Abstract
Data on the extension of urban areas are important for analyzing growth dynamics and to support the planning of transport and service provision. Satellite-based remote sensing has proven extremely useful, especially in cities that experience fast spatial growth. Different approaches to satellite-based mapping may, however, produce different results concerning urban categorization and delineation, often making direct comparison misleading. This study analyses four different satellite-based studies of urban land cover in Accra, Ghana and presents a new land cover map based on visual interpretation of segmented Sentinel-2 imagery. The methods and results, as well as the underlying definition of “urban”, are compared and discussed. One method identifies exclusively areas with man-made, impervious surfaces, such as roads and buildings, as proxies for urban extent. Other methods aim to identify a broader set of land cover types, including green spaces, which are treated as part of the mixed urban fabric. Further differences are found in the way urban fringe areas under development are classified depending on their degree of urbanization, and in the distance threshold values used for defining the urban agglomeration. For the most recent maps we identify a difference in the measured size of the Accra agglomeration of almost 100%.
Highlights
Accra, like many other cities in Sub-Saharan Africa (SSA), has experienced very rapid urban growth in terms of both area and population during the last twenty to thirty years
All area estimates for the Accra agglomeration that have been computed from the five datasets are shown in Table 1 and Figure 2
For the most recent maps for example we identify a difference in the estimated size of the Accra agglomeration of almost 100%
Summary
Like many other cities in Sub-Saharan Africa (SSA), has experienced very rapid urban growth in terms of both area and population during the last twenty to thirty years. Individual builders typically erect houses gradually over several years when funds are available, as mortgage schemes are less prevalent. Many of these large, newly urbanized areas of Accra develop into urban ‘deserts’ characterized for long periods by very little service provision, low accessibility, proneness to seasonal floods etc., thereby restricting their residents’. Most countries in Western Europe have strict spatio-statistical definitions that guide the mapping of urban areas, including the demarcation of town/city polygons [22]. The core of these definitions is a population and a density threshold. The term “naturally continuous” is explicitly used of areas where the distance between “houses” is less than
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