A Comparison of Satellite-Based Estimates of Urban Agglomeration Size for the Accra Area

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%.

Spatial and temporal dynamics of croplands in expanding West African cities

“…Accra, Ghana's capital and economic hub, has attracted migrants from other parts of the country for a long time, which has supported a rapid increase in urban population and horizontal growth (Arup and Cities Alliance, 2016; Møller‐Jensen, Allotey, Kofie, & Yankson, 2020). Inner‐urban farming to supply the city with fresh exotic vegetables has a long tradition in Accra (Anyane, 1963) especially where open spaces allow irrigation (Drechsel & Keraita, 2014).…”

“…Cities in West Africa are undergoing many land‐use changes, particularly the expansion of built‐up area, with implications on urban cropland. Related assessments of land‐use changes can face significant technical challenges and vary considerably even for one and the same city (Møller‐Jensen et al., 2020).…”

Built-in Flood Risk: the Intertwinement of Flood Risk and Unregulated Urban Expansion in African Cities

“…Field observations were systematically recorded with GPS track logs, photos and field notes. Field-based data collection is complemented by insights from a recent study (Møller-Jensen et al, 2020), which documents the gradual extension and development of the built-up area of Accra based on analysis of satellite images.…”

Urban encroachment in ecologically sensitive areas: drivers, impediments and consequences

“…The historical expansion of Accra is visualised through maps illustrating the expansion of the builtup area for the reference years 1991, 2000 and 2014 (see Møller-Jensen et al 2020 for a thorough discussion of the methodology). The encroachment upon ecologically sensitive areas within the selected sites is examined through visual inspection and comparison of historical satellite imagery from Google Earth.…”

“…The city has experienced significant spatial growth in the past decades. The expansion in fringe areas is largely unplanned and unregulated (Møller-Jensen et al 2020;Agyemang et al 2017;Akubia & Bruns 2019).…”