Part of the Geochemical Baseline Survey of the Environment (G-BASE) for south-west England.
This map is based on analysis of 3720 stream sediment samples collected at an average density of one per 2.5 km2 during the field campaigns of 2004 and 2012.
The image was generated in ArcGIS 10.1 using inverse distance weighting (IDW) of the nearest 12 samples within a variable search radius capped at 10 km. Cell size is 500 m. Detailed information on sampling and analytical methods are provided in the following published geochemical atlas:
Regional geochemistry of Wales and west-central England: stream sediment and soil. 2000. British Geological Survey, Keyworth, Nottingham. ISBN 0 85272 378 4.