User Guide: RADON POTENTIAL DATASET - England and Wales

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User Guide: RADON POTENTIAL DATASET - England and Wales This document provides information for users of the joint HPA-BGS RADON POTENTIAL DATASET for England and Wales. 1.

1 User Guide: RADON POTENTIAL DATASET - England and Wales This document provides information for users of the joint HPA-BGS RADON POTENTIAL DATASET for England and Wales. 1. Background to joint Health Protection Agency (HPA) - British Geological Survey (BGS) digital RADON POTENTIAL DATASET for England and Wales Radon is a natural radioactive gas, which enters buildings from the ground. Exposure to high concentrations increases the risk of lung cancer. The Health Protection Agency recommends that radon levels should be reduced in homes where the annual average is at or above 200 becquerels per cubic metre (200 Bq m -3 ). This is termed the Action Level. The Health Protection Agency defines radon Affected Areas as those with 1% chance or more of a house having a radon concentration at or above the Action Level of 200 Bq m -3. The joint HPA-BGS digital RADON POTENTIAL DATASET provides the current definitive map of radon Affected Areas in England and Wales. It will also allow an estimate to be made of the probability that an individual property in England and Wales is at or above the Action Level for radon. This information also provides an answer to one of the standard legal enquiries on house purchase in England and Wales, known as CON29 standard Enquiry of Local Authority; 3.13 Radon Gas: Location of the Property in a Radon Affected Area. The RADON POTENTIAL DATASET will also provide information on the level of protection required for new buildings under as described in the latest Building Research Establishment guidance on radon protective measures for new buildings (BR ). For Con29 enquiries the full address and post code or the National Grid coordinates of a property will be required to interrogate the RADON POTENTIAL DATASET which will provide an estimate of the probability that a particular property is at or above the Action Level for radon. For BR 211 enquiries, the dataset can also be searched using site outlines when no property addresses are available. For further information contact: Central Enquiries Radon Survey British Geological Survey Health Protection Agency Kingsley Dunham Centre Centre for Radiation, Chemical and Keyworth Environmental Hazards Nottingham Chilton, Didcot NG12 5GG Oxon OX11 0RQ Direct tel. +44(0) Direct tel. +44(0) Fax. +44(0) Fax. +44(0) enquiries@bgs.ac.uk radon@hpa.org.uk Web: Web: 2. What is the RADON POTENTIAL DATASET information for? It provides the answer to one of the standard legal enquiries on house purchase in England and Wales, known as CON29 Standard Enquiry of Local Authority; 3.13 Radon Gas: Location of the Property in a radon Affected Area. It provides information on the level of protection required for new buildings under BR 211 (2007) Radon: Guidance on protective measures for new buildings.. 08/12/ of 5 BGS NERC

2 3. Creation of the joint HPA-BGS RADON POTENTIAL DATASET Previously, the potential for high radon levels in UK houses has been mapped either on the basis of grouping the results of radon measurements in houses by grid squares or by geological units. In both cases, lognormal modelling of the distribution of radon concentrations was applied to allow the estimated proportion of houses at or above the UK radon Action Level (AL, 200 Bq m 3 ) to be mapped. Combining the grid square and geological mapping methods gives a more accurate map than either method can provide separately. The land area is first divided up using a combination of bedrock and superficial geological characteristics derived from BGS DiGMapGB-50 (1: scale) digital geological map data. Each different combination of geological characteristics may appear at the land surface in many discontinuous locations across the country. The Health Protection Agency has a database of over houses in which long-term measurements of radon concentration have been made, and whose locations are accurately known. Each of these measurements is allocated to the appropriate bedrock superficial geological combination underlying it. Taking each geological combination in turn, the spatial variation of radon potential is mapped, treating the combination as if it were continuous over the land area. All of the maps of radon potential within different geological combinations are then combined to produce a map of variation in radon potential over the whole land surface. This unbuffered dataset formed the raw data on which processing was carried out to ensure all end-users would achieve consistent results (see section 4). Further information is available in the Environmental Radon Newsletter (Issue 35 and Issue 51; and in Miles, J.C.H. and Appleton J.D., Mapping variation in radon potential both between and within geological units. Journal of Radiological Protection, 25, Technical Information The unbuffered radon potential dataset, described in section 3, was pre-processed using ArcGIS9.2 Service Pack 2: To include a 'geological data accuracy' buffer of 50 m to allow for the precision (uncertainty) of the geological lines as defined as part of the 1:50,000 scale DiGMapGB-50 product used to create the raw radon dataset. To include a 'residential property' buffer of 25 m, to allow for the average extent of homes located using Ordnance Survey ADDRESS-POINT coordinates. The geological data accuracy and residential property buffers were added together and the resultant 75-m buffer was applied sequentially to radon potential Class 6, 5, 4, 3 and 2 base radon potential data polygons (Table 1) derived from unbuffered radon potential data described in section 3. These buffered polygons, together with the Class 1 polygons, were converted to separate 25-m raster grids, which were then superimposed on each other to produce a final 25-m raster grid that gives the highest Class for each individual 25-m cell. Table 1: Radon Potential Classes and percentage bands Radon Potential Class Nominal percentage band Actual percentage band to to to to to to The radon potential dataset based on DigMapGB-50 does not quite extend to the coastline in some areas; either because reclamation of land has moved the coastline or because the 08/12/ of 5 BGS NERC

