent upon many things.
Indeed, the level of matching between the spatial, spectral, and temporal resolutions of the measurements and needs of the investigation will determine the usefulness of the derived data information. Spectral range and resolution as used here refer to the portion of the electromagnetic spectrum being used in the measurement. It must be appropriate to the questions being addressed, for example, thermal range for ﬁre detection, visible or infrared for vegetation health and conditi0on. Spatial resolution determines the level of detail that can be extracted concerning objects in a given scene. As a general rule it must correspond to the sizes of typical objects that must be separately identiﬁed. A resolution of a meter or less is needed to identify a single tree, a resolution of ﬁve meters for a single-family residence, a resolution of 50 meters might be suﬃcient for identiﬁcation of most agricultural ﬁelds in North America, while a resolution of 500 meters to a kilometer may be enough for studying ocean surface features. A necessary compromise between area to be covered and ground resolution of the measurement must take into account the volume of data which will be genera ted by, say, a large area coverage—millions of km —using high resolution data—one to 30 meters.
The frequency of the data acquisition must also match the natural frequency of change in the landscape and the physical, social, or cultural phenomena of interest. For traﬃc studies, timeframes of minutes can be important. Hourly observations might be of interest for certain meteorological conditions. Daily observations may be needed for an assessment of plant evapotranspiration associated with irrigated agricultural crops, while observation once every year could be suﬃcient for land cover change assessment.
2.2 Geographic Information Systems
The synergy between geographic information systems (GIS) and remote sensing comes into play here. To be interpreted accurately, remotely sensed data are often supplemented with other data. Often these ancillary geospatial data can be found or included in a GIS for analysis. But to be more valuable in decision-making contexts, GIS data layers should be up-to-date as is practical. Remotely sensed data are a key technology for updating many types of GIS data. Thus when environmental planners, resource managers, and public policy decision-makers want to measure, map, monitor, or model future scenarios in order to facilitate better management decision-making, remote sensing is being employed more and more within the context of a GIS as a decision support system.
3. Historical Development
In the most general sense, the ﬁeld of remote sensing begins with the invention of photography. Joseph Nicephore Niepce took the ﬁrst photograph of nature in 1827. It is a picture of the courtyard of his home in the village of Saint Loup de Varenne, France, with an exposure time of eight hours. In 1829 Niepce and Louis M. Daguerre signed a partnership agreement to work on a practical method of taking photographs. Niepce had been working on what he called Heliography, or sun drawing, and Daguerre on dioramas, which he constructed with the aid of a camera obscura. The process they pioneered was called direct positive image. The result of their collaboration, called the daguerreotype, was ﬁnalized in 1839. Also in 1839, an Englishman named William H. Fox-Talbot and a member of the Royal Academy invented the calotype (from the Greek: kalos for beautiful and typos for impression). Talbot’s positive negative process is essentially the same as that used today, yet at the time the images produced by the two processes were not comparable.
In 1858, Gaspar Felix Tournachon, also known as Nadar a Parisian photographer and balloonist, took the ﬁrst known aerial photograph from a captive balloon at an altitude of 1,200 feet over Paris. During the late 1870s, German foresters used aerial photography taken from balloons to both map and measure the aerial extent of tree species. This is an example of the earliest known application of both aerial photographic interpretation and photogrammetry (the science of making precise measurements from remotely sensed data).
The use of rockets to launch cameras ﬁrst occurred in 1906 when Albert Maul launched a rocket propelled by compressed air that was equipped with a camera. The camera took a picture from an altitude of 2,600 feet and the camera was then returned to the ground by parachute. In 1909, Wilbur Wright took one of the ﬁrst photographs from an airplane in Centrocelli, Italy. By 1915, cameras devoted to aerial photography were being produced. World War One saw an explosion in the use of aerial photographs. During the Meuse Argonne oﬀensive some 55,000 aerial photographs were taken and delivered to Allied Expeditionary Forces over a four-day period. During this period thousands of photo interpreters were trained. These interpreters laid the groundwork for the applications of aerial photographic interpretation that began to expand after the end of World War One.
