The last three posts looked at how the 24 individual sheets of John Rocque’s map of 1746 showing central London and its immediate environs, were stitched together to create a single image, how that image was then referenced to the real world and how the warping evident on the map following that process may be explained. This post considers how this map was then used to identify locations within 1746 London, and needs to start by re-stating what the purpose of the exercise was.
The project essentially produced two datasets; a single georeferenced image of the 1746 map that had originally been published as 24 separate sheets, and a set of points to indicate the location of all the named streets and places and the parish and ward in which they exist. This latter point data would provide a valuable resource for any other researchers of the period, because it would allow data gathered by street and parish – tax returns, average age of death, quantity of beer consumed etc. – to be plotted on a map. In short the project provided a resource whereby researchers can more easily engage with the significance of space and location in their research.
This existing data set also had to be extended such that there was a separate point for each combination of the spatial criteria we were using, (Ward, Parish and Street). Thus whereas the existing index would have a single point for the centre of a street, our aim would be to have 4 points for that street because it passes through 4 different parishes. Only in this way could we deliver the same degree of spatial resolution which the researcher had gone to some lengths to capture. For example, the street of Cheapside in centralLondon is some c.500 m long and traverses 7 parishes.
Thus the new data set had to under go two processes; the georeferencing of the existing point data and the extension of that dataset into the more comprehensive state. For both of these processes the first question to face was which map base ought to be used locate and create these datasets; the georeferenced Rocque map or the more accurate 1st/2nd edition OS data that came out some 50 years later.
Embedding the data in reliable mapping.
This question referred back to both the guiding principal of the project and a legacy the project’s data sets would provide. The principal was that whatever data sets were produce they had to be embedded in a geographical reality, while the legacy of the work, was that the data it produced would enable the easy creation of similar spatially aware indices of location for the area at earlier or later times.
Comparison of area around north Cheapside on 1st edition and Rocque maps
The stated principle effectively ruled out the use of the Rocque map as the main source of new data capture. While the georeferencing was generally accurate, the detail suffered as it invariably would where relatively primitive survey technique and a coarse style of depiction were employed. Thus an alley way shown on the Rocque map would not exactly overlie the same alleyway shown on the far more accurate ordnance survey data of some 50 years later. Where the divergence between the two representations was greater than the width of the feature itself, one is clearly in an untenable position.
The desired legacy also gave against the use of Rocque for new data capture, since if it were solely used, then one would be left with a good fit between the digitised data and the 1746 map, but it would be of limited use if one wished to create a similar index for another map – perhaps the Horwood map of 1799. Embedded within spatial data derived solely from the Rocque map, would be all that map’s peculiar collection of errors. The likely effect of this would be that all such data would need to be edited to agree with the 1799 map if not re-created from scratch. If the new data was based on digitisation from the early edition ordnance survey map, then that would remain the benchmark, the authoritative version of the man street lay out.
Processing the existing index of places
The existing index recorded the details of places on a per sheet basis, each of which needed to be transformed. Two approaches were possible here. Since we knew both where in the 8 x 3 matrix of published sheets each one existed, and where on each scan the actual area of cartography was located (i.e. excluding any borders), it would have been possible to calculate the pixel coordinates of a point within an image made up of all 24 sheets. The point data was transformed to the Ordnance survey grid using a simple two –point transformation, where the graphical and real-world geo-graphical coordinates for two points are determined and the entire data is transformed on this basis. This simple change can be compared with the more complex warping of spatial information which occurred when the Rocque map was georeferenced. This raster map is a continuous surface of data – like a photograph – and that surface can be stretched as if it were printed on a sheet of rubber, such that different locations are moved by different amounts. Conversely the point data is discrete; one point is independent of another and cannot be shifted in such a differential manner.
The second approach was more useful, which was simply to carry out the same two point transformation, but to do it for each sheet at a time, i.e. 24 operations. This was more beneficial since it limited error to that found in a single sheet. While point location would still degrade with distance from the pairs of points used in the transformation, on a sheet -by-sheet basis that distance and consequent degrading effect was less pronounced (See below)
Points plotted from existing index of places following, georeferenced on a per-sheet basis
However it is important to note that even this operation was not expected to drop one of the existing points into precisely the correct spot as shown on the existing Old Bailey Online map tiles. This point will be returned too.
