Viewshed Analysis for Visual Impact Assessment

by Liia Koiv-Haus, ASLA, AICP

The Kancamagus Highway in New Hampshire / image: Dyndez, CC BY-SA 4.0, via Wikimedia Commons

Visual Impact Assessments (VIAs) are a technical resource report produced for transportation projects that require consideration of the National Environmental Policy Act (NEPA). VIAs evaluate visual resources—scenic views or vistas—in a project study area and develop mitigation measures to reduce or mitigate a project’s negative impacts on visual resources. Because they require landscape design, revegetation and grading expertise, these reports are often completed by landscape architects. For example, a wildlife overpass might be designed to blend in with surrounding vegetation and topography as the result of mitigation commitments established by the VIA.

NEPA is generally required for projects involving federal lands, federal dollars, or a federal agency permit. Different federal agencies have different methods for visual assessment; the Federal Highway Administration (FHWA), US Army Corps of Engineers (USACE), Bureau of Ocean Energy Management (BOEM), Federal Energy Regulatory Commission (FERC), Natural Resources Conservation Service (NRCS), Bureau of Land Management (BLM), and United States Forest Service (USFS) all have varying requirements and guidance.

The 2015 FHWA Guidelines for the Visual Impact Assessment of Highway Projects provides nationwide guidance for Departments of Transportation (DOTs) completing VIAs. These guidelines state that “With the ever-increasing sophistication of computer modeling, adding vegetation and structures to [a] corridor’s topographic information to establish actual physical constraints will become increasingly possible and is preferred for the VIA.” The computer modeling discussed here is referred to as a “viewshed analysis.” A viewshed analysis is a computer algorithm or analysis using Geographic Information Systems (GIS) software that shows what area is visible from a certain location, taking into consideration obstructions like buildings, trees, and topography. The visible area is called a viewshed and is typically depicted on a map.

Viewshed in red, overlaid with viewpoint in green / image: GIS Stack Exchange, user contributions licensed under CC BY-SA. rev 2023.2.7.43224

Standard practice for VIA practitioners is to only run a viewshed analysis for complicated or controversial projects with a likelihood of adverse visual impacts—projects that would result in visual contrast due to the scale of transportation corridor widening or new alignments, elevated structures (bridges, interchanges, walls), grading requirements, or vegetation removal. Viewshed analysis is only conducted for select projects because it requires large elevation datasets that can be time consuming to download, are not available geographically at a consistent resolution, take up a lot of hard drive space, and require a specialized technical skillset to manipulate. But the alternative, a VIA produced without running a viewshed, relies on individuals’ interpretation of the landscape and views rather than standardized data-driven computer modeling using GIS, so it is innately more subjective. Furthermore, because the VIA practitioner must work within the limitations of Google Earth and site visits to gauge views, it is more difficult to evaluate views on privately owned properties that are inaccessible. These views can be more accurately represented in a viewshed analysis.

The more complicated VIAs that do involve a viewshed analysis are often run using digital elevation model (DEM) data, which only takes into account topography or the ground surface. A ground-based viewshed analysis is worth running for most projects because it provides a broad sense of what might be visible from a project location. However, a full viewshed analysis, as mentioned in the FHWA guidance, is preferred, and this type of analysis requires elevation data on vertical elements in the landscape that can only be obtained via Light Imaging Detection and Ranging (LiDAR) point cloud data. LiDAR is even more difficult to process than DEM data, but a standardized process for working with it can simplify and streamline analysis so it can be easily replicated. Once LiDAR-based information on vertical features in the landscape is added to a DEM, the resulting surface model is called a Digital Surface Model, or DSM. Depending on the quality level of the LiDAR data, one could also separate out vegetation elevations and building elevations to produce a separate Canopy Height Model (CHM) and building height model.

Graphic depiction of data layers in GIS / image: United States General Accounting Office (GAO)

In response to the FHWA Guidelines’ recommendation related to viewshed analysis, in late 2021, the Colorado Department of Transportation (CDOT) Landscape Architecture Section began developing a standardized viewshed analysis process to use in the scoping phase of VIAs. A data-driven visual analysis process has the potential to save DOTs thousands of dollars on each project, help visually communicate the impacts of a project to stakeholders and the design team, and add to project buy-in early in the planning phase. This process can be applied to other state DOT projects throughout the country.

Not only can this process be applied to state and federal transportation projects on a case-by-case basis, but it can be applied programmatically to Scenic and Historic Byways throughout a state to identify and prioritize adjacent lands for conservation planning. Once parcels are prioritized for conservation based on visibility from scenic and historic byways, states can work with local land trusts and other stakeholders to establish conservation easements. Visibility data can also be utilized by local and regional planning departments to develop viewshed protection or scenic viewshed overlay districts as part of zoning codes to limit building height in visually sensitive areas, encourage transfer of development rights (TDR) to shift development to less sensitive areas, and preserve scenic views for generations to come.

Liia Koiv-Haus, ASLA, AICP, is a Landscape Specialist for the Colorado Department of Transportation. She also serves as an officer for ASLA’s Landscape—Land Use Planning Professional Practice Network (PPN).

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