The name authorities used to build our Administrative Gazetteer generally contain no maps, or other information on locations in space. We have added boundary maps, stored as strings of coordinates within our database, from a number of sources:
- Boundaries for modern districts, counties and regions reported on by the 2001 Census are very accurate Ordnance Survey data provided through the Office for National Statistics and the General Register Office in Edinburgh, although our Statistical Atlas uses a "generalised" version of these boundaries to speed up map creation by the web site.
- Most historical boundaries in the system were researched by the Great Britain Historical GIS Project. Our main sources were the County Administrative Diagrams published at two miles to the inch scale by the Ordnance Survey in several editions, beginning around 1900. We extended our mapping back into the 19th century based on the lists of boundary changes in census and vital registration reports, plus the unpublished boundary maps prepared for the Registrar General and now held in class RG.18 in the National Archives. However, it was not possible to identify all the detached portions of parishes eliminated by the Divided Parishes Acts of 1876 and 1882.
- Our mapping of boundaries within the old County of London used six inch to the mile maps, while the extremely detailed mapping of the tiny parishes within the City of London was provided by Craig Spence, based on research he undertook at the Centre for Metropolitan History, University of London.
- Boundaries for Irish counties and baronies were created by the Centre for Data Digitisation and Analysis at the Queens' University, Belfast.
- Because our historic boundary maps were hand-digitised from original maps mostly published at one or two miles to the inch, it cannot be as accurate as modern Ordnance Survey data. Generally, features are accurate to between 100 and 200 metres. Note that most of the Ordnance Survey maps used to create our vector boundaries are available in our historical map library as images, and these are necessarily more accurate if all you want is a visual sense of the location of boundaries.
- The system includes boundaries for parishes, counties and the most important districts for statistical reporting. We have computed approximate locations for most other units based on their relationships with units that do have boundaries. For example, ancient districts like Hundreds and Wapentakes have locations based on the average of the centres of their component parishes, which lets us locate them on thumbnail maps of Britain.