Wireframe and ZTV Notes

WIREFRAME and ZTV DRAWINGS

Erection of domestic wind turbines

Field 414697 Ballaman, Ballnahowe Road, Port Erin, Isle of Man

June 2023

1 Introduction

• 1.1 Assessing the impact of a project before it is implemented is now an established procedure, to prevent avoidable damage to the environment and local communities. Impacts can be assessed using a variety of techniques by predicting changes to the local flora and fauna, to the economy, to society, to the health of individuals, and to the built environment. Some impacts are direct, and some are indirect. Some can be forecast precisely, and some are open to debate and interpretation.
• 1.2 Visual impacts on the local landscape can have both direct and indirect consequences. Increasingly, they are modelled to display the likely appearance of proposed projects, but the significance to viewers is more difficult to quantify.
• 1.3 The production of wireframe drawings presented as part of this application is a well-established technique, normally employed for larger, utility-scale wind farm developments. Despite the proposed development falling below the UK’s EIA threshold, the applicant has produced these drawings as an aid to decision-making.
• 1.4 Wireframe drawings should be used in combination with the Zone of Theoretical Visibility (ZTV) map, included in this application as Figure 7. Both the ZTV and the wireframe drawings presented in this application were produced using WindPRO™ software, version 3.6.366.
• 1.5 WindPRO™ is the industry-leading software suite for the design and planning of onshore wind projects. It is capable of wind data analysis, calculation of energy yields, quantification of uncertainties, assessment of site suitability, and calculation and visualization of environmental impacts.
• 1.6 Notes about the ZTV and the Wireframe drawings are provided below, to assist the reader in understanding their application and their limitations.

2 Producing ZTVs

• 2.1 Various computer software packages exist to allow wireframe drawings to be generated. Common to all of them is a requirement for terrain data, turbine data and the selection of viewpoints from which to generate the image.
• 2.2 ZTVs can be produced using either Digital Terrain Modelling (DTM) or Digital Surface Modelling (DSM). A DTM represents the ground surface as a mesh of points. This may form a regular grid of squares when seen on plan, known as a Square Grid DTM; or an irregular network of triangles, known as a TIN (Triangulated Irregular Network).
• 2.3 A Square Grid DTM cannot represent terrain features smaller than the cell size, for example a small knoll or outcrop. Such features are either

• lost between grid points or represented by one point only. For example, the OS Square Grid DTM product, ‘Terrain 5’, uses a 5m cell size and is interpolated from a TIN maintained by Ordnance Survey.
• 2.4 For this study, the ZTV was derived from a Digital Surface Model (DSM). This offers greater accuracy as it is derived from Lidar data provided by the Isle of Man’s Department for Infrastructure. The ‘cell’ size used in the DSM is 2.5m×2.5m – this is superior to the OS DTM5 product and allows minor topographic features such as trees, hedges, and buildings to be included in the calculation. The DSM therefore allows a more accurate representation of where the proposed development might theoretically be visible from.
• 2.5 Due to the small scale of the turbines proposed, the distance over which the ZTV was calculated was restricted to less than 10km. This is for two main reasons: first, beyond 10km the curvature of the earth begins to have an impact, the vertical difference being sufficient to hide a single-storey house, increasing further with distance; second, guidance recognises that for turbines that are 50m tall, the assessment should extend to 15km.1 The proposed turbines are less than a quarter of this size.

3 Producing Wireframe Drawings

• 3.1 Wireframe Drawings are one method of visual representation of the proposed development. Other methods include sketches, photomontages, diagrams and, in certain instances, 3d computer scale-models. It must be remembered that all visualisations can never exactly match reality. Nevertheless, they are accurate enough to allow decision-makers to understand potential impacts.
• 3.2 Wireframes are, in essence, line drawings of the terrain and the wind turbines. The terrain (the DSM) is presented as a mesh seen in perspective. This is a reliable representation of the landform, but it can result in the more distant parts of the scene becoming unreadable as the grid lines get closer together, eventually merging into solid colour.
• 3.3 Different colour has been used to highlight the wind turbines in contrast to the landform lines, especially in distant views where the effect of merging lines noted above often occurs, and where some turbines may only just be visible against the landform. Wind turbines are coloured red, with terrain coloured grey.
• 3.4 The ‘cells’ used in the wireframes to depict the terrain measure

• 2.5m×2.5m, which is the native format of the Lidar data, to ensure consistency and, importantly, replicability. The Lidar data was obtained from the Department for Infrastructure and is used under license.

1 Scottish Natural Heritage (Feb 2017) Visual Representations of Wind Farms, Version 2.2.

• 3.5 By comparing wireframes with the views on site, the assessor can make judgements on the likely visual impacts in a variety of environmental conditions, safe in the knowledge that the wireframes have not been subject to manipulation that cannot be quantified.

3.6 Instances where data has resulted in erroneous representations of features are identified wherever possible.