Building the Salem Witchcraft GIS

Digitizing the Upham Map

J. Quality and Usefulness of Control Points

With control points established, we now began the process of registration and rectification of the Upham map image. Registration refers to the process of linking an image to real geographic coordinates with GIS data. Rectification refers to the transformation process in which a registered image is "distorted" to orient it to true north. Both steps are necessary to produce georeferenced points from Upham's map. Registration is a trial-and-error process, an art, rather than a science. It essentially involves significant human judgment to account for uncertainty, and thus introduces more error.

Registration and rectification were performed in ArcInfo 7.2.1, which requires at least three widely dispersed control points. We have sixteen, but of unequal value and quality.

Several of these points, and particularly those on the northeastern, northwestern, and southwestern corners, represented large area features: ponds, hills, or islands within hills. It is difficult to match a point (e.g., a set of precise geographic coordinates) with a large area feature that could contain many such points. Imprecise matches of these points could result in a poor overall transformation.

Furthermore, at the outset, we could not be sure of the quality of Upham's points. If Upham incorrectly drew the distance between two sites we used as control points, linking his drawing to our data would involve an erroneous match.