Viewsheds are commonly used in terrain analysis, which is of interest to wireless communications, urban planning, archaeology, and military science. Viewsheds are often calculated for public areas — for example, from public roadways, public parks, or high-rise buildings — and frequently a goal in the designation of open space areas and green belts. Viewshed analyses are useful for trail design as they can associate the field of view to the landscape's scenic beauty along the trail and help with the placement of facilities to enhance user satisfaction.
So what do we mean by viewshed? In a nutshell, a viewshed is the geographical area that is visible from an observer location on the terrain (Figure 1). It includes all surrounding points that are in line-of-sight with that location and excludes points that are beyond the horizon or obstructed by terrain and other features (e.g., buildings, trees).
Figure 1. Simple Viewshed model (source MDPI).
In this blog, we briefly describe a series of viewshed analyses for the East Bluff Trail in Devils Lake State Park, Baraboo, Wisconsin, while hoping to show how they may be used to interpret the visual landscape along the entire length of the hiking trail.
In order to perform any viewshed analyses, we first need digital elevation data. Since a viewshed operation is only as accurate as the elevation data it is conducted on, we were to be able to obtain a publicly available high resolution digital elevation model (DEM) derived from 3DEP LiDAR point cloud data (source, USGS). We then created 30 somewhat evenly spaced points along the East Bluff Trail, located on the east side of Devils Lake, to act as observer locations for the analysis (see Figure 2 and virtual fly-over video below).
Figure 2. LiDAR derived terrain model with aerial photo overlay of Devils Lake State Park. The East Bluff Trail is the the right of the lake along with the 30 observer points used in the viewshed analysis.
Virtual fly-over (running from north to south) along the East Bluff Trail at Devils Lake State Park. The video was created using a LiDAR point cloud and aerial photo mosaic overlay. The East Bluff Trail is denoted as a faint red line running along and ahead of the video's point of view (Geographic Techniques LLC).
To make sense of where visibility differs along the trail, we divided the observer points into five regions (north, north central, central, south central, and south) and ran each region as a separate viewshed model. (Viewshed models can also be run for a single observer point, but was not considered for this project). Also, each observer point was given an offset height of 4 feet (above ground level) for each model run.
Viewshed Models
Figure 3 contains a series of viewshed model composites showing frequency, or visual magnitude, which offer a measure of viewshed connectedness along the trail. Colors show the number of East Bluff Trail observer points that can be seen at a particular location within the view frame of the map. Red colors indicate the highest number of sectional observer points, while green indicates the least. As one can see, the visibility aspects of the trail change from one section to the next as one moves along the trail.
Figure 3. Various viewshed model composites, starting in the north: (Top) North, North Central; (middle) Central, South Central; (bottom) South, and total combined (all observer points). Each image shows frequency of visibility to the selected observer points, from any location across the map area (green = lowest; red = highest).
In the northern section of the East Bluff Trail much of the visibility is to the northwest and toward the east. A bit further south, in the north central section, the western areas of the lake, west bluff, and areas to the southwest become more visible. A complete change occurs in the central region of the trail as the visibility is becomes mostly toward areas to the east. Traveling further south into the south central portions of the trail, visibility returns across the lake to the west bluff and southwest areas, with some areas visible to the east. The southern sections of the trail run mostly eastward, with most visibility toward the south bluff and south end of the lake.
Single line-of-sight (LOS) profiles can also help determine visibility at particular locations, regardless of viewshed. Figure 4 shows an elevation profile, created from LiDAR point cloud data, extending eastward from observer point 10. LOS is shown as a yellow highlighted dashed line. In this case, the viewshed extends out nearly 5000 feet, however, trees obstruct LOS near the observer point (top left) and towards the end of LOS (right).
Figure 4. LOS profile (yellow dashed line) utilizing a LiDAR point cloud, looking eastward from observation point 10. Brown indicates the ground reflectance elevation, and vegetation height is shown in green.
What implications do viewshed models have in recreational trail planning? For one, viewsheds can provide guidance for visual landscape interpretation along the length of an entire recreational trail (not just the waterfalls, for example, located at the end of the path). Secondly, although it may be difficult to see any scenery through trees in many situations, knowing the visibility beyond local obstructions can help identify strategic points that can be used for scenic viewing areas or the placement of observation towers. We should also note that viewsheds may not be the only tool used for trail planning — vegetation characterization, land use diversity, and slope analysis should also be considered in the planning, as well as maintenance, of recreational trails.
Hopefully, this blog sheds some light (no pun intended) on the importance of viewshed analyses to recreation trails. Please feel free to contact Geographic Techniques about how terrain analyses, including viewsheds, can help your organization with your trail planning and design projects.