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Map Digitisations: Methodological Foundations, Uncertainties, and Error Margins at the example of the Gough Map
Last modified: 2011-12-18
Abstract
Historic maps, while not necessarily spatially accurate, function to convey to the researcher a sense of place - they capture locations as they presented themselves at the time of map production and, maybe more importantly, within the conceptual framework of the mapmaker’s perception. The Gough Map—drawn around 1370 and later partly redrawn and otherwise amended—is an ideal subject for the study of digitisation techniques. Not only has it been reprinted and redrawn several times, its features were digitised twice, in 2005 and 2010, offering the ability to compare advances in a fast-moving field.
At the core of the paper lies a number of sets of comparative digitisations based on the author’s own photos of the Gough map; which offer the ability to directly evaluate the accuracy of areal calculations and distance measurements using raster and vector based digitisations of the same subject, different preparations of the source images (including image filters to bring out features of interest), different conceptual approaches to uncertainty (rather than drawing only the features that the researcher is certain they have identified, marking features as ‘certain’ ‘likely’ and ‘possible’ and applying the same scrutiny in secondary research to their potential identification). This is particularly relevant to the Gough map, which shows a recognisable, but incomplete network of distance lines (which may or may not be a map of roads—this is disputed in the literature) as well as a number of fainter lines which are not usually rendered as ‘roads’ in reproductions but which might nonetheless be of cartographic significance.
The second part of this paper thus seeks to integrate the knowledge gained through digitisation with topographic information (where the modern topographic map is adjusted to accommodate key points of the historical one and vice versa, again examining the advantages and disadvantages for each technique employed) as much as placing it in the context of medieval geographies, in other words the known roads, rivers, and important settlements which would have shaped the mapmaker’s image of his world.
At the core of the paper lies a number of sets of comparative digitisations based on the author’s own photos of the Gough map; which offer the ability to directly evaluate the accuracy of areal calculations and distance measurements using raster and vector based digitisations of the same subject, different preparations of the source images (including image filters to bring out features of interest), different conceptual approaches to uncertainty (rather than drawing only the features that the researcher is certain they have identified, marking features as ‘certain’ ‘likely’ and ‘possible’ and applying the same scrutiny in secondary research to their potential identification). This is particularly relevant to the Gough map, which shows a recognisable, but incomplete network of distance lines (which may or may not be a map of roads—this is disputed in the literature) as well as a number of fainter lines which are not usually rendered as ‘roads’ in reproductions but which might nonetheless be of cartographic significance.
The second part of this paper thus seeks to integrate the knowledge gained through digitisation with topographic information (where the modern topographic map is adjusted to accommodate key points of the historical one and vice versa, again examining the advantages and disadvantages for each technique employed) as much as placing it in the context of medieval geographies, in other words the known roads, rivers, and important settlements which would have shaped the mapmaker’s image of his world.
Keywords
Historic maps; map digitisation; Gough Map; best practice