Changes in administrative structures over time has profound implications for the organisation of topographically ordered research data. One example could be the numerous changes in the municipal structure in Denmark the last 150 years.

Mapping the huge amount of changes over the past 350 years, the DigDag project (Digital atlas of the Danish historical-administrative geography) has established a uniform research infrastructure: a digital cartographical skeleton for thematic mapping and analysis. This research infrastructure, available at www.digdag.dk, currently contains more than 70,000 GIS polygons showing the geographical extent of Danish administrative units in their varying historical forms.

Thus, for instance epidemiological data from the 19th century tied to – now obsolete and practically forgotten – "physicates" or "physicians' districts" can finally be analysed in a geographical context, and for instance historical censuses tied to an obsolete parish structure can now be depicted more accurately.

Digitisation of historical place-name data is a key to establishing an efficient search facility, and though not fully integrated yet, the place-name data resulting from the project can be accessed with enhanced search facilities via www.danmarksstednavne.dk. First and foremost, these 150,000 entries originate from a digitisation of the printed series Danmarks Stednavne.