A place in history: a guide to using GIS in historical research

CHAPTER 3: ACQUIRING SPATIAL DATA

3.4 Geo-referencing

Whether data have been scanned or digitised, their underlying coordinate scheme at this stage of the data capture process will usually be in inches or centimetres measured from the bottom left hand corner of the scanner or digitiser. Geo-referencing is the process by which these coordinates are converted into real-world coordinates on a projection system. This allows distances and areas to be calculated and data from different sources to be integrated.

Map projections are intricate and complicated. In a country such as Britain where the use of the National Grid is almost universal, a detailed understanding of projection systems is rarely necessary. In this guide only the briefest description will be given, further details can be found in works listed in the bibliography. The earth is a globe and locations on that globe are described using latitude, the number of degrees north or south of the equator, and longitude, the number of degrees east or west of the Greenwich meridian. Maps are flat sheets of paper. Projections are the translations used to convert from a curved earth to a flat map surface. Doing this involves distorting one or more of distances, angles, areas or shapes. A projection will also often convert from degrees of longitude and latitude to miles or kilometres from a particular location with longitude becoming the x-coordinate and latitude becoming the y-coordinate.

The British National Grid is a Transverse Mercator projection. The origin of its ellipsoid runs north-south at 2° west of the Greenwich meridian, approximating to the central spine of Britain. As one moves east or west from this line, distances in particular become increasingly distorted. Fortunately, as Britain is a long, thin country running approximately north-south these distortions are rarely significant at the kind of scales at which historians operate. Traditionally, Britain was subdivided into grid squares with sides of 100km. Each square was given a two-letter identifier and locations were expressed as distances in kilometres or metres from the south-west corner of the grid square. For example, NN is a grid square in the southern highlands of Scotland. Within a computer, this use of letter codes is clumsy. Instead a false origin is given for the whole country. This is a point south-west of the Scilly Isles that allows all locations on mainland Britain to be expressed to the nearest metre as non-negative integers of no more than six figures. Location 253000, 720000 is 253km east of the false origin and 720km north and is in the southern highlands. This structure allows easy calculations of the distances between any two points expressed using six-figure National Grid references.

Most GIS software packages make the process of geo-referencing appear quite straightforward. Using the source map the user finds the real-world coordinates of a number of reference points, usually four. These are also called tic points and are often the corners of the map sheet. These points are then digitised. The user is then prompted to type in the real-world coordinates of the reference points and the software uses these four points to convert every coordinate in the layer to real-world coordinates. Frequently the software will also prompt for a projection system at this stage and convert the layer accordingly. This means that the locations of all points on the layer will be expressed in National Grid coordinates or whatever other coordinate system has been selected, and that all distances measured on the layer will be in metres, kilometres, or whatever referencing unit is used.

While this is relatively easy using modern maps where coordinate grids are shown and projection information is readily available, it can be difficult using older maps. Where no coordinate grid is provided, this can be worked around by finding reference points on the source map that are also mapped on modern maps (appropriate features may include churches, lighthouses, trig-points or railway stations). The coordinates of these points can then be found using modern maps that do include a coordinate grid. For the sake of accuracy the modern maps used should preferably be larger scale than the source map. If no information is provided on map projections then books such as Delano-Smith and Kain (1999), Harley (1975), Oliver (1993), Owen and Pilbeam (1992) are good sources of information. For work on small study areas the impact of projections will be so small that they may not be worth bothering with.