On Edges

I use a lot of subtle effects in my mapping work. Knockouts, inner glows, blurs, and other such tricks help me separate land from water, or keep my text legible. Until recently, I thought of all of these tools separately, each one for use in its own situation. However, in the last year or so, I’ve slowly come to understand how they’re all related to each other, and how, when I’m using any of these effects, I’m getting at the same overall idea: the manipulation of edges.


By an edge, I mean a shift between colors. The stronger the contrast between the two colors, the stronger the edge. Likewise, the more abrupt the shift, the stronger the edge.

I’m sure there are more refined definitions, used by people who study visual perception, but I’m making this up as I go along.

In any case, edges help our brains separate objects (or perhaps we might say graphical marks) from each other and understand their shapes. In fact, I recall reading once that our brains even have special cells that detect edges. In the example above, the sudden shift in color sets off those edge-detecting cells, and they then allow you to perceive the shape of a circle, as a separate object on the page.

As I have come to understand, many of the cartographic tricks that I regularly use are, in essence, about making edges stronger or weaker. 

An Example: Glows

Let’s consider glows. Here I have two islands, one with a glow, and one without:

On the left, the edge between the two shapes is weak because there’s not a lot of contrast between the colors, so it’s not easy to perceive (though you can probably manage it). On the right, the edge is stronger. Let’s zoom in on what’s going on:

The glow has darkened the color of the land by a small amount at its edge. This means that the contrast between land and water is much greater there than it was, and the edge is thus stronger and easier to detect. The glow then fades away quickly, so that most of the land polygon remains the original color. In essence, we’ve changed the color of the land temporarily, just to give the edge a boost and help our brains tune in to the shape.

It’s also quite possible to use glows to make edges weaker. Here again I have two islands:

The one on the left has no glow, but has a good color contrast and so the edge can be perceived. On the right, the glow has muddied things. By darkening the water along the coastline, I’ve made the water’s color closer to that of the land. Thus I have weakened the contrast and made the edge harder to see.

Understanding all this has been very helpful to me. For a long time, I knew that glows could help one part of my map stand out against another, but I would apply them rather naively. I would often accidentally make an edge worse, as above, because I either put my glow on the wrong side, or made the polygon darker when I should have made it lighter. Understanding that glows are all about edge enhancement has helped me pick the correct option the first time.

But glows are just one example. There are plenty of other little effects I use in my work that are also about edges. And we can break them down into two groups: techniques that enhance edges, and those that reduce them.

Edge Enhancements

Let’s quickly go through the list.

Inner & Outer Glows

We just went through these, so you probably get the idea. While they are generally used to increase edges, you could also use them to weaken edges, but that’s not usually how they get applied. Note that outer & inner glows are really the same thing, but are separated in software like Adobe Illustrator, where the terms just refer to whether the glow effect goes on the outside or the inside of the polygon you’re applying it to.

Drop Shadows

These are actually just glows that are slightly offset:

Once again, I’ve simply darkened the edge of the island, thus improving the contrast between the land and the water and making the edge stronger. The island on the left relies on a contrast between pale yellow and pale blue. The island on the right is defined by the much better contrast between pale yellow and dark grey (which then fades down into the pale blue). The fact that the shadow is a little heavier on one side of the circle also tricks our brains into perceiving a little 3D effect, but that’s a separate matter.


Sometimes people put a stroke around their text to help it stand out against a busy background.

Notice that the text on the right is easier to read. Here, I haven’t really enhanced the old edge so much as I’ve replaced it with a new one entirely. On the left, there’s a weak edge between the dark green text and the busy green background. On the right, there’s now a strong edge between the green text and the white stroke around the text. And then there’s another strong edge between the halo and the background. So we’ve effectively doubled the number of edges and strengthened them, meaning the shapes of the letters are much easier to pick out.

Sometimes I (and other mappers) use the term “text halo” to refer instead to an outer glow or drop shadow effect applied to text. It still works to make text legible by enhancing edges, but it’s subtler (I’ve exaggerated it a bit here so you can see it easily):

It draws less visual attention because there are fewer edges. There’s still a strengthened edge between the text and the rest of the map, but now there’s no hard edge between the halo and the background. That’s been made to gradually fade away.

Edge Reductions

Sometimes we want to make edges weaker, instead. Why? Because they draw attention. Here’s an example I used in a recent tutorial:

Notice how the text can easily be read when it’s against a light or dark grey background, but when the background gets striped, things get more difficult. Those stripes make edges that demand your brain’s attention. They act as a form of noise, making it hard for you to pick out the important edges: the ones that define the shape of the words.

Sometimes we want to get rid of competing edges that pull attention away from the ones that we want readers to notice.


In the above example, the solution was to blur the background:

By blurring things, the stripe edges get weaker and draw less attention, letting us focus better on the harder edges of the text.


Another solution (again, often used with text) is to do a knockout. This simply hides features that are getting in the way:

The red lines vanish near the text (and the black dots, for that matter). Now the only edges that are in the vicinity of the text are the ones formed between the text and the background, meaning there’s nothing interfering with our ability to perceive the shapes of the letters.


A feather is often not much different than a blur. Instead of giving an object a hard edge, you can let it fade away gradually, on one side or all around. Here, I’ve feathered one side of a dashed line:

There’s a hard edge on one side of the line, but a weak edge on the other. In this particular case, I used the distinction to subtly indicate to the reader which area was inside of the polygon (the area bounded by the feathered edge) and which was the outside (the area bounded by the hard edge).

Parting Thoughts

Blurs are sort of like feathers, and feathers are sort of like glows, if you think about it (a glow is just a stroke with a feathered side). Again, these various techniques, each with their own name, are all aspects of the same central idea: strengthening or weakening the contrast along an edge.

And that’s really the point of this whole post. Most of these techniques are probably already in your toolkit, or are at least not unfamiliar to you. However, I think it’s valuable to not only understand that a tool can make your work better, but to understand why it does so. For my own part, I think that understanding the bigger picture of how these techniques fit together will help me use them more intelligently in the future.

3 thoughts on “On Edges

  1. Very clear explanations. Thank you. I make maps for forest management plans for work. The maps have to be black and white, and the scale and most of the symbology is prescribed. The maps can become cluttered and difficult to read. I look forward to experimenting with some of these ideas.

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