The National Library of Scotland, British Library, David Rumsey Map Collection and other institutions use the online Georeferencer application for their collaborative map georeferencing projects. Due to differences in scale, projection and accuracy the historical map will not match a modern reference map. Imagine the historical map was printed on a rubber sheet, which you can stretch and pin it down to the modern map at known points. By the magic of mapping technology, Georeferencer does all the calculations behind the transformation.
An example from the British Library is a map of the parish of Thompson, a village in rural Norfolk, England a few miles from my home. It was published in a book about the history of Thompson published in 1892, but it was redrawn from an earlier map dated to around 1725 which shows open fields, a farming system from medieval times.
Having selected this map and opened it in the British Library georeferencer, my first job was to locate Thompson on the modern map and zoom in so I could see features marked on both maps. I started with the church, a good bet in rural England where churches may stand for centuries. Other features required some interpretation and local knowledge. Rivers and roads may change course so are not good candidates for control points. In this case the parish boundary has remained the same so points where it intersects other features or sharply changes direction are helpful. I needed to use several of the supplied reference maps and satellite images to find enough features for 10 control points.
|Control points on the Thompson map on the left match reference maps and satellite image on the right.|
The visualize tab gives 2D, 3D and side by side viewing options. The 2D option lays the historical map over Google maps, and the slider changes the transparency of the historical layer. The 3D option opens Google Earth in a small window, so you would do better to click on the 'Open KML in Google Earth' link. Notice many of the field boundaries follow older features. I had not realised that Thompson Water is a modern feature!
|Overlay of Thompson map in Google Earth|
A harder example, from The National Library of Scotland, is a map of Islay, an island off the west coast of Scotland, dated 1654. Many of the place names on the historical do not appear reference maps. Variant spellings of place names add to the challenge. Notice that the historical map appears distorted compared to the modern map.
|Comparison of Islay map on left with modern map right, with control points|
Eventually I managed to locate 36 control points. In the process I gained an insight that I would never have realised otherwise. This map was drawn from the perspective of someone at sea. The relative positions of settlements as seen from the sea is emphasized. At the time this map was made, sea travel predominated. Geographic accuracy is compromised and, even with a lot of correction, only achieves a poor match to the modern map.
|Transformations of Islay map, increasing correction from left to right. Left:affine, centre:polynomial, right: TPS|
All maps lie because representing the earth's surface on a flat piece of paper always requires compromise. Although we think of the earth as a sphere, in reality it is flattened at the poles and very lumpy. The mathematical transformation of geographic co-ordinates onto a flat plane is called a projection. Projections preserve one of area, distance or direction while distorting the other two. The most familiar Mercator projection preserves direction, important for nautical navigation, but distorts the area of objects near the poles.
Ready to explore for yourself? Go and have fun discovering things were not as you thought.