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Visual Impact assessments – GIS

Visual Impact Assessments for potential hydro schemes

We have recently carried out a number of Visual Impact Assessments for run of river hydro schemes which are being planned in the Loch Lomond and Trossachs National Park.

Each site needed to be assessed to find out how visible the intake, powerhouse and penstock would be from potential visual receptors. The three sites are situated within 5 kilometres of each other so a cumulative assessment of the sites was also required.

The Zone of Theoretical Visibility (ZTV),  also known as Zone of Visual Influence (ZVI), was generated out on all 3 aspects of each hydro scheme. This is a computer generated tool to identify the likely (or theoretical) extent of visibility of the development.  This was carried out using Viewshed analysis in ArcGIS 10.1 using a 10m elevation model – DTM (Digital Terrain Model) ensuring the elevation of all the development was set to a proposed height. The DTM is a bare-earth model and consequently does not feature buildings, vegetation or other boundaries which may have a significant effect on the visibility of the development. This is why it is known as a Zone of Theoretical Visibility. Neither does the ZTV take account of the effects of distance in reducing the significance of a development.

After the maps were developed potential visual receptors were marked, such as local houses, view points, car parks and tracks used by the public. The coordinates were noted and these were then visited and photographs taken to see if there would potentially be any visual impact at that point.

A cumulative assessment was then undertaken of the three new powerhouses and two other powerhouses already located within a 5 km radius of the potential new sites. This indicated where the powerhouses could be seen from and how many could be seen from that point.

ZTV maps have a tendency to present an extreme view of intervisibility – in terms of pure line-of-sight, many developments can be seen across a wide area. However, it is important to recognise that their actual visual significance is dependent on a number of factors, such as scale, setting and the nature of the viewing experience. Thus, the benefits and limitations of ZTV should be understood by all parties – it is simply a useful tool to illustrate and explain the effects of a development.

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