Two years ago, I wrote an article called “Adding Realistic Coastlines to a Map“. I was asked to re-post it in the tutorial section at the Cartographer’s Guild where, shortly after doing so, it received an award. Since then, this article has been, by far, the most viewed post on this site. Given the interest it has received, I think it is high time I revisited this tutorial.

Most techniques that I have seen are overly destructive to the original shape of the coastline and produce numerous lakes throughout the interior. Neither of these effects were acceptable for what I was trying to achieve, so I set out to create a new technique that would give my coastline a more organic feel without drastically altering the original shape. I believe this method achieves that nicely.

Adobe Photoshop 7.0 was used to create this tutorial. A modified version for GIMP is also included.

Getting Started

Start out with a white landmass on a black background. For this tutorial, I am using a 600×400 image and have drawn a simple ellipsoid for a landmass. I thought this was the best way to focus on the process without being distracted by the shape of a specific landmass.

Step 1 – Add Noise

We want to “muddy up” the image by adding some noise.

Filter | Noise | Add Noise

Be sure to select Gaussian distribution and checkmark Monochromatic. I have Amount set to 100%.

Step 2 – Blur the Image

Now we blur the image. I find that the Gaussian blur best creates the effect I need.

Filter | Blur | Gaussian Blur

I have radius set to 2.0 pixels

Step 3 – Adjust Threshold

Image | Adjustments | Threshold

A value of 150-175 should give good results. I am using a value of 160.

This is where the magic happens. Everything else is just to enable and enhance this step. Don’t worry about what’s going on inside the landmass. That will disappear in the next step. What we are doing here is creating “holes” around the edges of the landmass. When we use the Magic Wand Tool in the next step, the selection will seep in through these holes. Notice in the image below that the selection can only seep in so far before it encounters a barrier. That’s the entire key to this technique. The more you adjust the threshold, the further in the selection can go.

Step 4 – Magic Wand Selection & Fill

Select the Magic Wand Tool. Make sure Contiguous is checkmarked. I use a tolerance of 0 and uncheckmark Anti-Aliased and Use All Layers. Use the magic wand tool in the black area outside the landmass.

Press SHIFT-CTRL-I to invert the selection

Fill selection with White (Edit | Fill | White)

That’s it! It’s an amazingly simple process to create realistic coastlines using this method.


The first two steps (add noise and blur) affect how the third step (adjust threshold) will work. There are different types of noise and blurring effects, as well as various settings for each. Changing these settings will change the final outcome.

Adjusting the threshold is the core of this technique. Different values here will have a huge impact on what you can achieve.

Another variation isn’t quite so obvious. Start with an image smaller than what you want, apply noise and blur, and then increase the image size before adjusting the threshold. This will change the scale of the effect.

Throughout this tutorial I have focused on modifying coastlines. However, this same technique can also be applied to hills, mountains, forests, etc.

GIMP Method

RobA (at Cartographer’s Guild) converted this process to work with the GIMP:

  • Filter | Noise | HSV Noise (Boldness 1, Hue 0, Sat 0, Value 255).
  • Edit | Fade (Darken Only)
  • Filter | Blur |Gaussian Blur (2 or adjust to taste). *(1 will create very fine distressing and the larger the blur, the larger the effect)
  • Colors | Threshold (adjust to taste). *(I found 160 is a good start, up to 185 for a glacier coastline)
  • Fuzzy Select Tool (Threshold 0), click in black surrounding area.
  • Ctrl-I to invert the selection
  • Fill with White.