Painting height-maps for satellite-style maps

[theleast]

A few people have asked me how I made my regional map of The Burning Archipelago, so I decided to make this tutorial. Keep in mind that I'm fairly new at this and am still learning as I go. If you know of a better way to achieve something, please speak up!

Overview

This is for a satellite-style map, leaning towards realism. I'll be painting a height-map in Photoshop, then using that height-map to generate 3D lighting and paint the final coloured version. Here's the end result:


NB: I'm assuming that the reader has some basic proficiency in the use of Photoshop.

Step 1: Find your references
It's important to know the scale of your map early on. I knew that wanted a pair of islands roughly 60-120km long, so I went to Google Earth to find real examples. I settled on Stewart Island in Southern New Zealand, because it's roughly the right size, and also features similar climate and geology to my islands. I took a screencap and saved it for reference:


Step 2: Define the coastline
This where I would normally start with a quick sketch, but I already had my larger regional map to work with, so instead I grabbed part of it, scaled it up and dropped into Adobe Illustrator. I then reduced the opacity of the reference and used the pencil tool to draw the coastline over the top of the reference. By setting the pencil accuracy to the finest option, and using my wacom tablet, I was able to quickly hand-draw some random, jagged coastline:


Once completed, I removed the strokes and set the fill to 100% black. I then alt-dragged the illustrator file into Photoshop:

This became the base coastline that defined the rest of the process. This could also be drawn or painted in Photoshop, but I like the extra control that Illustrator offers when it comes to drawing complex natural shapes.

Step 3: Paint the height-map
A height-map is a greyscale image that defines how high each pixel is in relation to the other pixels. Black pixels are the lowest, white pixels are the highest. By painting a height-map we are physically defining the contours of our terrain, and laying down the foundation for everything that comes next. For height-maps in Photoshop, I work at 4096x4096 resolution, with the image mode set to Greyscale, 16-Bits/Channel. Your computer might have trouble working with such a large file, so scale it down if you need to, but try to keep these settings as high as possible to avoid ugly artifacts later in the process.

To start off, I created a new layer in PS, created a layer-mask based on the coastlines, filled that layer with black, set its blending mode to screen and then applied the following blending options:

Feel free to tweak these settings to your preference, but take note of the "Technique". Make sure it's set to "Precise" to ensure that the effect follows the edges of the island as precisely as possible, otherwise it will be soft, muddy and useless.

This creates a very quick and dirty height-map for the islands. The island edges are low, and there are ridges running up the centre of the island:


I then used the eraser too set to 1% to chip away at this layer, so the ridges didn't uniformly follow the edges of the islands:


These ridges are still very uniform though, and lack the randomness of nature. To achieve this randomness, I created a new layer using the previous one as a clipping mask. I used the filter 'Render Clouds' to fill the layer with soft noise, and then applied 'Difference Clouds' multiple times to the layer to add further iterations on the detail:



I then set this layer to "Overlay" and tweaked the opacity. This gave the ridges a nice, random appearance:


Finally, to fill the open planes, I looked to real-world topography. I used NASA's free Earth height-maps to find areas that had comparable terrain to my islands, and dropped them on top of my ridge layer, set to 'Screen'. I then used black brushes to remove a lot of this data from the edges of the islands and tone it all down, otherwise the islands would have been abnormally lumpy. I also hand-painted some rivers (in black) and a volcanic peak (in white):



After lots of experimentation and tweaking, I finally had a working height-map, with the sea set to 100% black and the highest mountain peak at 100% white. The next step was to take this greyscale data and make it 3D...