I've finally completed and published the next article in my world-building series.
The topic this time is height fields, and specifically building up a base height field according your own design, as opposed to just generating a random one.
The next article, detailing my recipe for erosion in Wilbur, is already in the works. Hopefully it will appear quite soon.
The Cartography of Thorfinn Tait: World-building: Building Base Height Fields
Here's a little clip from the middle of the article:
Now it's time to start on the terrain itself. Make three new layers at the bottom of the file, underneath Altitude Scaling. Name them Mountains, Hills, and Base Terrain, and place them in that order top to bottom. Create layer groups for each of these layers (Control/Command G), and name them Mountains, Hills and Base Terrain too.
Hit D to reset palette colours to black and white, then Filter / Render / Clouds in all three layers. For the mountains layer, optionally hold Alt/Option while clicking in the menu to render clouds; this will give higher contrast clouds.
Optionally, you can render the clouds at half size and scale them up, or at double size and scale them down. Each white cloud will become a peak in your final map, so in this way you can control how big or small the mountains will be. Your choice will depend on the scale of your image. For Calidar, I rendered clouds in a separate file that was double the dimensions of the main map, then scaled them down and copy-pasted the layer in.
For all three layers, Filter / Render / Difference Clouds, and repeat until you get a random texture to your liking. I'm partial to the valley-like channels you get with either one or three renders of difference clouds, so I tend to do just one step of difference clouds for hills and mountains, and either one or three for base terrain. Experiment and find something you like. Remember that at any stage you can also invert the clouds to get a different effect.
Add a levels adjustment layer (Layer / New Adjustment Layer / Levels, or click the icon in the Adjustments panel) above each of the three terrain layers. Right click on each level layer and select Create Clipping Mask (or Alt/Option click the layer) so that the levels only affect the layer immediately below. Then dial in the following settings:
Mountains: 0/1.40/188, Output Levels: 175/255
Hills: 0/1.80/195, Output Levels: 87/191
Base Terrain: 0/1.00/240, Output Levels: 1/104
Your file is now ready to go. Time to start shaping the terrain.