In the past few months I’ve picked up the R language for digital analysis of historical sources. It’s one of the most powerful digital humanities tools that I’ve learned, because it’s useful for so many tasks, especially quantitative analysis and mapping.

While there are several mapping packages for R, including the excellent ggmap, the boundaries that these packages come with are all contemporary boundaries. While it’s acceptable to plot data on physical map of the United States, as I did for my maps of Catholic dioceses in North America, it’s far more interesting to be able to include the changing political boundaries of the United States.

The data for those historic state boundaries are available from the National Historical Geographic Information System, which “provides, free of charge, aggregate census data and GIS-compatible boundary files for the United States between 1790 and 2011.” The NHGIS has an extraordinarily useful collection of historical data, including a fair amount of data on religion, which I’ll write about later. Among their offerings are shapefiles for state boundaries by decade, beginning in 1790. These shapefiles are easy to use in a GIS program like QGIS.

The shapefiles don’t, however, work out of the box for R. The shapefiles are associated with a particular map projection, and to work with ggmap and ggplot2, they need to be converted to the more common WGS84 projection. This conversion is much too time consuming to be performed on-the-fly. The shapefiles also need to be converted to a data frame to be plotted. For doing work in R, the most convenient form this boundary data could take is as an R object for easy loading.

I’ve written an R script to convert these shapefiles to R objects in the correct projection as part of my project on the historical demography of American religions. You can download the shapefiles from the NHGIS and use the script perform the conversion yourself. But since this is non-trivial both in terms of the required dependencies (see the script’s documentation) and in the amount of time it takes to process the files, you can download the already converted boundary data:

• historic-state-boundaries.zip (500 MB)

The NHGIS has kindly granted me permission to share these converted files, but if you use them, please be sure to cite the NHGIS. (Please cite me too!) The NHGIS has also asked that you not redistribute these converted files yourself, but instead point people here or to their own site. This is the citation you can use for the NHGIS:

Minnesota Population Center. National Historical Geographic Information System: Version 2.0. Minneapolis, MN: University of Minnesota 2011. http://www.nhgis.org

You can load this boundary data into R using the R.utils package, like so:

<span class="n">library</span><span class="p">(</span><span class="n">R.utils</span><span class="p">)</span>
<span class="n">states_1790</span> <span class="o"><-</span> <span class="n">loadObject</span><span class="p">(</span><span class="s2">"path/to/us.state.1790.Rdata"</span><span class="p">)</span>

<span class="n">library</span><span class="p">(</span><span class="n">ggplot2</span><span class="p">)</span>
<span class="n">ggplot</span><span class="p">()</span> <span class="o">+</span>
<span class="n">geom_polygon</span><span class="p">(</span><span class="n">data</span> <span class="o">=</span> <span class="n">states_1790</span><span class="p">,</span> <span class="n">fill</span> <span class="o">=</span> <span class="s1">'gray'</span><span class="p">,</span> <span class="n">color</span> <span class="o">=</span> <span class="s1">'blue'</span><span class="p">,</span> <span class="n">alpha</span><span class="o">=</span><span class="m">0.3</span><span class="p">,</span>
             <span class="n">aes</span><span class="p">(</span><span class="n">x</span> <span class="o">=</span> <span class="n">long</span><span class="p">,</span> <span class="n">y</span> <span class="o">=</span> <span class="n">lat</span><span class="p">,</span> <span class="n">group</span> <span class="o">=</span> <span class="n">group</span><span class="p">))</span>

If you’re using R for historical mapping, please try out this data and let me know if you have any problems. Thanks to the NHGIS for providing the data in the first place.