A scale-adjusted measure of “Urban sprawl” using nighttime satellite imagery

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Abstract

“Urban Sprawl” is a growing concern of citizens, environmental organizations, and governments. Negative impacts often attributed to urban sprawl are traffic congestion, loss of open space, and increased pollutant runoff into natural waterways. Definitions of “Urban Sprawl” range from local patterns of land use and development to aggregate measures of per capita land consumption for given contiguous urban areas (UA). This research creates a measure of per capita land use consumption as an aggregate index for the spatially contiguous urban areas of the conterminous United States with population of 50,000 or greater. Nighttime satellite imagery obtained by the Defense Meteorological Satellite Program's Operational Linescan System (DMSP OLS) is used as a proxy measure of urban extent. The corresponding population of these urban areas is derived from a grid of the block group level data from the 1990 U.S. Census. These numbers are used to develop a regression equation between Ln(Urban Area) and Ln(Urban Population). The ‘scale-adjustment’ mentioned in the title characterizes the “Urban Sprawl” of each of the urban areas by how far above or below they are on the “Sprawl Line” determined by this regression. This “Sprawl Line” allows for a more fair comparison of “Urban Sprawl” between larger and smaller metropolitan areas because a simple measure of per capita land consumption or population density does not account for the natural increase in aggregate population density that occurs as cities grow in population. Cities that have more “Urban Sprawl” by this measure tended to be inland and Midwestern cities such as Minneapolis–St. Paul, Atlanta, Dallas–Ft. Worth, St. Louis, and Kansas City. Surprisingly, west coast cities including Los Angeles had some of the lowest levels of “Urban Sprawl” by this measure. There were many low light levels seen in the nighttime imagery around these major urban areas that were not included in either of the two definitions of urban extent used in this study. These areas may represent a growing commuter-shed of urban workers who do not live in the urban core but nonetheless contribute to many of the impacts typically attributed to “Urban Sprawl”. “Urban Sprawl” is difficult to define precisely partly because public perception of sprawl is likely derived from local land use planning decisions, spatio-demographic change in growing urban areas, and changing values and social mores resulting from differential rates of international migration to the urban areas of the United States. Nonetheless, the aggregate measures derived here are somewhat different than similar previously used measures in that they are ‘scale-adjusted’; also, the spatial patterns of “Urban Sprawl” shown here shed some insight and raise interesting questions about how the dynamics of “Urban Sprawl” are changing.

Introduction

The issue of what is commonly referred to as “Urban sprawl” is gaining increasing attention and concern from citizens, environmental organizations, and governments (http://www.sierraclub.org/sprawl/; http://www.vtsprawl.org/index3.htm; (Benfield et al., 2001)). Concerns are raised about the impact urban sprawl has on the loss of open space, traffic congestion, and energy consumption. Nonetheless, specific, measurable, and generally accepted definitions of urban sprawl are difficult to find. William Whyte's 1958 definition of urban sprawl referred to patterns of urban development (“…the leapfrog nature of urban growth…”) (Whyte, 1958). Others have defined “Urban Sprawl” based simply on the aggregate population density of a given urban area Fulton et al., 2001, Kolankiewicz & Beck, 2001. It is very likely that “Urban Sprawl” happens to some extent in specific areas of most cities. It could be argued that “Urban Sprawl” is similar to pornography in that it is difficult to define but ‘You know it when you see it’. It could be argued that “Urban Sprawl” is a multi-dimensional phenomenon that needs to be characterized with several variables. Nonetheless, this research focuses on providing a single, scale-adjusted (population corrected), aggregate indicator of “Urban Sprawl” for all urban areas of population greater than 50,000 in the conterminous United States.

