The Programming Historian (Dec 2013)
Georeferencing in QGIS 2.0
Abstract
In this lesson, you will learn how to georeference historical maps so that they may be added to a GIS as a raster layer. Georeferencing is required for anyone who wants to accurately digitize data found on a paper map, and since historians work mostly in the realm of paper, georeferencing is one of our most commonly used tools. The technique uses a series of control points to give a two-dimensional object like a paper map the real world coordinates it needs to align with the three-dimensional features of the earth in GIS software (in Intro to Google Maps and Google Earth we saw an ‘overlay’ which is a Google Earth shortcut version of georeferencing). Georeferencing a historical map requires a knowledge of both the geography and the history of the place you are studying to ensure accuracy. The built and natural landscapes change over time, and it is important to confirm that the location of your control points — whether they be houses, intersections, or even towns — have remained constant. Entering control points in a GIS is easy, but behind the scenes, georeferencing uses complex transformation and compression processes. These are used to correct the distortions and inaccuracies found in many historical maps and stretch the maps so that they fit geographic coordinates. In cartography this is known as rubber-sheeting because it treats the map as if it were made of rubber and the control points as if they were tacks ‘pinning’ the historical document to a three dimensional surface like the globe. To offer some examples of georeferenced historical maps, we prepared some National Topographic Series maps hosted on the University of Toronto Map Library website courtesy of Marcel Fortin, and we overlaid them on a Google web map. Viewers can adjust the transparency with the slider bar on the top right, view the historical map as an overlay on terrain or satellite images, or click ‘Earth’ to switch into Google Earth mode and see 3D elevation and modern buildings (in Halifax and Dartmouth). Note: these historical images are large and will appear on the screen slowly, especially as you zoom into the Google map.
