This paper investigates diatopic variation in a historical corpus of German. Based on equivalent word forms from different language areas, replacement rules and mappings are derived which describe the relations between these word forms. These rules and mappings are then interpreted as reflections of morphological, phonological or graphemic variation. Based on sample rules and mappings, we show that our approach can replicate results from historical linguistics. While previous studies were restricted to predefined word lists, or confined to single authors or texts, our approach uses a much wider range of data available in historical corpora.