Abstract
Accurate detection of spatial patterns of urban growth is crucial to the analysis of urban growth processes. A common practice is to use post-classification change analysis, overlaying multiple independently derived land cover layers. This approach is problematic as propagation of classification errors can lead to overestimation of change by an order of magnitude. This paper contributes to the growing literature on change classification using pixel-based time series analysis. In particular, we have developed a method that identifies change in the urban fabric at the pixel level based on breaks in the seasonal and year-on-year trend of the normalised difference vegetation index (NDVI). The method is applied to a case study area in the south of England that is characterised by high levels of cloud cover. The study uses the Landsat data archive over the period 1984–2018. The performance of the method was assessed using 500 ground truth points. These points were randomly selected and manually assessed for change using high-resolution earth observation imagery. The method identifies pixels where a land cover change occurred with a user’s accuracy of change 45.3 ± 4.45% and outperforms a post-classification analysis of an otherwise more advanced land cover product, which achieved a user’s accuracy of 17.8 ± 3.42%. This method performs better where changes exhibit large differences in NDVI dynamics amongst land cover types, such as the transition from agricultural to suburban, and less so where small differences of NDVI are observed, such as changes in land cover within pixels that are densely built up already. The method proved relatively robust for outliers and missing data, for example, in the case of high levels of cloud cover, but does rely on a period of data availability before and after the change event. Future developments to improve the method are to incorporate spectral information other than NDVI and to consider multiple change events per pixel over the analysed period.
Highlights
Global urbanisation and population growth puts pressure on environmental systems, and provides opportunities for development [1]
No harmonisation between Landsat sensors was performed as during preliminary analysis, the normalised difference vegetation index (NDVI) calculated from surface reflectance images was observed to have a negligible impact on long-term average trends
NDVI was chosen as the single metric for change detection; as a vegetation index it is subject to periodic cycles and is a indication of urbanisation
Summary
Global urbanisation and population growth puts pressure on environmental systems, and provides opportunities for development [1]. The detection, classification, and characterisation of urban growth patterns is crucial to the effective management of urbanisation pressures [2]. A common approach to land cover change is post-classification comparison (PCC). In this approach, land cover classifications are produced independently for the same study area for two or more moments in time. Differences between the layers are interpreted as change over time [6]. This approach is problematic, because it means that misclassifications are likely to be registered as a change.
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