Scale factor and image resolution: some cartographic considerations

The preservation of our cartographic heritage has long advocated the transformation of historical maps or, more generally, of paper maps produced by analogue methods into digital format. The development of GIS techniques and cartographic databases has allowed increasingly rapid georeferencing of scanned maps into global datums like WGS84. The prerequisite for good georeferencing is, however, good digital transformation of the paper map format. This is of course a technical issue, but it also has some mapping implications connected to cartographic generalization theory. The subject of this paper is to connect the well-recognized cartographic generalization concept (the graphicism error) to the resolution of the scanned image (measured in SPI). The core of this paper’s issue must be of course clear to all the technicians that are involved in map digitalizations, for instance at the several public cartographic archives. Not only: because of the fact that most of the current maps are in digital format, we think that the given concept should also be taught to medium/high level students of Cartography and to base level GIS students. After a short introduction on some technical features linked to the scanning process (DPI, PPI, SPI), the scale factor of a map is briefly recalled. Then the numerical relation between the scanning resolution and the scale of the paper map is given. Awareness of this relation is useful to avoid scanner accuracy superseding the accuracy of the scanned map.