REMOTE SENSING

Information on characteristics of landscape and human settlement can be derived from aerial photography and satellite imagery. The technology and procedures involved are known as remote sensing.

Resolution and Bandwidth

The images produced by remote sensing can be classified by resolution and bandwidth. Resolution refers to the size of the image captured by the smallest pixel (picture element) in the image. The highest resolution data commercially available from satellite-based sensors as of around 2000 is one meter–that is, the smallest pixel in the image corresponds to an area of 1m by 1m on the ground. More detailed imagery typically requires the use of aerial photography. Bandwidth refers to the wavelengths recorded by the sensor. They may be panchromatic (producing black-and-white images) or specific to certain parts of the spectrum, such as visible red, green, blue, and near-infrared bands, or other (longer or shorter) bands that are not visible to the naked eye. If information is recorded for two or more wavelength bands, the image is described as multispectral.

Each of the various land cover categories on the earth's surface, whether natural or built, has a distinctive "spectral signature," the combination of wavelength values characteristic of that category of surface (such as bare soil, a specific type of vegetation, water, or an impervious surface like asphalt or concrete) but not of others. The aim in using remotely sensed data is to associate each pixel with a particular type of land cover. The higher the resolution (i.e., the smaller the pixel size), the more likely it is that the pixel will include only one type of land cover, and hence can be unambiguously categorized. The process of categorization is a crucial operation involving sophisticated statistical procedures, including new approaches drawing on "fuzzy" analysis.

Major producers of high resolution panchromatic and multispectral satellite imagery include the Landsat Thematic Mapper images and IKONOS (USA), Spot (France), Indian Remote Sensing (India), and Spin-2 (Russia). The Earth Observing-1 (EO-1) satellite launched by NASA in 2000 is capable of measuring 220 spectral bands at 30m resolution.

Remote Sensing in Demographic Analysis

For demographic analysis it is necessary to combine the data from remotely sensed images with locallevel information from censuses, vital statistics, surveys, or administrative data. This is done within a geographic information system (GIS), which allows the matching of two or more sources of data for each small geographic area such as a census tract.

Remote sensing has been used in a variety of ways in demographic analysis, particularly for estimating population size and distribution, assessing the human impact on the natural environment, and examining characteristics of urban settlement.

A number of remotely sensed characteristics can be used to indicate the spatial distribution of population. The visible and near-infrared emissions of nighttime lights is one such indicator. A broader array of characteristics was drawn on in producing the Land Scan Global Population 1998 Database developed by Oak Ridge National Laboratory, Tennessee. This database has a resolution of 30 arc seconds–approximately 1 km². Population data for larger geographic units come from ground-based sources, but the allocation over these 1 km cells is made on the basis of remotely detected characteristics of those cells: land cover, road proximity, slope, and nighttime lights.

Monitoring changes in the natural environment is one of the major uses of remote imagery. Such changes are often linked to expansion of human settlement and economic activity. The important case of deforestation in the Amazon region is explored by Tom Evans and Emilio Moran (2002) and Steven Walsh and Kelly Crews-Meyer (2002).

In urban applications, by use of a variety of indicators remote imaging allows the specification of gradations of "urbanness," avoiding the conventional urban-rural dichotomy. The ways in which remote sensing can be used in urban areas are summarized by Jean-Paul Donnay, Michael Barnsley, and Paul Longley (2001). A case study of Cairo is given by Tarek Rashed and colleagues (2001).

Remote sensing cannot reveal what people on the ground are doing. It offers only clues to that behavior, and proxies for variables such as the level of economic development or degree of urbanization that in turn may be predictors of that behavior. Its usefulness is as a tool that extends the spatial scope of social science analysis.

The value to demography of remotely sensed imagery depends not only on the quality and quantity of data that can be derived but also on the uses to which it can be put. To expand the latter requires further investigation of the connection between built and social environments and a better understanding of the spatial dimensions of human behavior. These are active areas of research. Advances in GIS techniques and applications will also promote greater use of remote sensing. It seems likely that remote sensing imagery will eventually be seen as a routine additional source of demographic information.

BIBLIOGRAPHY

Donnay, Jean-Paul, Michael J. Barnsley, and Paul A. Longley. 2001. Remote Sensing and Urban Analysis. London: Taylor and Francis.

Entwisle, Barbara, Ronald R. Rindfuss, Stephan J. Walsh, Tom P. Evans, and Sara R. Curran. 1997. "Geographic Information Systems, Spatial Network Analysis, and Contraceptive Choice." Demography 34: 171–188.

Evans, Tom P., and Emilio F. Moran. 2002. "Spatial Integration of Social and Biophysical Factors Related to Landcover Change." In Population and Environment: Methods of Analysis, Supplement to Population and Development Review, Vol. 28, ed. Wolfgang Lutz, Alexia Prskawetz, and Warren C. Sanderson. New York: Population Council.

Jensen, John R., and Dave C. Cowen. 1999. "Remote Sensing of Urban/Suburban Infrastructure and Social-economic Attributes." Photogrammetric Engineering and Remote Sensing 65: 611–624.

Liverman, Diana, Emilio F. Moran, Ronald R. Rindfuss, and Paul C. Stern, eds. 1998. People and Pixels: Linking Remote Sensing and Social Science. Washington, D.C.: National Academy Press.

Rashed, Tarek, John R. Weeks, M. Saad Gadalla, and Allan G. Hill. 2001. "Revealing the Anatomy of Cities Through Spectral Mixture Analysis of Multispectral Imagery: A Case Study of the Greater Cairo Region, Egypt." Geocarto International 16: 5–16.

Sutton, Paul. 1997. "Modeling Population Density with Night-Time Satellite Imagery and GIS." Computing, Environment and Urban Systems 21: 227–244.

Walsh, Steven J., and Kelly Crews-Meyer, eds. 2002. Linking People, Place, and Society. Boston: Kluwer Academic Publishers.

Weeks, John R., M. Saad Gadalla, Tarek Rashed, James Stanforth, and Allan G. Hill. 2000. "Spatial Variability in Fertility in Menoufia, Egypt, Assessed Through the Application of Remote Sensing and GIS Technologies." Environment and Planning A32: 695–714.

INTERNET RESOURCE.

Land Scan Global Population Database. Oak Ridge National Laboratory. 1998. <http://www.ornl.gov/gist/projects/LandScan/SIMPLE/smaps.htm>.