Accessibility

The accessibility analysis answers, for every populated 1 km grid cell, how far away the nearest settlement of a given kind is, both in travel time and in network distance. The template is Analyses/Accessibility/get_Settlement_Access_grids_T, prepared per continent and run per country.

Variants

Six destination sets are evaluated. The first three are based on settlement size, the last three on position in the settlement hierarchy.

Variant	Destinations
villages	all settlements of type above 10, that is villages and larger
towns	type above 22, that is towns and cities
cities	type above 29, that is cities
loc_ctr	settlements that are a centre at 15 minutes
reg_ctr	settlements that are a centre at 30 minutes
nat_ctr	settlements that are a centre at 60 minutes

Destinations are restricted to the country being processed. The result therefore describes accessibility within a country, and a cell near a border is not served by a town on the other side of it.

The computation

Grid cells are connected to the network in the same way as in Catchments: snapped to the nearest node, accepted within 100 metres, split into connected and unconnected cells with their separated unit kept.

For each variant, two routings run from all destinations of that variant at once:

• one with travel time as impedance, giving the travel time to the nearest destination for every network node;
• one with link length as impedance, giving the network distance along the route to the nearest destination.

Connected cells read their values from their node. Cells that end up without a value take the value of the nearest cell that has one.

Unconnected cells are handled per separated unit, as in the catchment analysis, but here the detour is priced rather than ignored. The straight line distance from the unconnected cell to the connected cell it borrows from is added to the network distance, and that same distance divided by the low default speed of 30 km/h is added to the travel time. An unconnected cell is therefore never reported as being as accessible as its connected neighbour.

Output

Results are written as GeoTIFF, per country, on a grid that is cut to the bounding box of the country and aligned to the population raster in World Mollweide. The path is %LocalDataProjDir%/Settlement_Accessibility/<settlement set>/<continent>/<country>/, with:

• tt/<variant>.tif, travel time in minutes to the nearest destination of the variant;
• td/<variant>.tif, network distance in kilometres along the same route.

Twelve rasters per country, six of each. Storing them cut to the country rather than on a global grid keeps the files small, at the cost of having to mosaic them afterwards.