With projected urbanization concentrated in low-income countries, improved knowledge on settlement distribution, location and size is important for the assessment of environmental, socioeconomic and demographic change. The urban maps examined here represent the first steps in filling this knowledge gap.
Two principal research questions were defined at the outset of this paper: (i) can global urban maps be used accurately at a national scale for a low-income country?, and (ii) does the processing involved in producing a national-level 25 m spatial resolution urban map produce significant enough accuracy increases to justify its adoption over global-level maps at a national scale?
In many low-income regions of the World, the only up-to-date maps of urban extents are those produced on a global-scale. However, in response to research question (i), the various inaccuracies highlighted show that to conduct research or form policy decisions at a national-level based solely on urban data from the AVHRR, DCW, DMSP-OLS, GRUMP-UE and MODIS global maps in their current incarnations would require very careful interpretation. Results here emphasise clearly that such maps were produced on a global scale with the aim of being used in global studies, and thus should be used exclusively for such purposes.
This study has also shown that a semi-automated approach to settlement mapping based on medium spatial resolution satellite imagery can produce a map of comparable accuracy to one produced through visual interpretation and ground survey. In response to research question (ii), results show that such a map exhibits much greater accuracy in urban delineation justifying adoption over any global map tested here. In low-income regions where the speed of urbanisation is creating the greatest need for contemporary maps and where resources are not available for map update via traditional survey-based methodologies, this may represent a cost-effective viable alternative.
At a time when urbanization in low-income countries is receiving much attention (UN Habitat 2003
; UN Habitat 2004
), satellite imagery has an important role to play in aiding the production of urban maps for city-, regional-, national- and global-scale analysis. Users of urban maps need to be made aware of limitations, and providers should remain flexible in their ability to update, adapt and incorporate new information, imagery and approaches. If this can be achieved, then satellite imagery-derived urban maps will prove vital tools in extending the study of, among others, population (Hay et al. 2005
), childhood mortality (Balk et al. 2004
) and malaria burden (Hay et al. 2004
; Hay et al. 2005
) across the low-income regions of the World. Continued successful usage will in turn provide feedback on optimal approaches for mapping improvement.