The Atlas

The survey

Six passes over the same number, in this order.

1. 01

   Ground

   Layered value noise, warped by more noise so the coast never comes out circular, multiplied by one large landmass and a couple of smaller ones. Ridged noise is mixed in to give the interior mountains a spine.

2. 02

   Sea

   Rather than picking a height and hoping, the water is poured to whatever level leaves the intended fraction of the map dry. Every world therefore has a coastline worth looking at. The slider moves that level.

3. 03

   Weather

   A wind direction is drawn, and any ground sheltered behind higher ground upwind gets less rain. Temperature falls with latitude and with altitude. Rain and heat together decide whether a hillside is forest or desert.

4. 04

   Water

   Every cell drains to its lowest neighbour. Sorting the map from high to low and passing water down the chain gives each cell the size of the catchment above it — where that total gets large enough, a river is drawn. Where the water arrives somewhere with nowhere lower to go, a lake.

5. 05

   People

   Every land cell is scored for how much anyone would want to live on it: flat, fed by a river, on the coast, not frozen, not sand. The best sites are taken in turn, kept apart from each other, and named.

6. 06

   Roads

   The towns are joined into one network by shortest total distance, and each link is then routed cell by cell across the terrain by Dijkstra's algorithm, paying heavily for climbing and for getting wet. The roads find the passes on their own.