Tag: Thematic Maps

What is Value-by-Alpha anyway?

As some of you know, we are currently producing a Digital Atlas of American History. While working on one of the maps for the Atlas, I was searching for a better way of showing foreign-born population other than your run of the mill choropleth map. I stumbled upon a paper written by Robert Roth, Andy Woodruff, and Zachary Johnson titled “Value-by-alpha Maps: An Alternative Technique to the Cartogram.” You can read more about it in Andy Woodruff’s blog. After reading the paper and Andy’s blog I got really excited about trying this for our foreign-born population map.

Though choropleth mapping and area cartograms are two of the most common techniques for mapping thematic variables such as foreign-born population, each have significant drawbacks. Choropleth maps fail to distinguish between areas of high and low population. Area cartograms address that issue but can be difficult to interpret given the spatial distortions they introduce. Roth et al. (2010) have developed a new method for developing thematic maps: value-by-alpha mapping.  In the case of foreign-born population maps the value-by-alpha technique uses varying opacities to highlight areas of high population density and deemphasizes areas of low population density. This equalizes foreign-born population based on density and show the percentage of population in each county that were born outside the US—all while preserving both shape and topology. Utilizing this method for foreign-born population effectively highlights high density areas with large foreign-born populations, showing patterns that would likely be missed with traditional choropleth or area cartogram mapping techniques. 2010_foreign_born

My first attempt involved using ArcGIS. Achieving this in Arc is a little problematic, but nonetheless you get a pretty cool map. My one issue is that Arc really limits you on the color range and transparency values you can assign to a layer without some finagling. Andrew Wheeler has a great tutorial in his blog about how to do this in ArcGIS. I found a way to get around these limits in Arc but only after going with my second attempt. To do this you need to calculate in the attribute table a transparency value assigned to each value based on population density and assign that calculated value using the Display Expression tab under Layer Properties. Here are the results using the method outlined in Andrew’s Blog.

The second attempt involved using Leaflet and JavaScript. After discussing the first method with our director Rob, we decided this would be better achieved using JavaScript and Leaflet. JavaScript allows you to give each individual population density value a transparency and color value, where Arc makes you clump these into categories. Rob helped out a great deal with this method since my programs skills are very minimal. This gave us the greatest detail and really highlighted areas of high population density and high foreign-born population. The percent foreign born value is equalized by population density using alpha channels. This visually weights the map and neutralizes areas with low population density. Here is the same map as above but using JavaScript. 2010 Outlined


Though their are a couple of ways to do the Value-by-Alpha method, we found that the JavaScript approach gave us the most granular results and really conveyed what we were trying to show with the foreign-born data. Below is the final poster we presented at this years VAMLIS Conference.


Main Poster 04.psd

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