Occasionally, I like to go through the QGIS plugins and see what’s new. There are all sorts of handy tools in there, but one in particular caught my eye this time. The Historical Map plugin, by Nicolas Karasiak & Antoine Lomellini, takes on a specific challenge: extracting data from old maps. Of course, you can always digitize this data, but it can be quite time consuming. The plugin leverages image classification techniques to speed up the process. The nice thing about historical maps (such as USGS topos) is that there are usually fairly discrete color codings. Discrete colors takes a lot of the guesswork out of the image classification process, but the techniques can be pretty intimidating if you haven’t done them before.
As some of you know, most of my life for the last two years has revolved around predicting the locations of small headwater streams that are not captured by the venerable National Hydrography Dataset. The NHD is a fantastic resource overall, but it does have its shortcomings, particularly in small, ephemeral channels. Usually, the limitations are fairly predictable, as they result from the intersection of limits of scale, subjective cartographic choices, and deliberate design rules that make a map good for one purpose but not necessarily others. Sometimes, those shortomings manifest themselves in unexpected ways.