The 1:63 360 / 1:50 000 scale map series are the most useful scale for most purposes. They provide almost complete coverage of onshore Great Britain. The BGS collection of 1:63 360 and 1:50 000 scale maps comprises two map series: - Geological Survey of England and Wales 1:63 360 / 1:50 000 Geological Map Series [New Series]. These maps are based on the Ordnance Survey One-inch New Series topographic basemaps and provide almost complete coverage of England and Wales, with the exception of sheet 180 (Knighton). The quarter-sheets of 1:63 360 Old Series sheets 91 to 110 coincide with sheets 1 to 73 of the New Series maps. These earlier maps often carry two sheet numbers which refer to the Old Series and the New Series. - Geological Survey of Scotland 1:63 360 / 1:50 000 Geological Map Series. These maps are based on the Ordnance Survey First, Second, Third and Fourth editions of the One-inch map of Scotland. The maps used the most recent topographic basemap available at the time. In the Western Isles, one-inch mapping was abandoned and replaced by maps at 1:100 000 scale, which are associated with this series. Sheets were traditionally issued at 1:63 360 scale, with the first 1:50 000 maps appearing in 1972. Sheets at 1:50 000 scale may be either facsimile enlargements of an existing 1:63 360 sheets, or may contain new geology and cartography. The latter bear the additional series designation '1:50 000 series'. Within the Scottish series, new mapping at 1:50 000 scale was split into east and west sheets. For example, the original one-inch sheet 32 became 1:50 000 sheets 32E and 32W. A number of irregular sheets were also introduced with the new 1:50 000 scale mapping. There are a number of irregular special sheets within both series. Geological maps represent a geologist's compiled interpretation of the geology of an area. A geologist will consider the data available at the time, including measurements and observations collected during field campaigns, as well as their knowledge of geological processes and the geological context to create a model of the geology of an area. This model is then fitted to a topographic basemap and drawn up at the appropriate scale, with generalization if necessary, to create a geological map, which is a representation of the geological model. Explanatory notes and vertical and horizontal cross sections may be published with the map. Geological maps may be created to show various aspects of the geology, or themes. The most common map themes held by BGS are solid (later referred to as bedrock) and drift (later referred to as superficial). These maps are, for the most part, hard-copy paper records stored in the National Geoscience Data Centre (NGDC) and are delivered as digital scans through the BGS website.
