Maps are navigational aids or information sources created by cartographers. Maps provide us with a two-dimensional representation of a physical place. Settlers like the Donnellys, for instance, utilized maps to determine the lands owned by the Canada Company that were available for purchase.

Historians utilize maps for a number of reasons. Most obviously, maps provide a visual image of a location, whether that location is as large as the world, or as small as a murder scene. Maps can also provide us with information that can allow us to draw certain inferences from other sources available to us.

Some of the maps on this website, such as the one of Middlesex County, are primary sources in that they were created at or close to the time under study. Other maps, however, are secondary sources in that specialists for this site created them. They have been included in this site as "sources" rather than in the “Interpretations" section however, because they are based on primary sources. For instance, Roland Longpre used primary as well as secondary data to develop the maps/diagrams of the murder scenes at the Donnellys’ homestead and William’s home.

One might assume that maps can always be trusted, but since maps are the cartographer’s conception of a location, they are not unbiased, neutral sources. A cartographer might make mistakes, or even purposely leave out details on a map. Like any other source, maps should be questioned and evaluated. Moreover, maps are sometimes difficult to interpret since they are two-dimensional representations of a three-dimensional space. Fortunately, you will also find a three-dimensional reconstruction of the Donnelly homestead on this website. As computer applications become more powerful, perhaps two-dimensional maps will become a thing of the past.

Many libraries and archives have map collections that are generally separate from the main collections.