From paper to pixels - digitized maps of vascular plant distributions in Denmark in the early 20th century

Historic data are essential for quantifying changes in biodiversity and their drivers. Much legacy data remains undigitized in archives, such as natural history museums, representing untapped resources for assessing past biodiversity patterns. In 1904, the first systematic survey of species distributions of the entire Danish vascular flora was initiated. Field surveys were essentially conducted in the early decades of the 20^th century. Results were published as black and white printed maps (1931-1976). We used automated georeferencing and image classification to extract data from all published maps. We present 1340 historic distribution maps of vascular plant species, subspecies and varieties in Denmark at 10 × 10 km resolution. These maps represent the Danish flora of native and archaeophyte species known at the time, except a dozen ubiquitous native species. By comparison with modern distribution data, long-term increases and declines can be accurately quantified, contributing to large-scale analysis of national, European or global plant diversity changes in the modern era.