VISIBILITY ASSESSMENT - SENSITIVE LANDSCAPES
Zone of Theoretical Visibility (ZTV) mapping offers a model of the landscape surfaces that are visible from an observation point. It is often a very important aspect of the environmental planning process.
Natural Resource Wales (NRW) instructed Geo Smart Decisions to create and present strategic evidence on the visibility of National Parks and Areas of Outstanding Natural Beauty AONB) to their surrounding landscapes in Wales. The key and majority audience for this work are those considering visual impact issues for strategic planning purposes. The approach applied uses computer GIS software (‘Geographical Information Systems’) to compute viewsheds or ‘Zones of Theoretical Visibility’ (ZTVs) in nine Designated Landscapes with settings in Wales, these were: Snowdonia National Park, Brecon Beacons National Park, Pembrokeshire Coast National Park, Wye Valley AONB (partly in Wales), Shropshire Hills AONB (wholly in England), Llŷn AONB, Clwydian Range and Dee Valley AONB, Gower AONB and Anglesey AONB.
ZTV calculation can provide a binary (visible or non-visible) product delineating regions theoretically visible for a human observer located at specific vantage points. Secondly, for regions that are not directly visible to the human observer, an estimate of the height (above ground level) required for an object to become visible can also be created. ZTV analysis can either be carried out on a single observer point, or the combination of many. When undertaking the latter analysis, binary viewsheds can become cumulative, combining the visible areas from all observer points. The cumulate viewshed can subsequently form heatmaps, showing regions visible from multiple observer points located throughout the landscape. Whilst the models assume a ‘bare earth’, i.e., ones that do not contain surface features such a vegetation artificial structures, they do account for factors such as atmospheric aerosol conditions and the curvature of the earth resulting in broad yet strategic estimates.
To create these data, observer points had to first be generated across the landscapes within and surrounding national parks and areas of outstanding natural beauty. In typical ZTV deployments, these points are selected manually, based on the location development. However, due to the project’s national scale, comprising of nine specific areas and their surrounding regions, a method for automatically generating observer points at critical locations had to be developed. The points were derived utilising the topographic features extracted from the digital elevation model used in the ZTV calculation to identify observer points that identified the total viewable area with the least number of points. To refine this methodology, extensive testing was conducted refining the appropriate number of points, the resolution of the DEM and various available ZTV algorithms, working closely with the client to create a bespoke model that identified regions that corresponded with previous work. The resultant method generated thousands of observer points, for which, multiple ZTVs were computed to identify visible areas, estimates of heights required to become visible, and visibility heatmaps.
To further enhance the project, GSD also provided analysis specific to NRW’s LANDMAP Visual and Sensory aspect areas. This then provided NRW with complete coverage of theoretical visibility over an entire country, alongside a national visual impact model for protected areas in a first-of-its-kind product. Project outputs and the final report are in the public domain and can be obtained from NRW.