One of the first steps in any visual impact assessment is to determine the preliminary area of potential effect (APE). To determine the APE, we must understand the geographic range of potential visibility, which typically requires a zone of visual influence (ZVI), or viewshed analysis. A viewshed analysis is a geographic information systems (GIS) analysis that uses topographic data to determine the availability of a clear line of sight from a viewer’s position and height, to a specific project’s position and height.
Generally, this basic calculation is still the foundation for all viewshed analyses completed today. However, the data sources and input capacity have changed dramatically, introducing a level of accuracy that is unprecedented in the industry. Traditional viewshed analysis was an accurate indicator of locations where a feature would not be visible. However, identifying areas of actual visibility was more speculative. With the technology available today, viewshed analysis can now tell us where a feature or project will be visible with astonishing accuracy.
Since the 1980’s, technology for viewshed analysis has rapidly evolved to allow both project sponsors and regulatory bodies to understand and predict the visibility of planned development projects with increasing accuracy and precision. This presentation will outline the history of viewshed analysis methods over the past few decades and expand upon the current frontiers of the technology. We will present a case study focusing on a proposed wind farm located on the outer continental shelf in the Atlantic Ocean to explain how recent advances in raw data availability, processing power, and analytical techniques have allowed exponential improvements in the quality and utility of viewshed analysis, in terms of identifying and evaluating potential visual effects on aesthetic resources within the project APE. We will compare viewsheds produced for the same study area using three different analytical techniques, representing three milestones in the evolution of viewshed analysis technology. These include:
- analysis based on bare earth topography (Bare Earth Viewshed);
- analysis based on topography plus mapped forest vegetation areas (Vegetation Viewshed); and
- analysis using raw lidar derived digital surface models (DSMs), which reflect topography, vegetation, and structures (DSM Viewshed).
Finally, we discuss how a lidar-based viewshed analysis informs the latter stages of a visual impact assessment and allows the consultant to focus efforts on resources that are included in the project APE.
The case study will include specific examples of how viewshed analysis is a powerful tool in predicting where impacts are likely to occur and the degree to which they should be further investigated. The viewshed analysis allows for a more focused application of field review, photography, visual simulations, and expert evaluation. The case study will demonstrate how lidar-based viewshed analysis allowed for a massive reduction in the investigation area, which allowed stakeholders, government agencies, and consultants to focus on the most likely affected resources, producing a substantially refined visual impact assessment.
Perkins, Gordon W., "The Application of Lidar Data for Determining the Area of Potential Effect Associated with Offshore Wind Projects on the Outer Continental Shelf" (2019). Visual Resource Stewardship Conference. 13.