Uncovering merging and joining as urban expansion processes

Urban expansion studies typically distinguish three main processes: infill, edge expansion, and outlying development. These general processes do not specifically capture another type of urban expansion: the clumping or coalescing of multiple individual urban areas (patches) into one larger urban agglomeration. This paper develops a new Urban Patch Expansion (UPE) framework that characterises coalescence as an urban expansion process that occurs simultaneously with the three commonly distinguished processes. We characterise two types of coalescence: merging and joining. Merging is defined as the amalgamation of at least two initial patches whose combined area is smaller than the newly added urban area merging them, while joining refers to cases where the total area of the initial urban patches is larger than the newly developed urban area connecting them. In an application of this framework, we analyse urban development in the functional urban area of Bandung (Indonesia) in the period 1975–2015 focussing on the variation in urban development processes along the gradient from the urban centre to the most outward edges of the urban agglomeration. The analysis employs the latest release of the Global Human Settlement Layer (GHSL) that provides a highly detailed rasterised account of urban development since 1975. This new spatial analysis approach enriches our understanding of the spatio-temporal dynamics of urban expansion by providing a more specific account of when and where specific development types prevail. Our research characterises urban development in the Bandung area as a sequence of phases. Outlying development is the most important process in the initial period (1975–1990), highlighting a phase of diffusion. From 1990 coalescence sets in, with joining as the prevalent process in the second period (1990–2000). Infill and edge expansion are steady processes throughout the analysis period, where the latter gradually increases in importance as the urban area continues to grow. Both processes further compact the urban landscape. The final period (2000–2015) shows approximately equal shares of joining, infill and edge expansion. Joining can thus be seen as the first step in the transition from the diffusion to the coalescence stage, whereas the infilling of urban voids and the expansion of city at its edges reflect a more continuous growth process. We observe a similar development sequence over space: edge expansion dominates the zone between 5 and 9 km from the core, joining in the zone up to 17 km and outlying beyond that. The latter process, however, became less important over time. Our approach can be used to compare refined development trajectories in different periods and regions and thus supports our understanding of urban growth.