3 coastline used on the paper map from which the digital geology was captured does not quite reach the actual coastline due to the scale of the mapping. In order to obtain radon potential values in these areas, values were extrapolated by expanding the raster grid by 200 m based on the adjacent cells. 4.5 The raster grid was then converted back to a vector dataset as a shapefile without simplification of the cell boundaries. 4.6 The final RADON POTENTIAL shapefile was intersected with the British National Grid 5 km tile boundaries to split large polygons, and multipart polygons were converted to single part, to improve the performance of the final dataset. 4.7 During testing it was found that some AddressPoints joined to the buffered radon class dataset returned a value of 0 instead of a radon potential class value. This was due to the fact that if a point lies exactly on the boundary between polygons no value is returned in ArcGIS. It was discovered that some GIS systems do return a value in this situation and some do not. Also if a value is returned, it is not known which of the two possible values (from the two polygons either side of the polygon boundary) is going to be returned. To ensure consistency between different systems the dataset was moved by of a metre in both the x and y directions. This then ensures that no AddressPoint can ever lie directly on top of a boundary between polygons. As it has been decided to use AddressPoints precise (rounded) to 1 metre, the final shifted dataset will have no boundaries that have coordinates exactly on metre boundaries. This is the final RADON POTENTIAL DATASET. 4.8 When using AddressPoints against the data, they should be rounded to1 metre using the following standard practice: 0.5 and above should be rounded up and (9 recurring) and below should be rounded down. In the case of an AddressPoint, where only one digit occurs after the decimal point 0.5, 0.6, 0.7, 0.8, 0.9 should be rounded up and 0.4, 0.3, 0.2, 0.1 should be rounded down. This pre-processing of the data should ensure that all RADON POTENTIAL DATASET users will obtain the same result when using ADDRESS-POINT data rounded to 1 metre as described in section 4.8. Data users should not apply any further buffer to the rounded ADDRESS-POINT coordinates, for example to define property extent, because a 25-m buffer for this purpose is already included in the RADON POTENTIAL DATASET. Additionally, any other forms of spatial search against the data, e.g., measured grid references or site polygons, do not need to be buffered. 4.9 When making spatial searches against the data (whether using site or building outlines) the highest value polygon encountered in the search area should be used for the overall result. As the data is pre-buffered it is not necessary to buffer any search area other than to allow for any inaccuracies in defining the site outline. This pre-processed RADON POTENTIAL DATASET has been subject to rigorous quality checks involving BGS and HPA personnel. IMPORTANT NOTE REGARDING ADDRESSPOINT POSITIONAL ACCURACY: At the date of this document, the BGS is investigating how best to deal with the positional accuracy flagging within AddressPoint (as described in the OS User Guide). It is likely that Addresspoints flagged up by OS as inaccurately located (e.g. because their position is interim and due for update) will have to be rejected for searches, and users recommended to seek an alternative location method (e.g. grid reference, site plan). Please contact the BGS for more information. 08/12/ of 5 BGS NERC

4 IMPORTANT NOTE REGARDING GIS FORMAT CONVERSION: Spatial datasets are supplied by the BGS in a variety of different GIS formats. Due to the differences in precision of different formats and to small changes in precision during translation between formats, the absolute position of features in different GIS systems may vary by a few millimetres on the ground. It is strongly recommended that the data is used in the format supplied and not converted to other GIS formats as the errors can be cumulative. These issues of precision may appear to be minor but can lead to different answers being reported by different GIS software solutions. 5. Data History The spatial data for the RADON POTENTIAL DATASET are primarily derived from the 1: scale digital geological map of Great Britain, DiGMapGB-50. There are some areas of 1: scale data, for example, in upland areas of Wales, where larger scale data are not available. BGS is continually surveying and resurveying areas of Britain, improving and updating the geological maps. These updates are made to DiGMapGB-50 annually. The RADON POTENTIAL DATASET is based upon the DiGMapGB-50 Version 3 (2006) bedrock, superficial and artificial data sets. Each data layer is rectified to align with British National Grid origin. The BGS and the HPA are committed to improving the RADON POTENTIAL DATASET as more information becomes available. 6. Limitations 6.1 The RADON POTENTIAL DATASET has been developed at 1: scale and must not be used at larger scales. 6.2 A 75-m buffer has been applied to the 1: scale base radon potential data. All spatial searches of the RADON POTENTIAL DATASET should be done without applying any additional buffer. 6.3 The RADON POTENTIAL DATASET consists of vector polygons and is available in a range of GIS formats, including ArcGIS (.shp), ArcInfo Coverages and MapInfo (.tab). More specialised formats may be available but may incur additional processing costs. 6.4 The RADON POTENTIAL DATASET is concerned with radon potential related to NATURAL geological sources only. The data do NOT cover the impacts of man-made features, apart from the impact of ironstone mining over the Northampton Sand Formation. 6.5 The RADON POTENTIAL DATASET is based on, and limited to, an interpretation of the records in the possession of The British Geological Survey and The Health Protection Agency at the time the base radon potential data set was created (2006). 6.6 An indication of high radon potential does not necessarily mean that an individual property will have a high radon concentration. The only way to find out whether a property is in fact above, at or below the Action Level is to carry out a radon measurement. Guidance on measuring radon can be obtained from the HPA ( 6.7 All address searches against the data should be made using Ordnance Survey ADDRESS-POINT coordinates rounded to 1 metre as described in Section Address searches against the data should take into account the positional accuracy flags within the ADDRESS-POINT database, and users recommended to seek alternative location methods for inaccurately located addresses (see note above). 08/12/ of 5 BGS NERC

5 7. References BR 211: Radon: guidance on protective measures for new buildings.2007 edition. ISBN (Watford: IHS BRE Press) Miles, J.C.H. and Appleton J.D., Mapping variation in radon potential both between and within geological units. Journal of Radiological Protection, 25, /12/ of 5 BGS NERC

Radon levels in the area surrounding East Northants Resource Management Facility

Radon levels in the area surrounding East Northants Resource Management Facility Summary Radon information for the area and villages around the ENRMF has been obtained. The information shows that in the