In the early 1920s the ﬁrst books and articles on the applications of aerial photo interpretation began to be published. Lee’s 1922 The Face of the Earth As Seen From the Air work ‘shows familiar scenes from a new angle,’ and contains chapters on mountain features, coastal mud ﬂats, plains, and submerged landforms. Joerg’s 1927 work, ‘The use of the air plane in city geography,’ was published in the Annals of the Association of American Geographers. This work focused on the application of aerial photography for urban studies. In 1927 Bergen published his work on ‘Aerial surveys for city planning’ in the Transactions of the American Society of Civil Engineers, and in 1930 the American Geographical Society published the work by Platt and Johnson, ‘Peru from the Air.’ While providing a regional overview of the country this work also focuses on archeological sites. Of a more general nature is the work by Fairchild (1927) in the May issue of the Annals of the American Academy of Political and Social Sciences entitled: ‘Aerial photography, its development and future.’ This research paper discusses potential applications of aerial photography from highway planning to water supply management. During this period, however, by far the foremost applications of aerial photography discussed in the literature are for mapping and geological exploration, particularly on the uses of aerial photography in oil exploration, structural and engineering geological analyses, land feature mapping, and soils mapping.
It is in the late 1920s and early 1930s that the agencies of the US Government begin to make signiﬁcant use of aerial photography for operational applications. At this time the Agricultural Adjustment Administration began to systematically photograph farm and ranch lands across the entire USA. This use of aerial photography for documenting agriculturally active areas has continued at ﬁve to 10-year intervals since this time. Thishistorical record has been very signiﬁcant in many types of studies, but has been particularly useful in a variety of legal actions where the documentation of land use and/or land cover conditions and changes through time are in question. At the same time, the US Forest Service began to use aerial photography to document much of the nation’s timber reserves. Studies such as this began in other nations as well, notably Canada. In this timeframe the US Geological Survey began to photograph many areas of the country in order to make topographical and geological maps based upon photogrammetric techniques and interpretation. The Tennessee Valley Authority and other regional agencies began to use aerial photography for planning purposes, as did state and metropolitan planning agencies. The Master Plan of Residential Land Use, published in 1943 by the Chicago Planning Commission, was based largely on data interpreted from aerial photographs.
World War Two led to developments in aerial photography, with a special interest in new and higher resolution ﬁlm emulsions and thermal infrared imagery. Kodak patented the ﬁrst false color infrared sensitive ﬁlm in 1946. The 1950s saw developments in technology that allowed multispectral imagery to be taken. Thermal infrared and side-looking airborne radar systems began to be acquired by the US military. Around this time, research personnel at the US Oﬃce of Naval Research coined the term ‘remote sensing’ to cover the expanding range of sensor systems and technologies for acquiring images of the world around us. This was the same year that the Soviet Union launched Sputnik, the ﬁrst Earth-orbiting satellite. Civilian use of remotely sensed data during this period produced some of the ﬁrst studies on the application of aerial photography for population estimation.
As early as 1946 the US Government began to pursue an operational space-based imaging system when the US Air Force asked the RAND Corporation to explore the possibility of launching an object into orbit. An early experiment in satellite land remote sensing was conducted from the Explorer 6 platform in 1959. The synoptic images of Earth’s weather patterns acquired by the Television and Infrared Observational Satellite (TIROS-1), launched in April of 1960, captivated the American public and demonstrated the capability and necessity of earth observation from space. Also in 1960 the ﬁrst experimental spy satellite, a component of what would later become known as the Corona Program, was launched and, shortly thereafter, classiﬁed as secret. These data were withheld from the public until 1995, when the imagery and documentation were released to the public under Executive Order 12951. In 1965 NASA initiated the Earth Resources Survey Program (ERS). This early program sought to develop methods and applications for remote sensing. The Department of Agriculture researched the use of remote sensing for geological, hydrological, geographical, and cartographical applications. In 1966 the Department of Commerce began to participate in the ERS program by establishing the Environmental Sciences Group within the Environmental Sciences Administration (the predecessor to NOAA). Also in 1966, the Department of the Interior initiated the multi-agency Earth Resources Observation Satellite Program, which studied the potential of Earth-observing satellites for natural resources applications.