Studies using aggregate population density as an indicator of “Urban Sprawl” have typically used ‘urban area’ designations of the U.S. Census along with corresponding population figures to determine an average population density for urbanized areas within the US Fulton et al., 2001, Kolankiewicz & Beck, 2001. These aggregate measures of sprawl suffer from two problems: (1) problems associated with measurements of the areal extent of an urban area, and (2) the nonlinear variation of the aggregate population density of urban areas as a function of total population. Remotely sensed images of urban environments have great potential for delineating urban areas. GIS coverages of urban environments suffer from arbitrary administrative boundaries used in conjunction with housing unit density or population density thresholds. Nighttime imagery has some advantages over daytime imagery in that it is measuring emitted rather than reflected radiation, this avoids some classification problems in separating developed vs. nondeveloped land cover. This research utilizes a ‘scale-adjusted’ measure of “Urban Sprawl” that addressees the nonlinearity problem and uses two ‘thresholds’ of nighttime satellite imagery as a means of measuring the areal extent of urban areas in the United States.

The urban extent of cities varies as a nonlinear function of their total population Nordbeck, 1965, Stewart & Warntz, 1958, Tobler, 1969. This has also been demonstrated using nighttime satellite imagery as a proxy measure of urban areal extent both nationally and globally Sutton et al., 1997, Sutton et al., 2001. Typically, as cities grow their aggregate population density increases; consequently, the aggregate population density of large cities like Los Angeles and Chicago will be higher than the aggregate population density of smaller cities such as Portland and Kansas City. However, this does not imply that Los Angeles and Chicago suffer less from “Urban Sprawl” than Portland or Kansas City. Any aggregate measure of “Urban Sprawl” for an urban area should be scale-adjusted by the total population of that urban area.

The U.S. Census defines the urban area (UA) of a city each time a census takes place. A UA is designated for all central cities with a population in excess of 50,000. The urban areas designated by the census do not always correspond with land cover maps derived from satellite imagery (Vogelmann et al., 1998). This study uses nighttime satellite imagery provided by the Defense Meteorological Satellite Program's Operational Linescan System (DMSP OLS) to measure the areal extent of the urban areas of the conterminous United States. Imhoff et al. (1997) have used the DMSP OLS imagery in similar ways. The DMSP OLS imagery is compared to a gridded population density dataset derived from the 1990 U.S. Census (Meij, 1995). This comparison results in measures of both the areal extent and the population of all the urban areas in the conterminous United States. These numbers are then used to calculate improved aggregate measures of urban sprawl.

Section snippets

Data and methods

The data required to develop a ‘scale-adjusted’ measure of urban sprawl are simply: (1) the areal extent of urban areas, (2) the corresponding population of those urban areas, and (3) a formula describing the relationship between the population and areal extent of these urban areas. The data used to obtain areal extent and population are: (1) a radiance calibrated DMSP OLS image of the United States (Elvidge et al., 1998), and (2) a grid of population density derived from the U.S. Census (Meij,

Results and analysis

The regression line on the scatterplots of the Ln(Area) vs. Ln(Population) relationship represents a scale-adjusted “Sprawl Line”. The line itself represents the average relationship between the areal extent and population of urban areas in the conterminous United States. It should be noted that the nature of this sprawl line is specific to the United States. A ‘Sprawl Line’ for other countries will generally have a higher intercept for countries with lower GDP per capita (in general poorer

Discussion

These aggregate indicators of “Urban Sprawl” for the urban areas of the United States do not suggest that there are no areas of “Urban Sprawl” within urban areas above the “Sprawl Line”; however, these results are comparable yet different than previously determined aggregate numbers because they are scale-adjusted. It is very likely that the green urban areas in Fig. 5, Fig. 6 that are well above the “Sprawl Line” contain smaller areas within them that most people would characterize as “Urban

Conclusion

Clearly, measuring “Urban Sprawl” is a daunting task. Many people decide “Urban Sprawl” is happening in their backyard based on perceived negative experiences such as traffic congestion, changing demographics, or overall population growth that is correctly or incorrectly attributed to “Urban Sprawl”. This investigation presents a measure of “Urban Sprawl” that is scale-adjusted to the total population of an urban area and uses nighttime satellite imagery as an objective and uniform measure of

Acknowledgements

The author would like to gratefully acknowledge insightful suggestions, comments, and criticisms from the anonymous reviewers of this article.

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