The contribution of a GIS-based landscape assessment model to a scientifically rigorous approach to visual impact assessment

Abstract

Visual impact assessments (VIAs) rely primarily on key observation points (KOP). However, the analysis is highly dependent on the selected viewpoints. An analysis using a GIS-based landscape assessment model (LAM) would be an informative complement to a KOP analysis by providing results that cover the complete study area.A LAM based on intrinsic landscape attributes is presented that includes four components: visual quality, visibility, visual magnitude and visual impact. The model is applied to two proposed alternatives for the Northern Pass, a 300 km long 345 kV transmission line in New Hampshire, USA, and an existing transmission line in the shared corridor. The validity of the four model components is tested against field observations and the evaluation of photographic simulations. Correlations are generally above 0.50, indicating high validity (Cohen, 1988). A new metric is introduced that combines the mean value with the size of the viewshed.Effect size (Cohen’s d) is used to evaluate the cumulative and incremental visual impacts within three geographical sections, the White Mountain National Forest—a nationally recognized scenic resource—and the project as a whole. In general, the results indicate that the visual impact of the existing transmission line is large, and while the incremental impact of the proposed alternatives are smaller than the existing impact, they are sufficient to indicate that the cumulative impact is unreasonable.The final discussion considers the difficulties in determining when an impact is unreasonable, and how the LAM might be extended by including viewer sensitivity and scenic resource concern.