During the 1960s and into the mid-1970s NASA and a number of other Federal Agencies focused a signiﬁcant amount of attention on applied remote sensing research. This was, in part, aimed at justifying the need for civil land satellite remote sensing systems and cataloging the variety of fundamental and applied science questions that could be addressed by such systems. It is during this period that eﬀorts began to develop computer-assisted image interpretation techniques. Results of these activities, and the state-of-theart practice of remote sensing during this time period, are documented in the American Society of Photogrammetry’s ﬁrst edition of the Manual of Remote Sensing (1975), as well as other sources.
The ﬁrst Earth Resources Technology Satellite (ERTS-1) was designed originally as a research tool. When it was launched in July of 1972, however, this satellite ushered in a brief era of emphasis by NASA on the demonstration of operational applications using this series of satellites. Through 1974, ERTS-1 (later renamed Landsat 1) transmitted over 100,000 terrestrial images covering 75 percent of the Earth’s land surface. Between 1972 and 1978, major research eﬀorts utilizing this new civil land remote sensing data were conducted in agriculture, forestry, water resources, and geology. Other notable regional-scale applications research in the 1970s included the Large Area Crop Inventory Experiment (LACIE); Agricultural and Resource Inventory Surveys Through Aerospace Remote Sensing (AgRISTARS); and the Western Snow Melt Application System Veriﬁcation Test (ASVT).
In 1975, Landsat 2 was launched and the Ford Administration approved budget appropriations for Landsat 3, which was launched in 1978. In March of 1979, President Carter announced the Administration’s commitment to maintaining the continuity of civilian terrestrial satellite remotely sensed data and recommended the transition of the program to the private sector. This initiated the process of program commercialization that would come to fruition in the mid-1980s. In 1981, the Reagan Administration accelerated the pace of Landsat commercialization by essentially rejecting the Carter Administration’s commitment to an operational civilian land remote sensing program and advocating the immediate end of government funding for the Landsat Program.
By the middle of the 1980s, it was clear that satellite remote sensing could provide globally consistent, spatially disaggregated, and temporally repetitive coverage of the land surface of the Earth. Landsat 4, with the ﬁrst Thematic Mapper instrument, was launched in 1982. Higher spatial resolution multispectral imagery was now publicly available and could be applied by scientists and professionals to the assessment of terrestrial ecosystems and land cover. A number of researchers began to develop techniques to employ the spectral information from Landsat to measure green vegetation status, and phenology, as well as interannual variation in these parameters. These studies then led researchers to the development of vegetation measures that provide an index of condition and, in part, describe absorption of sunlight, photosynthetic capacity, and evaporation rates. With this research, the science community began to understand that space-based Earth imaging can and does provide such information concerning the spatial and temporal patterns associated with land processes. During this period of capability demonstration, researchers noted that the possibility of developing fully integrated land–ocean–atmosphere monitoring and modeling capabilities were essentially realized.
In March 1984, Landsat 5 was launched and in July of that year the US Congress provided the legislative authority to transfer the Landsat program to the private sector. Thus began the nearly decade long period of ‘commercialization’ of the US civil land remote sensing program. Commercialization (and the subsequent price increases for Landsat data) caused a shift in scientiﬁc research emphasis. Researchers began to move away from Landsat imagery and began to design experiments utilizing NOAA polar orbiting meteorological satellites and their Advanced Very High Resolution Radiometer (AVHRR) sensor system as a primary data source for scientiﬁc research. This shift was the result of increased costs, greater restrictions on sharing and more limited availability of Landsat data to both the basic and applied land remote sensing community.
