Why georeference historical maps?
Georeferencing early maps is becoming steadily easier and cheaper, and provides several new ways of using and understanding maps. By Georeferencing maps, you can visualise and compare maps in new ways, improve retrieval and indexing methods, further their use, and develop different means of understanding and interpreting them.
There is also a DLib paper about Georeferencer covering this subject: Georeferencer: Crowdsourced Georeferencing for Map Library Collections, Volume 18, Number 11/12, November/December 2012
The Wikipedia definition of Georeferencing summarises the main purposes of georeferencing and the methods involved. C. Balletti's 'Georeference in the analysis of the geometric content of early maps' is also a helpful introduction.
There are several advantages of georeferencing historical maps:
- Georeferencing allows you to compare maps directly with other georeferenced maps. You can compare historical maps to present-day maps of the same location, or compare them to other historical maps at different points in time. The Visualising Urban Geographies project illustrates this for Maps of Edinburgh.
- Through geo-referencing, early maps can be delivered through better or more familiar interfaces. Georeferenced maps can be easily integrated with Google Maps and Google Earth or related modern mapping applications. This takes advantage of the growing familiarity with these search tools and their user interfaces. It is also possible to allow historical maps to be delivered into other websites through using open standards, such as the OGC Web Map Service (WMS) protocol. Also, through the digital mosaicking and joining of individual map sheets, georeferencing allows the delivery of seamed versions of separate paper maps in an easier format to use.
- Georeferencing allows new ways of integrating early maps with other spatial information. The Tracks in Time Leeds Tithe Map project uses the georeferencing of tithe maps to examine social and economic information from tithe schedules. The Russian navy 18th century maps of Beiruit models historical urban spaces by georeferencing historical maps and overlaying them with height information. The AddressingHistory project illustrates the potential of linking together georeferenced street directories and maps for Edinburgh.
- Georeferencing can lead towards a better means of accessing early maps, through improved search and retrieval. For example, once georeferenced, numerical geospatial metadata can allow better searching of historical maps, as well as their greater visibility to global geographic search and retrieval services.
- Georeferencing can allow new ways of understanding the content of early maps. Once georeferenced, map geometry can be analysed (for example, using the MapAnalyst program for examining planimetric accuracy). John Hessler's Warping History blog provides detailed examples of how cartometric analysis can be usefully applied to further the understanding of historical maps.
Why be cautious about georeferencing historical maps?
There are some important methodological and practical reservations about the value of georeferencing historical maps:
- Georeferencing misrepresents and distorts maps by explicitly transforming or warping them. These historical maps were not intended to be changed and viewed in this way. As such, it can misuse or misinterpret historic maps, visualising them in ways that their makers and users would probably never have intended, and lead to false ideas and assumptions about the maps themselves. Georeferencing does not necessarily improve a historical map or make it more accurate. In the course of transforming the original map, georeferencing changes lines and shapes, the distances between objects, the map's aesthetics and its value as a cultural object.
- The georeferencing process overemphasises geometric accuracy or inaccuracy of historic maps and therefore downplays all other aspects of their content - their symbolism, placenames, the way they represent features, their purpose and usage, ideology, political and cultural role and bias, and other meanings. Taken on its own, georeferencing can therefore reinforce a positivist, modern and simplistic concept that the history of mapping is simply a history of improvements in accuracy over time, ending up with present day maps that are best of all, as well as the misconception that more accurate maps are 'better' maps. Both of these ideas have rightly been discredited.
Some of these issues were usefully discussed on MapHist in March 2009. Whilst they are important reservations, georeferencing arguably still has a role for particular maps in particular circumstances, provided these reservations are borne in mind. What may be useful and appropriate for certain 19th or 20th century maps may not be useful or appropriate for 16th or 17th century maps. The AHDS Guide to Using GIS in Historical Research has good further information about using georeferenced maps and other information in historical applications. The NLS Map images website shows all maps first and foremost in their authentic, original state as ungeoreferenced images. The Georeferencer application allows ways of creating and viewing georeferenced versions as an added extra, only for those who wish to explore this technique and visualise the maps in this different way.
This is only a brief look at this topic and we would be interested to hear your views. Please e-mail firstname.lastname@example.org to provide general comments/feedback.