The Locating London’s Past blog has a pair of fascinating new posts by Peter Rauxloh about the process of creating the ‘new’ version of Rocque’s 1746 map.
However, georeferencing an historic map is generally more tricky than a modern aerial photograph, for two reasons. First is the obvious fact that it can be challenging to identify pairs of long-lived common points on both the historic map and that produced 200 years later. Secondly, while the accuracy of the aerial photograph or infra-red scan is generally effected by systematic linear machine or design errors like poor camera calibration or inadequate resolution, the optical ground survey that created the old map, had to contend with both systematic error such as poor instrument adjustment or a stretched measuring chain, and a panoply of non-linear sporadic sources of gross and minor error which can occur during the survey itself and in the technique and materials used for its depiction.
The last post looked at how the single composite scan made from the 24 separate sheets of John Rocque’s 1746 map of central London, were fitted in place over moden mapping using the process named georeferencing. When applied to historic maps, this process will frequently distort their original orthogonal form, as the image is wrestled into its correct location. This posts examines a possible explanation of the warping that took place when the Rocque map was subject to this process.