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5 Strategic Noise Maps
5.1 Introduction to Noise Mapping
A noise map is analogous to a weather map, but instead of showing a temperature or percentage cloud cover it show noise levels in terms of coloured contour bands. Also the noise levels represent the noise to be experienced within a certain area over a given period of time. Therefore the level may vary throughout the day or even on a daily basis as the noise map will display levels based on annual averages.
The maps produced in response to the END are strategic maps, i.e. the noise levels are predicted using a 10m grid spacing at a receptor height of 4m above ground level, and consequently it will not be possible to use the maps to determine the noise level outside any given property. It is important to remember that the noise maps are strategic and they will be used accordingly.
Noise maps are produced by specialised computer software that calculate the noise level at a specific point as it spreads out from the sources of noise that have been included. The software can take account of features which affect the spread of noise such as buildings and the shape of the ground ( e.g. cuttings and embankments), and whether the ground is acoustically absorbent ( e.g. fields) or reflective (concrete or water).
The data used in the mapping process can be regarded as follow:
- Map data (topological and topographic)
- Attribute data
Map data is essential to identify the geographical location of objects and features to be included in the modelling assessment. Attribute data are data such as road and rail traffic flows etc.
A lot of this work is done using GIS techniques (Geographical Information Systems). A GIS may be described as a system of computer software, hardware and data, and personnel to manipulate, analyse and present information that is geo-referenced ( i.e. tied to a spatial location). Once the actual three dimensional model is created using these techniques the attribute data is added and the model processed using the specialised computer software.
The completed map can then be interrogated to determine the number of people or properties within each noise bands, the number of people properties exposed to noise above any given level etc. The noise maps can also be used to consider the effect of certain actions such as resurfacing a road or introducing noise barriers.
5.2 Limitations of Strategic Mapping
As was stated above the noise maps are based on predicted noise levels using a 10m grid spacing at a receptor height of 4m above ground level. The value of the "grid" is determined by the centre point of the grid and therefore in reality there may be some variation within the grid. Also, with a receptor point at 4m above ground level and the average "ear" at about 1.2-1.5m above ground level it should be clear that the strategic maps cannot be used to determine the level for any specific property. It would therefore be a mistake to try to categorise any site at ground floor level in terms of the Noise Exposure Categories given in the Scottish Executive Planning Advice Note 56 ( PAN 56) ( http://www.scotland.gov.uk/library/pan/pan56-00.htm). Once again it is important to remember that the noise maps are strategic and they will be used accordingly.
5.3 Data Sources for Mapping
It was explained in Section 5.1 above that two types of data are used in the creation of the noise maps; namely map data and attribute data. The map data sources are as follows; Ordinance Survey Data (MasterMap, AddressPoint, ITN, PointX, Nextmap data, Agglomeration Polygon Layer (from the Scottish Executive and Land Use Constraints polygons. The attribute data sources were as follows, Transport Model for Scotland ( TMfS) and, SERIS data from Transport Scotland (roads), Actraff data Deltarail, Geogis and Engineering Network Model ( ENM)model from Network Rail and finally .csv result files from the airport authorities.
5.4 Consolidated Maps
The term 'consolidated' in this context means a map showing the total noise level at a location from the various sources being mapped. The consolidated maps, as required by the Regulations is the combination of the noise maps for each of the road, rail, industrial and aircraft maps within the agglomerations. The consolidated maps are produced simply as a logarithmic sum of the separate source sound levels and are not subject to response or effects based corrections in any shape or form. It is important therefore to remember that any differences in community response between the different noise sources present at the same sound should only be interpreted in that light.
5.5 Identification of Candidate Noise Management Areas ( CNMA)
A prioritisation matrix is to be developed to determine the Candidate Noise Management Areas ( CNMA). This matrix will take into account many factors some of which are as follows; the level of noise, the relevant contributions from each of the different noise sources, the relative effects of different noise sources present, insofar as they are known or might be known to be different at similar sound levels, the number of people and properties exposed, population density, type of area, landuse, any trigger or target levels contained with existing legislation/guidance, distance of properties from main transportation links and recommended target levels contained within existing legislation guidance. Once a CNMA is identified further action will be considered as appropriate.
5.6 Analysis of Maps
Maps will be analysed using GIS tools and this will facilitate an understanding of the number of people exposed to different noise levels, the number of buildings affected, and the number of properties within agglomerations that have a quiet facade. From this analysis it should be possible to define relative quiet areas within agglomerations and areas where there are excessive noise levels. For technical reasons, it is easier to identify areas of high noise level than quieter or tranquil areas. However, areas of relative quiet within agglomerations can in principle be shown on noise maps.
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