The US Government would not regain operational control of the Landsat program until the late 1990s. Essentially, though the law changed in 1992, the governmental control of any orbital assets did not occur until Landsat 7 was launched in April 1999. The commercial pricing and data sharing restrictions prior to this transition back to government pricing had a negative eﬀect on the developing research and application programs at the time which sought to utilize satellite imagery but could not employ it due to the high data cost. It would not be until the twenty-ﬁrst century that the Landsat program would begin to come under the control of an operational land oriented federal agency, the United States Geological Survey (USGS).
Despite these diﬃculties, research using Landsat data during this time included work in many disciplines and application areas. Organizations important in formulating the types of research conducted at this time include the Intergovernmental Panel on Climate Change (IPCC), the International Council of Scientiﬁc Unions (ICSU), the International Geosphere Biosphere Program (IGBP), and the Human Dimensions of Global Change (HDGC) project. Of particular signiﬁcance was the research that validated the use of multitemporal Landsat and other data to monitor and assess tropical deforestation; and also unambiguously demonstrated the value of time series data to provide information on change occurring in every area of the world.
Other countries entered the space imaging arena with the launch of the French Systeme Probatoire de la Observation de la Terre (SPOT) satellite in 1986 and the Indian Remote Sensing Satellite (IRS-1) in 1988. In 1990, the French launched SPOT 2; a year later the European Radar Satellite (ERS-1) was launched, aimed primarily at oceanographic applications, followed shortly after by the Russian radar satellite Almaz; 1992 saw the launch by the Japanese of the ﬁrst Japanese Earth Resources Satellite (JERS-1). In 1995, ERS-2 was launched and Canada launched Radarsat, with a primary objective of aiding navigation of shipping in the far north.
The most recent advances, at the end of the twentieth century, involved the launch by US agencies and organizations of Landsat 7 with the Enhanced Thematic Mapper (ETM ) system, the Shuttle Interferometeric Synthetic Aperature Radar (IFSAR) mission, Earth Observing System-1 (now named Terra), and the one-meter commercial IKONOS satellite (all in 1999). The Landsat ETM data are already proving useful in a variety of studies from ﬁre fuels mapping to deforestation and urban change monitoring. IFSAR data will be employed to improve the quality of topographical maps between about 60 N and 60 S of the Equator. Terra data is being employed to give continental to global-scale land cover change information with a temporal frequency never before available. IKONOS data give civil researchers the opportunity to see features and objects from space that could only be seen if aerial photography or intelligence satellite data were available to the user. These satellites will add immeasurably to our ability to measure, map, monitor, and model our Earth system as never before.
4. Future Directions
What is even more important is that by 2006, some 20 governments and organizations are expected to launch civilian and commercial high spatial resolution satellites in an attempt to capture a share of the growing market for remote sensing imagery. These satellites will have sensor systems with spatial resolutions that range from several kilometers to less than a meter. It is clear from developments such as these that a number of the long-standing restrictions that the military and intelligence community placed on satellite remote sensing are being put aside as technology advances and becomes more accessible, creating greater transparency among nations of the world. With this increasing transparency the opportunity for both positive and negative uses of remotely sensed data increase. On the one hand it may be easier to detect and identify violations of international agreements, while on the other it may be easier for states to gain tactically important information on their neighboring states.
There is already a growing interest in these data among social, economic, and behavioral users. This interest stems from the realization that whereas remotely sensed data cannot provide all the data information needed to resolve or answer given questions it may, when combined with other information, allow data to be more reliably interpreted across space and through time. Indeed, an important aspect of remote sensing is its value as a historical record— applications in legal proceedings involving environmental contamination and ownership liability are one example. There is a growing diversity of applications to which remotely sensed data may be applied, in social, economic, or behavioral science, from population estimation, famine early warning and disaster assessments to inventorying industrial capacity, assessing transportation infrastructure, and monitoring of environmental laws and treaties.