Extending the data set
This component of work became the largest element, since it required that the extent of all thoroughfares present in 1746 be mapped, thus an entire street network had to be derived. Returning to the example of Cheapside, our aim was to create 8 separate points for each parish this street traverses, and to do this we needed to start with a polygon which would represent that thoroughfare. This would allow us apply a geoprocessing operation where by polygons representing parishes would be intersected with the polygon representing Cheapside to produce 8 new baby polygons whose centre could be calculated.
So expansion of the place data set began with the digitisation of a connected network of street lines from the ordnance survey maps, to include all routes shown on the Rocque map. Lines would be traced along routes and be ‘snapped’ at one or both ends to another line in the network.
Completed street network for main area covered by Rocque's map.
The street network was a line data set, but for each street, two other items of data were recorded. The first was an average width of the street, the second a categorisation of the routes precedence within a street hierarchy. There were 7 levels of thoroughfare defined.
|Main thoroughfare – outsized e.g. Holborn,Cheapsidetypically 20m width. This code is also to be used for those roads that run around squares and similar entities.
|Main thoroughfare – standard width (c.15m)
|Secondary thoroughfare – standard width (c.10m)
|Tertiary thoroughfare – standard width (c.6m)
|Alley way – i.e. a narrow route running between two streets or street and place polygon (1-2m)
|Cul-de-sac plus area – i.e. a dead end but one which opens out into a court. The wider area is captured as a polygon and will be amalgamated with class 6 streets (typically 1-2m but up to 10 for a Mews)
|Cul-de-sac – i.e. a simple dead-end. (1-2m but up to 10 for a Mews)
Detail of street network showing colour graded street classifications
This was accompanied by the creation of a polygon layer that recorded all the areas (squares, notable buildings, yards etc.) which were shown on Rocque, including on occasion, portions of street where there was a short but marked increase in width. These polygons were classed as either standalone (e.g. a churchyard or notable building) or connected, i.e. a wider area too which a narrower thoroughfare led, for example a yard.
The Holborn viaduct dilemma
As described, the street network and place polygons were digitised from the 1st and 2nd OS mapping, with the warped Rocque map acting as a guide as to which roads to digitise (since there were many more of them by the early 19th century). However of course, the reverse was also true; the Rocque map would often show detail which had been obliterated by later developments. A good example of this is the re-working of the area around Holborn viaduct. In such cases we only had recourse to digitise from the Rocque map itself. Aware of the degree of warping that this map exhibited following its georeferencing, a leaf was taken out the method adopted for the processing of the existing place point index, and individual, focussed georeferencing operations were performed.
This began by delineating with a temporary polygon a problem area – like Holborn viaduct – as requiring special attention. Then the warped Rocque map was turned on, and would display the area at the time in roughly the right place. At this point the map was georeferenced again, but solely using common points around the area of change which yielded an improved fit between it and the OS data in just that area. The quality of fit outside that area being irrelevant.
These width and street rank of the roads were recorded so that two GIS operation could be performed on the data. The first used the width to create a polygon from the centre line of each street to which any connected polygons would be amalgamated. The street rank, was then used to determine which polygon ought to be trimmed by another(Fig) so that overlapping was minimised. Where streets of the same rank joined (as shown in the top left of the image ) no trimming took place.
Street lines expanded to polygons based on recorded width
Street polygons trimmed on the basis of the hierarchy of routes
The reason these buffer and trim operations took place was to yield a clean set of polygons that could then be intersected with the parish and ward polygons also being prepared. Having streets represented as polygons, as opposed to lines enabled us to generate both a more accurate centroid for each street and parish overlap, but more importantly it enabled us to see many of them in the first place. In 1746 and indeed today both ecclesiastical and administrative boundaries will often follow a street centreline; if streets were only represented using lines, we would miss the fact that one half of a street is in one parish and the other is in a different one.
A final point here is that as discussed at the outset, the ultimate purpose of the operation was to enable the creation of an accurate point data set. The street centrelines, buffered out to a standard width, do not, and were not meant to trace the fine edge of the actual street edge as shown on OS or Rocque maps; rather they are an approximation to those edges. This was both an expedient approach to achieve the desired goal, but more pragmatically the 1 inch to 200 foot scale adopted by Rocque was simply too coarse to capture such edge detail. Even with perfect accuracy, at this scale wobbles in the street edge of less than 2.4 meters could not really be shown.
By Patrick Manix of Motco Limited
GIS shorthand for the operation whereby one piece of vector data is connected
to another, i.e. there is no chance of having a miss-match.
This focus on boundary accuracy was another reason why the digital capture programme had to take place using the OS rather than the georeferenced Rocque mapping.