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The Joint Research Centre (JRC) of the European Commission wanted to develop projections of how built-up land and population levels will likely change locally across the globe. The first objective of this work was to provide the UN Population Division with projections of the population by Degree of Urbanisation level up to 2100. The UN Population Division would like to include these global projections in their World Urbanization Prospects publication foreseen for early 2025. The second objective is to develop a flexible and efficient model that can integrate other population projections and the global SSP scenarios. These projections must be done spatially explicitly and on a fine spatial resolution (entailing 1 km2 grid cells). Given the complexity of the task, this work required a dedicated GIS-based platform that allows trained users to create, recreate, and adapt projections in a computationally efficient way. In 2024, the JRC and Object Vision developed this model in the GeoDMS software.
The model is based on open data and the open-source GeoDMS platform. The modelling process requires up to 120Gb of memory for large continents. System requirements are a recent Windows machine with at least 64Gb RAM (or a very large page file). For development purposes, the model is typically ran for six continents with fixed parameters, but the application allows for substantial modification and customisation. For detailed instructions on how to obtain the model, set it up, and run it, see our tutorial page.
The model downscales exogenously defined built-up land and population totals on 1km2 grid cells globally. Built-up is defined as the fraction of land covered by buildings; building heights are not taken into account. Population is defined as a discrete number of people, without discerning sex or age. Only grid cells are included that are not fully covered by water or ice, that are within country borders, and that are maximally 7km away from a grid cell that contains some built-up development. In the reference application the exogenous inputs are defined for a custom set of regional boundaries that are called 'functional areas'. In addition, a parametrized share of the region's population is re-allocated by the model to model internal migration. The model then first allocates additional built-up land. This is done by a linear downscaling of additional land over a grid map of values that combines the locational suitability of every grid cell, a balancing factor, and the expected additional built-up given the prior level of built-up development in the grid cell. Locational suitability is described as a logit probability that is estimated using calibration routines. Expected additional built-up is labelled as an ET-function ('Expected Toename') and is estimated from historical decennial built-up development.
The model mechanics section describes the model's inner workings in more detail.