Basic geo-referencing workflow

For an introduction to georeferencing, the Wikipedia definition of Georeferencing summarises the main purpose and methods involved. To view a project using georeferencing, C. Balletti's Georeference in the analysis of the geometric content of early maps provides a succesfull example.

In these help guides, we will illustrate three methods of Georeferencing: using ArcGIS software, using Quantum GIS (QGIS) software, and using the online Georeferencer. The steps below detail the basic workflow that will be followed using any of these methods.

  1. Cropping your images. Crop out the non-geographic parts of your map (ie. the margins or blank areas beyond the map’s boundaries). This is not essential but can create a tidier result. It is especially useful for multi-sheet and series maps. Some useful free software for editing and cropping images includes ImageMagick, Irfanview and GIMP.
  2. Fitting your historic map to the real-world. To fit your historic map to its present day real-world location, you need to give it both a suitable coordinate system and transformation method. Control Points are then used to correctly position your historic map.
    • Chosing your Coordinate Reference System. The first decision you must make is which coordinate system you will work in. This choice is particularly relevant to how you intend to use your georeferenced historic map. It affects how it is geographically displayed and can dictate the interfaces it is able to be used within. In the United Kingdom, the British National Grid is often easiest to use, and allows integration with Ordnance Survey gazetteers and mapping. Internationally, the Global Mercator projection on a spherical approximation of the Earth (usually termed the Spherical Mercator) is widely used (eg. in Google Maps and Earth, Microsoft Bing, Yahoo Maps). Further information on Coordinate Reference Systems.
    • Chosing your Transformation Method. The second decision you must make is to choose a transformation method. This choice dictates how your historic map will be transformed into your output georeferenced map image. Different Transformation Methods control how your historical map is positioned, stretched or warped to fit modern mapping. For example, the affine transformation rotates and enlarges the map, preserving the essential shape of the original. The polynomial or adjust transformation employs rubber sheeting or warping. The spline transformation, as a true rubber sheeting method, positions the source control points exactly to the target control points, optimising local accuracy (of the control points), rather than global accuracy. The adjust transformation uses a mixture of the spline and affine transformation, taking account of the control points within a broader context, so optimises for both global and local accuracy (Boutoura & Livieratos, 2006).
    • Adding Control Points. The next stage is to add Control Points. You place sets of Controls Points at chosen known locations on your historic map, and at the equivalent corresponding locations on the modern map you are using as your georeferencing base. These Control points effectively link the locations you chose to place them at, and they position your historic map, correctly aligned according to your modern map.
  3. Saving your Georeferencing. Once the historic map has been transformed to its new position, and you are happy with the accuracy of the results (sometimes called the 'image registration') you need to save the georeferencing information you have created. Depending on which georeferencing method you use, you can either save a seperate file that contains the georeferencing information for your historic map, or you can save a new georeferenced version of your historic map that has the georeferencing information embeded.

The following links provide detailed workflows for how to georeference a map using the desktop ArcGIS and QuantumGIS (QGIS) software, as well as using the online Georeferencer.