Fig. 2

Lineage and application frequency of gravity model developments. Displayed in light blue is the base gravity model, displayed in blue are the conceptual developments derived from it (2SFCA method, Kernel Density method), displayed in dark blue are categories of methodological developments derived from the 2SFCA method. Number of access measures that applied the respective methodological development is given in gray boxes, bubble sizes are weighted by frequency of application. Each applied measure was assigned to either the base gravity model, the 2SFCA method family, or the Kernel Density method (= mutually exclusive). In addition, for all 324 2SFCA-type measures, the methodological development(s) applied were identified. One 2SFCA-type measure can implement more than one methodological development (= not mutually exclusive). Therefore, the sum of 2SFCA-type methodological developments applied exceeds the number of measures classified as 2SFCA-type. 2SFCA Two-Step Floating Catchment Area, E2SFCA Enhanced Two-Step Floating Catchment Area, V2SFCA Variable Two-Step Floating Catchment Area, SPAR Spatial Access Ratio, 3SFCA Three-Step Floating Catchment Area, M2SFCA Modified Two-Step Floating Catchment Area, MM-2SFCA Multi-Mode Two-Step Floating Catchment Area