Determination of thresholds of visual impact: the case of wind turbines

Visual impact assessment using GIS (geographical information system) based viewshed mapping is commonly employed on major projects. However, there is typically little analysis of the possible range of impact based on the size of the introduced landscape element, its contrast with the surroundings, or the effect of typical levels of atmospheric scattering on the perceived contrast. A landscape element which is increasingly the subject of aesthetic scrutiny and visual analysis is the wind-energy turbine or-when installed in large numbers-the wind farm. I take the wind turbine as my subject partly because of its recent significance and also because, as a moving element, it is a special case. I report an Internet-based experiment to determine the relative perceived size of a turbine, image analysis to determine its typical contrast level, and the effect of atmospheric scattering on this contrast. With these three factors and equations derived in an earlier study, estimates are made for the probability of turbine detection, recognition, and visual impact at distances up to 30 km.