In the 2e AD&D Birthright campaign settling, political domains such as kingdoms and nations are subdivided into provinces. Birthright provinces are typically about 900 square miles in size and have a maximum population of 100,000 people limited by terrain. Provinces are the smallest unit of land in Birthright 's strategic level of play. They are also a game mechanic abstraction rather than an attempt to closely model a historical entity.
...on the 25 Mile Hex Scale
From the last post, we established the maximum self-contained population density of our High Middle Ages era culture. It was determined that 750 able-bodied adults could be supported in one 5 mile hex which includes rural and urban dwellers. This assumes that our ideal hex is in a temperate climate and located on fertile land. Let's tackle land first.
First let's define some terrain types:
Plains - are open flat lands well suited to farming.
Deserts - are open flat lands unsuited to farming due to an arid climate.
Badlands - are extremely rough, rocky terrain with poor topsoil and often subject to flash flooding.
Woodlands - are flat lands with light tree cover which does not block sunlight from reaching the surface.
Forests - are flat lands with heavy tree cover whose shade covers the ground layer.
Marshes - are flat low lands near coasts and rivers subject to heavy flooding.
Swamps - are flat low lands with greater expanses of water than a marsh and more tree cover.
Hills - are rocky low land masses extending above the surrounding terrain.
Grassy Hills - are low land masses extending above the surrounding terrain with decent topsoil. They can include tree cover.
Mountains - are high land masses extending above the surrounding terrain. They can include tree cover.
High Mountains - are very high land masses extending above the surrounding terrain. They are above the tree line.
Now let's determine the land available for agriculture:
We are going to assume that each 25 mile hex never really reaches its full population potential. For example, plains are given a maximum of 23 5 miles hexes available for farming rather than 25 to allow for cities or towns and to account for small patches of unsuitable land and land use inefficiencies.
Maximum number of 5 mile hexes available for farming in a 25 mile hex:
Grassy Hills, 5
Forested Hills, 5
Forested Mountains, 2
High Mountains, 1
Of these hexes, there is at least a minimum of 13 of the 25 available 5 mile sub hexes are given over to the primary terrain type. This is true also of the 5 mile sub hexes and the 1 mile sub hexes, etc. (It's hexes all the way down...) The computer programmer in me wants to generate a Ruby script with an randomly variable recursive algorithm which determines land use down to the pebble or until the stack overflows but fortunately I've gotten that sort of thing out of my system elsewhere. I'm setting the modifier statically at .75 for now.
If you're wondering where these numbers came from, it was by a melange of shallow wikipedia research, guesswork and reverse engineering to fit my preexisting map. (In truth, I did have to fix my map in several places.) If you wish to replace these numbers with some other numbers more to you and your maps' liking, you now have a framework with which to get started.
So the maximum population per 25 mile hex looks like this:
Grassy Hills: 4500.0
Wooded Hills: 2812.5
Wooded Mountains: 1125.0
High Mountains: 562.5
For the sake of example, let's look at the Domain of Lissone, from my DIY campaign world called Moranth:
There four woodland hexes each with 14 5 mile hex farming plots available.
4 * 14 = 56 farm plots
Next we have four plains hexes each with 23 5 mile hex farming plots available.
4 * 23 = 92 farm plots + 56 farm plots = 148 farm plots
Now we have two forest hexes each with 10 5 mile hex farming plots available.
2 * 10 = 20 farm plots + 148 farm plot = 168 farm plots.
Lastly we multiply the 168 farm plots by 750 people and then multiply that by the .75 that represents minor terrain variance.
168 * 750 * .75 = 94500
This is a little under the ideal Provincial maximum population of 100,000 but that's what happens when you draw the map first. ;)