Accuracy of BGS legacy digital geological map data

This report forms part of the output from the British Geological Survey’s Digital Geological Map of Great Britain (DiGMapGB) project which is responsible for providing nationwide digital geological map data at a range of scales. It summarises various aspects of map-making, including traditional geological surveying and digitisation which combine together with other factors to determine the ‘accuracy’ of digital geological data. The report is intended primarily for GIS users of the data, especially nongeologists. First, it describes what a geological map and a digital geological map are. The particular meanings of ‘accuracy’ and ‘legacy’ used in the context of this report are then explained. The ‘accuracy statement’ first issued for digital map data in 2001, is provided. As this covers only the spatial and cartographical aspects the need for a fuller explanation of the many factors that contribute to the overall accuracy of the geological linework is noted. Second, the report provides basic information about the DiGMapGB datasets, notes the many users and illustrates the need to understand data accuracy for correct analysis. Third, there is background information on making geological maps, some principles of their survey and compilation, the different types of geological boundary, the selection and generalisation process, and comments on the reliability of geological interpretations. For those seeking a greater level of technical detail the accuracy of Ordnance Survey topographical base maps is then described in cartographical and positional terms and related to GPS accuracy. This is followed by a similar discussion of the positional accuracy of BGS geological maps at different scales including specifications for the accuracy of digitisation. Final concluding remarks compare the significance of the various factors that control the accuracy of digital geological map data allowing users to assess their relative importance