Common term: Urban centre; Plural: Urban centres; Technical term: High density cluster/High density clusters
Short definition: An urban centre is a cluster of contiguous grid cells of 1 km2 with a density of at least 1500 inhabitants per km2 and a minimum population of 50 000.
Longer description: We define an urban centre as a cluster of non-diagonal contiguous cells having more than 50 000 people and a population density of 1500 inhabitants per km2 or more.
The calculation goes in three steps (Figure below):
- First, we select all cells that have a land density of more than 1500 inhabitants per sq km. These are high-density cells.
- Second, these cells will be combined in contiguous groups, that is to say for each high-density cell, we will check if it touches another high-density cell (not including diagonal touching). If so, that cell is added to the same group.
- Third, we then apply an iterative gap-filling algorithm that fills up the holes within the cluster and smoothens sharp borders. Only the clusters with a population of more than 50 000 inhabitants are kept. These are the Urban centres.