There have been a great many advances in the ﬁeld of remote sensing over the years since the invention of the camera. Yet, from an analysis of the materials covering the ﬁeld of remote sensing, it is clearly evident that users interested in questions in the physical and biological sciences have been (and continue to be) able to use these data more than users in the social, economic, and behavioral sciences.
The reasons for this disparity are many and varied. They range from the inability of remote sensing to provide needed data information to practitioners in the social, economic, and behavioral sciences to an unwillingness on the part of users in these areas to take the time to learn both the capabilities and limitations of the information that can be extracted from remotely sensed data. For some applications the spatial and temporal scales of data acquisition are not appropriate; for others the models to translate the data collected into meaningful information speciﬁc to a given need have yet to be developed. Remote sensing cannot see what is in the human mind. It can, however, document actions that we as human beings take in response to some stimuli. For example, it cannot tell us what crop a farmer may plant in a given year, but it can, in many instances, given appropriate dates of image acquisition and timely interpretation, tell us what has been planted before the crop is harvested. Indeed, given adequate ancillary data and models it can also give us an indication of how good the farmer’s harvest may be.
As our ability to collect data at higher spatial, spectral, and temporal resolutions increases, so too does our ability to derive useful information from remote sensing data. Remote sensing will never replace other, more traditional methods of data collection, such as census and survey instruments. Yet it is a proven, powerful method of collecting data which, when properly interpreted and evaluated, can provide a wide variety of useful information. Information which, when taken alone or when augmented by other data information/collected by more traditional means, can create more useful data sets for social, economic, and behavioral scientists.
5. Organizations That Pro Ide Remotely Sensed Data
- Center for International Earth Science Information Network (CIESIN)
- US National Aeronautics and Space Administration (NASA)
- US Geological Survey (USGS)
- SPOT Image
- US National Oceanographic and Atmospheric Administration (NOAA)
- Space Imaging
- Bergen E T 1927 Aerial surveys for city planners. Transaction of the American Society of Civil Engineers 90
- Chicago Planning Commission 1943 Master Plan of Residential Land Use. Chicago Planning Commission, Chicago
- Estes J E, Ehlers M, Malingreau J-P, Noble I, Raper J, Sellman A, Star J L, Weber J 1992 Advanced Data Acquisition and Analysis Technologies for Sustainable Development. MAB Digest 12, UNESCO, Paris
- Fairchild S M 1927 Aerial photography, its development and future. Annals of the American Academy of Political and Social Sciences 131 (May)
- Jensen J R 1996 Introductory Digital Image Processing: A Remote Sensing Perspective 2nd edn. Prentice Hall, Upper Saddle River, NJ
- Jensen J R 2000 Remote Sensing of the Environment: An Earth Resource Perspective. Prentice Hall, Upper Saddle River, NJ
- Joerg E L E 1923 The use of the air plane in city geography. Annals of the Association of American Geographers 13(4)
- Lee W T 1922 The Face of the Earth As Seen from the Air: A Study in the Application of Airplane Photography to Geography. American Geographical Society, New York
- Liverman D, Moran E F, Rindfuss R R, Stern P C (eds.) 1998 People and Pixels: Linking Remote Sensing and Social Science. National Academy Press, Washington, DC
- National Research Council 1970 Remote Sensing with Special Reference to Agricutlure and Forestry. National Academy of Sciences, Washington, DC
- Philipson W (ed.) 1997 Manual of Photographic Interpretation. 2nd edn American Society for Photogrammetry and Remote Sensing, Bethesda, MD
- Platt R R, Johnson G R 1930 Peru from the Air. American Geographical Society, New York
- Reeves R G, Anson A, Landen D (eds.) 1975 Manual of Remote Sensing. American Society of Photogrammetry, Falls Church, VA