The coloured copy of Horwood’s Plan used to produce the base map for this site is part of the British Library’s Crace Collection, a large collection of visual materials relating to London collected by the Victorian designer Frederick Crace, now divided between the BL and the British Museum. The library kindly provided images of the map’s thirty-two sheets as 250 dpi .tiffs and gave permission for the map’s use on this site.
I assembled the images of the individual sheets into one large file using the free GNU Image Manipulation Program (GIMP). A small number of minor transforms were used to make the images match up relatively neatly, but I did not attempt a perfect synthesis, partly due to the difficulty and partly as the map’s sheet divisions are interesting in themselves, so I didn’t see any particular value in trying to cover them up.
Once I had a large .png image exported from GIMP, I used a very straightforward piece of software called MapTiler to convert the image into a tiled map. MapTiler is proprietary software authored by Klokan Technologies. A free version can deal with small files and adds a watermark to the maps produced; for my purposes, MapTiler Plus was necessary. The program includes a visual georeferencing tool which lets you to pin the map to a contemporary world map; this adds co-ordinates which allow it to be accurately superimposed over other maps which use the WGS84 standard. I added co-ordinates to my image by placing a large number of pins in surviving monuments and at road junctions; while I’m not wholly happy with the alignment of the Limehouse Cut, in most cases, the correlation between the Plan and its modern equivalents is relatively good.
The tiled file produced by MapTiler is hosted on this site (which was built using WordPress). The base map can be accessed directly in a number of formats if you’re keen to explore it (Google Maps/Leaflet/Openlayers), but for the purposes of producing the annotated versions I’m using to research and explore Romantic-period London, I’m employing a plugin called Maps Marker Pro. This lets me add markers containing text and images using the WordPress dashboard and provides some neat means for organising groupings into layers and for managing overlays.
For more information on using the map interface, see this page.