The last post explained how scans of the 24 individual sheets of John Rocque’s 1746 map of centralLondonhad been fitted together so they comprised a single image. In this post the process by which that image was taken and fitted to the real world is discussed. That process is called geoeferencing.
Georeferencing is the process by which an electronic image of the earth is located onto the earth in the right place, so that the features it depicts overlie the same features shown on a current measured reality. At the start of the process the image’s dimensions are purely graphical ones measured in pixels i.e. the number cells it has across and down. At the end of the process those same pixels have a geographic coordinate value and represent a certain distance on the ground. In archaeology, georeferencing is commonly used to overlay remotely sensed images taken from air or space in both visible and non-visible spectrums. When done correctly, the process allows the features shown on one image to overlay the same features on the map too which it is referenced. However the great benefit of georeferencing to archaeology, is that it allows features which are not shown on the modern map to be correctly positioned and investigated. An aerial photograph captures a low sun picking out the undulations of a medieval field system, an infra-red image isolates the tell-tale sign of outcropping prehistoric peats along an eroding river bank, an airborne radar provides a detailed image of elevation, within which the previously hidden circuit of an iron-age hill fort emerges from beneath dense tree cover.
Historic maps are also a common subject for georeferencing by archaeologists, the landscape features they depict are often absent from their modern counterparts, making them invaluable when trying to gain an understanding of a site’s history. Such images are georefernced in exactly the same way as modern ones, by identifying pairs of points which are visible on both the un-referenced image and the modern map , and using them to calculate how much the image must be rotated and scaled and where it should be located in the modern coordinate system.
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. What this means in practice is that the accuracy errors in the aerial photograph are less common, and more easily spotted and mitigated. A square vertical aerial photograph will typically still describe a square shape on the ground once it has been georefernced. The less consistent error present in an historic map will typically require it to be warped in order to fit to the modern measured reality. Thus the orthogonal scan of an historic map plate will typically have a distinctly irregular, warped shape after it has been georefernced.
The georeferencing operation carried out for the project involved identifying some 48 common points with which the transformation of the image took place. As each pair of points is added to the set, the computer provides an average measure of disagreement between them, and this allows one to reject those pairs which greatly increase this value. To understand how this is achieved we can imagine a perfectly square plan of a Roman fort shown on an antiquarian plan. We may wish to georeference the old plan and do it using a, modern ordnance survey map which depicts the same feature. To determine if the attempt to identify the same four corners of the fort on both layers has been successful, we can look at the respective lengths between the points. The ratio of distances between points on both maps should be the same, in other words, we are asking, do the four points on each map describe the same square shape, if they do the average measure of disagreement will be low. Thus the difference in the combined average ratio between points from each map, is then a measure of how accurate we have been in identifying common points.
When georeferencing the Rocque map, it is interesting to note that greater levels of agreement were achieved when making use of the junctions of road centrelines as the common points rather than the corners of notable buildings. The latter are a common and useful target for georeferencing, since buildings like churches tend to endure and so provide useful common reference points. However the basic technique Rocque employed for his map, were angular and distance measurements made at street level with theodolite and chain and the detail of buildings that lined those roads were more sketchy. In the funding proposal Rocque and his engraver John Pine made for the map, they note that its scale of 200 feet to the inch would allow:
“an exact description of all the Squares, Streets, Courts and Alleys in their true proportions.. [plus] ground plots of the several Churches, Halls publick Buildings and considerable Houses and Gardens.” 
A depicted scale of 1 inch to 200 feet, means that a pen width on the printed map of 1mm equates to c.2.4m on the ground. Churches were engraved with a 1mm line or greater and there would need to be very little error in initial measurement of final engraving for such edges to wobble by twice or even three times this amount. Street centre lines on the other hand, have the inherent accuracy of the long sightlines that created them, and while the edges would be subject to the same degree of wobble in depiction, their centres should be more robust.
It is important to remember however there will always be error during a georeferencing operation and indeed when surveying in general. Accepting that error will inevitably occur when measuring things, and the consequent need to mitigate that error through good practice, planning and as we will see, measuring the same thing in more than one way, are all marks of a well planned survey and a competent surveyor. This is true be their instrument of choice a tape measure, total station or survey-grade GPS receiver. A history charting the development of modern spatial survey method, could take as its organising device, the recognition, mitigation, minimisation and dispersal of error.
Once a set of points had been chosen, the scanned Rocque map was then transformed, the effect of the operation being seen in the figures below. In the last post we described errors that grew out of differential shrinkage of the paper on which the individual map plates were printed. However the error in the Rocque map that cause it to be so clearly warped in order to fit the geographical reality, have a wholly different source, and one that comments on the fascinating state of surveying in the UK at the start of the 18th Century.
The next post will examine why it is that the map is warped so markedly.
 Hyde, R 2003 ‘PortrayingLondon Mid-Century – John Rocque and the Brothers Buck’ inLondon 1753, S O’Connell Ed pp28-38
An detailed blow by blow account of the area is given in Rachel Hewit’s excellent and readable publication Map of a Nation: A biography of the Ordnance Survey’ Granta Books 2010