Georeferencing Images: An Exercise in (Relative) Frustration

Geographic information systems (GIS) are used by dozens of different industries, including shipping corporations, public utilities, property developers, militaries, telecom services, and agriculturists. GIS technologies are also used by humanists and academic researchers to explore spatial relationships and visualize and find patterns in information that cannot be seen in pure data. One of the most useful GIS technologies, especially for historians, is georeferencing. Georeferencing older maps allows scholars to “modernize” historical documents and make them more usable by contemporary readers, as well as increasing their accessibility by hosting them online.

Georeferencing maps requires a GIS computer program such as Google Earth, Google Maps Engine Lite, ArcGIS, QGIS, or a service such as MapTiler designed specifically for georeferencing. Through the georeferencing process, spatial analysts “layer” different maps on top of each other, ultimately associating spatial locations on these maps with real-world coordinates. The GIS user designates control points on both the historical map (usually a static image file of some kind) and a modern basemap of the same spatial area. This basemap can be a static image as well, but is more often a proprietary dynamic map delivered through Google Earth, Google Maps, Open Street Map, US Topographic Maps, National Geographic Maps, or other services. These maps are “live” online and nearly infinitely scalable, drawing from databases housing petabytes of information and image. These control points map street intersections, buildings, or geographic features on historical maps to their counterparts in real life using specific coordinate reference systems (CRS). These coordinate reference systems define how their associated maps are projected; each CRS uses a different projection system that displays the Earth’s surface differently (eg Mercator, Gall-Peters, Winkel-Tripel). Designating a CRS allows the software to link the two points and fit the historical map to the basemap. While accurate historical maps may fit nearly perfectly and only require a few control points to be accurately georeferenced, others require many points and may experience significant warping as the map’s scale is distorted to minimize its inaccuracies and fit it to the basemap.

I decided to georeference a map of Paisley, the hometown of Robert Tannahill, my primary research subject. I know this area of Scotland relatively well because of its proximity to Glasgow, where I studied abroad during fall 2013 and returned to on a research grant during summer 2014. I was able to visit Paisley on several occasions. Tannahill died in 1810, so I wanted to find a map created as close to his death as possible. John Wood’s 1828 map fit the bill perfectly. Because the resolution of the map available for public download on the National Library of Scotland website was too low, and because I potentially intend to use it for a public website, I purchased access to the image from the NLS. This gave me a massive 20MB .JPG file – perfect for using with a GIS program.

I tried several software solutions, all of which have various faults and weaknesses. I first tried to use perhaps the simplest, Google Maps Engine Lite. While this allows users to create points and import CSV data, there is no utility for georeferencing an image (raster) file. I then used MapTiler; the interface was simple and the most usable for georeferencing of any product I tested. However, I found that there was no way to remove watermarks or incorporate multiple map layers without purchasing the product. I was also somewhat unimpressed with MapTiler’s warping abilities. In exasperation, I turned to a more powerful solution: QGIS.

QGIS is the open source alternative to the main industry standard GIS program, Esri’s ArcGIS. QGIS has most of ArcGIS’ functionality and even performs faster than ArcGIS on many benchmarks. While I haven’t had enough of an opportunity to test the entire program, I did try to use QGIS to georeference my map of Paisley. I was able to select a basemap relatively easily. I started out with Google Maps, but QGIS failed to allow me to scale past 1:25,000. I changed to Bing Street Maps, which provided more detailed resolution, though the load times were still noticeable. I was able to create control points, though the interface was somewhat clunky (clicking back and forth between two fullscreen windows). However, when I attempted to actually perform the georeference transformation and overlay the images, QGIS failed to function entirely, despite my attempts to change numerous settings and configurations. I would like to install QGIS on another machine and attempt the process again, but have not yet had the luxury of doing so.

In desperation I turned to a free trail of Esri’s ArcGIS. The installation process for ArcGIS was abysmal; it required multiple massive install files and authentications because of how tightly Esri controls the software. The programs are unhelpfully titled; I had to install ArcGIS Desktop Pro, ArcMap, ArcView, and many other arcs. Once I did manage to activate the trial and begin working in ArcMap, I found it easy to add basemap data (Open Street Map) and my raster image. The georeferencing process was still slightly awkward; you essentially place the raster image over the basemap, which requires you to zoom to that location, and then create reference points in one screen rather than splitscreen. If you are working with a shapefile basemap, this would probably work very well, but both my basemap and my raster image are opaque; when the raster image is overlaid on the base image it obscures the basemap entirely. I was able to change the opacity of the raster image to allow me to see both at once, but this was less user-friendly than either MapTiler or QGIS in my opinion. However, the georeferencing accuracy in ArcGIS was far higher than either of those programs; I could see the raster image being transformed and warped with each control point I added. ArcMap even provided a table of the control points with residuals – essentially data about the estimated error of each point. By deleting control points with high residuals and adding more points, I was able to quickly make georeference my image with high accuracy.

This is where the fun ended. Initially, ArcMap seemed promising because I had previously used ArcGIS Online to create and embed location frequency maps. However, Esri’s desktop suite and ArcGIS Online apparently share very few similarities. While some layers can be added to both, raster files, even when georeferenced, are not one of these layer types. I was unable to publish my map to ArcGIS Online, though I tried Esri’s recommended methods of sharing it as a Map Service and Map Package. These uploaded to the site, but couldn’t be used by ArcGIS Online – their only use is to be downloaded by other ArcGIS Desktop users. Apparently if you have a full scale ArcServer installation you can host maps like I created, but that is beyond what I was willing to do for this project. Frustrated, I exported my map as a static image (below) and quit ArcMap.

Paisley_ArcMap

I then returned to MapTiler. I added many more control points than in my first iteration. While the transformation was still weak compared to ArcGIS’, it provided far more hosting options than Esri – and even included a slick transparency slider option. I was able to remake the map, store the completed files on my Google Drive, and embed the map in my website in under an hour. While the finished result is imperfect, it is, at least, a finished and digital hosted map! I hope to eventually learn more fully featured GIS editors such as QGIS, but cannot fault MapTiler for what it is – easy to use, single-purposed, and fast.

*Update* MapTiler is somewhat difficult to extend in terms of functionality. However, after a LOT of tinkering, I was able to use the Google Maps output (not the standard index.html page) and add a custom layer of markers on the map with some JavaScript. I also added an infoWindow that displays images when you click on the markers. Unfortunately this means that I’ve lost the wonderful opacity slider, but I’ll be working to add that back in the future as well. Here’s the result – make sure to load the “unsafe scripts” (My JavaScript) to see it.

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