Georeferencing historic maps involves using control points to attach coordinates to the maps, thus defining their physical locations on the earth. This links the map to a coordinate system and projection so that it can be viewed alongside (or on top of) current maps or other historical maps.
The Georeferencing Process
Georeferencing involves the following steps:
- establishing control points on the map
- attaching known geographic coordinates to each control point
- choosing a projection and coordinate system for the map
- increasing accuracy by reducing the differences between the actual and program-predicted coordinates of control points
For North Carolina Maps, this process generally involves finding a current, previously georeferenced map of the area covered by the historic map. We found many of these maps as shapefiles through TIGER/Line or through the NC Department of Transportation's GIS Unit. Some towns and cities also have their own GIS units offering georeferenced maps; otherwise, the applicable portions of county maps were used. Once we found applicable current maps, we then georeferenced the historic map itself using hundreds of control points in a GIS program, most often Global Mapper. A large number of control points is necessary due to the inaccuracies of older maps (disproportionate distances, creative mapping, unusual projections or lack of projection, straightening of lines, etc.) as well as the substantial changes in identifiable points (roads, intersections, cities, bodies of water, etc.) that have occurred over time. We have attempted to minimize distortion of the historic maps while maximizing accuracy.
Many GIS tools are available for georeferencing; we focused on Global Mapper and ArcMap (from ArcGIS).
Maps Selected for Georeferencing
In order to maintain a level of accuracy, we began by georeferencing early 20th-century highway maps, county-level post office maps and soil surveys, and older, hand-drawn maps of cities and towns. Older, hand-drawn maps that cover larger areas, such as counties or regions, are much more difficult to georeference. Even if they appear detailed and highly accurate, they are often skewed, and the creators likely have taken liberties in straightening streets or borders; these differences are negligible on a small scale, but the larger the area covered, the greater the visible discrepancies. Likewise, older maps covering a very small area, such as a mine or a neighborhood, are often challenging to georeference due to an insufficient number of control points.
Maps that are particularly good to georeference include railroad maps, as railroads have shifted minimally since the early 19th century, soil surveys, as they are usually highly accurate, and any city or town map with a gridded downtown.