Denmark seen from the air -: a unique open-data repository for historical aerial imagery

Since 2012, the Royal Danish Library has digitized more than 1.5 million historical aerial images from its collection of analog aerial photographs. The digitized imagery consists of both vertical and oblique images originally recorded by several private companies and government agencies between the 1930s and 2010, which means that the data provides a high temporal and spatial coverage of most places in Denmark. All images have been geo-located by the help by volunteers using an online crowdsourcing platform and is freely accessible at the platform and through an Application Programming Interface (API). The archive of digitized and geolocated images offers a unique historical repository, which documents both urban and rural spaces in Denmark throughout the 20th century. Previously these aerial images have, in their analog form, been used in land use and land cover mapping, aerial archeology and for the identification of environmental risk (identification of historical dumpsites and petrol stations). However, as digital, geo-located and open data, these images can represent an interesting historical data-source for other remote sensing work, for example, as reference data for use with satellite (e.g. 1970s – 2010 Landsat and SPOT) imagery and “gap-filling” in longitudinal studies. This poster will present examples of image types held in the repository and how to retrieve data from the platform and API. Moreover, the poster will also reflect on quality of the data and the use of volunteers in data processing.