Axis maps blog is sharing an interesting technique for adding another dimension of data to maps by using transparency.  Somewhat like a cartogram, they’ve taken each county in the US and modified it based on population density, but instead of stretching it to a new size, they’ve increased the alpha transparency to make more populated areas brighter and less populated areas faded out.

It looks like a challenge to get the color mapping right, especially in light of data that has a few outliers compared to a large area of uniform low density like population.  Perhaps a logarithmic scale would be more useful for data like this.

Regardless it’s a novel highlighting method, and definitely one to keep in the toolbox in the future.  I could also see this being useful for highlighting in non-map visualizations such as treemaps.

There are a few more examples over at the original post, so check it out!