The project set out to design a network of shipping routes which could be used to visualise the density of shipping traffic moving from place to place. However, although a great deal of information is available from historic archives regarding shipping traffic, the descriptions of the routes themselves often remain vague. Generally, there is insufficient information to produce an exact representation of the paths that individual vessels actually took. It was possible, however, to provide an artificial model with which to characterise the trade relationships between ports, which can be used as a basis for plotting more exact information as and when it becomes available.
The route chosen by a vessel is dependent on a series of factors including; weather, currents, tides, politics, the reason for the voyage itself and the character of vessel technology at the time. The network of routes was drawn using a web of line segments which were located to take into account known hazards which had been historically recorded, such as shallows and obstructions on the seabed.
The routes network was created using the following attributes:
UK High Water and Low Water marks provided by Ordnance Survey;
Modern 5 metre depth contour;
5 kilometre offset line from High Water contour;
Modern and historic nautical charts;
Modern and historic sailing directions.
Mapping was initially conducted using a combination of AutoCAD software for drawing and ArcView 8.3 GIS software for buffering, visualisation and querying. AutoCAD was used to produce a series of overlays containing shipping routes, modern high water and low water contours, and a modern 5m contour to allow for an incremental approach in the production of layers compatible with the GIS.
The artificial nature of the network meant that the further offshore the routes extended, the more likely they were to misrepresent the possible routes taken by ships. In order to limit this uncertainty the network was cut by a fifty kilometre buffer from the coast. The routes themselves were given a 5km buffer to avoid the erroneous impression that all vessels travelled from place to place along exactly the same line. In reality there may have been any number of possible routes followed by vessels between ports. A 5km buffer was chosen as there are many ports and routes around the UK that lie in close proximity and the use of a larger buffer may have led to difficulties in differentiating one route from another. The buffer also allowed the number of vessels recorded as travelling along particular routes to be graphically displayed while avoiding making inferences about the exact line each vessel took.
This approach, however, could only be applied to commercial traffic that tended to travel between ports and was recorded by port administration. Little evidence is available for naval voyages or small scale shipping from administrative records pre-1730. Naval records tend to describe movements between anchorages rather than ports and this meant that the movements of naval fleets and vessels were difficult to define. However, considering anchorages as quasi-ports would allow these routes to be mapped.
A further problem encountered in mapping the route network was the lack of documentary sources that recorded both the point of departure and the destination. In addition, many sources record only general locations, such as ‘Devon and Cornwall’ or ‘the South Coast’, and not the specific ports. In order to map a complete route, both the origin and destination must be identified as a specific geo-referenced point. It was thus necessary to classify the records within the database as complete or incomplete routes, so that only the complete routes would be carried through to the GIS for mapping.
Many sources record the journeys of fleets of ships but do not specify the number of vessels operating within these fleets. Therefore, it was also necessary to divide the data into fleet routes and vessel routes so the information could be represented and queried separately within the GIS and thus avoid distortion of the route network