London has a long history of map making. We are lucky to have so many maps that document London’s development over the centuries, which often provide our archaeologists with vital clues about what to expect when we break ground on site, and help us to interpret what we find. However, because of changes in map making technology and convention, they are often considerably more difficult to use and navigate than modern maps.

We have set out to address this through our collaboration on Layers of London, a new digital platform which will allow visitors to peel back maps spanning 300 years of London’s history, to reveal the city changing over time. We have carefully georectified 9 of these maps - in other words, pulling them around in a Geographic Information System (GIS) so that they can be laid on top of an Ordnance Survey map.

To do this, we identified hundreds of known calibration points like old churches and street junctions to anchor the historic maps to. We also created GIS datasets of the historic roads, places, ward boundaries and parish boundaries from each map. In the GIS, each road and place is represented by a polygon, and we were able to build complex and reusable ArcGIS models to streamline and automate the process of creating these. The resulting maps and searchable, downloadable GIS datasets will be invaluable resources for historians and archaeologists wishing to explore London’s development. Three factors in particular made reaching this stage a formidable challenge:

Developments in printing technology

Copper printing plate sizes were limited in the past, so some larger maps had to spread to over 30 sheets of paper. Once high-resolution images of these often-delicate pages had been obtained, we frequently found that the edges of sheets often didn’t match up, due to error in the survey and cartographic processes, which were historically more of a craft discipline than now, and with extremely variable quality control pre-19th century. Our photography team have carefully resolved these discrepancies, digitally stitching the sheets of each map together in a mammoth act of tessellation.

Changes in map conventions

Historic maps did not always follow specific rules and conventions in the way that ours do today. As a result, some older maps were drawn in receding (not vertical) view and with very pictorial, 3D representations of structures. For these, we have had to devise specific workarounds for the website, since georectifying alone would have produced a very distorted image.

Developments in survey technology

On a 1658 map drawn by Richard Newcourt, Lambeth Palace is represented so far north of its actual location that the map image became illegible when we georectified it. The swampy Lambeth marshes would probably have been very difficult for the 17th century survey team to measure across accurately using just the simple theodolites, compass bearings, metal chains and trigonometry at their disposal, especially in the absence of useful high points of reference nearby! Our solution was to cut and paste Lambeth southwards closer to where it should have been on the GIS image – the only instance throughout the project where we were forced to alter the graphic itself. 

There is also no sign on the map of the Civil War forts and Lines of Communication which would presumably have still surrounded London to some degree at that time, reminding us that any map, like history, is only ever a selective interpretation of the real world.

The Layers of London project is generously supported by National Lottery players through the Heritage Lottery Fund, and by the Stavros Niarchos Foundation, the Ford Britain Trust, and the IHR Trust. To find out more or contribute visit: www.layersoflondon.org.

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