Model mechanics

This section describes what the model actually computes, in the order in which the steps depend on each other. Each step writes its results to disk, so that later steps read a stored result instead of recomputing it.

Order of the steps

1. Geography: countries, separated country parts, continents and functional areas are prepared and stored. Everything downstream is iterated over these units.
2. Source data: population grid cells, settlement polygons with population weighted centroids, and OSM road lines are read, filtered and stored in fast GeoDMS .fss files.
3. Network construction: per continent, the OSM lines are turned into a connected, routable and compressed link and node set. This is the heaviest step and its output is the basis of everything after it.
4. Settlement hierarchies: per continent, settlements are ranked by the travel time threshold up to which they are the largest reachable settlement.
5. Catchments: per country, populated grid cells are assigned to the nearest centre and dissolved into cleaned catchment polygons.
6. Functional rural areas: per country, catchments are merged into larger functional areas under population and travel time targets.
7. Accessibility: per country, travel time and network distance grids to the nearest settlement of each type.
8. Travel distance proxy: per country, a spatial interaction model producing potential accessibility, average travel distances and modelled link flows.
9. Road lengths: road length per grid cell by road class, computed directly from the uncompressed network.

Configuration layout

The GeoDMS configuration lives in cfg/ and is entered through cfg/main.dms. The tree mirrors the steps above:

Item	Contents
ModelParameters	all settings for a run, see Model parameters
Units	metric and time units, including the s_f seconds unit used for impedance
Geometries	projections and the base grids
Classifications	continents, settlement types, travel time thresholds, OSM road types
SourceData	regional units, population, settlements, infrastructure
NetworkSetup	selection of origins and destinations per continent and the call into the network templates
Analyses	settlement hierarchies, catchments, functional rural areas, accessibility, travel proxy, road lengths
Templates	the generic network construction and connectivity templates

MakeUnlinkedData at the top of main.dms groups the data preparation steps that have to be run once, in order, before any analysis can start.

Projections

Three coordinate systems are used side by side, each for what it is good at.

• WGS84 (EPSG:4326) is the base projection. Source data are read in it and most geometries are stored in it.
• Web Mercator (EPSG:3857) is used for the network itself. All node coordinates, link geometries and the connect operations that attach origins and destinations to the network work in this projection.
• World Mollweide (ESRI:54009) is used wherever area matters: the population grid, the dissolving of catchment polygons and all area statistics. A derived integer grid of 1000 metre cells (WorldMollweide_km) is used as the working grid when polygons are unioned.

Because these conversions happen in many places, the configuration requires a GeoDMS version with the corrected coordinate order in conversions.