Visibility analysis takes into consideration what can be seen by an observer from a given location. Key terrain, observation posts, and other locations can be used to assess capabilities (what can be seen) and vulnerabilities (what cannot be seen).
In this exercise, a sample scenario was recreated wherein a land developer is interested in building a luxurious high-rise mixed-use (residential/commercial) building within the city of Salzburg. The developer recently acquired a land property along Vogelweiderstraße and they are planning to build a 100m building on it.
While the city is eager to develop that part of the city, they are quite hesitant to approve the project because of the visual impact of the building on the current view from several vantage points in the city. Since the height of the building is quite high, the City Planning Office wants to explore the different scenarios that might arise in relation to the city's view, before even approving the said project. To be specific, they wanted to know if the building will be visible from two popular tourist vantage point (Modern Museum and Festung Hohenzsalzburg). Also, they wanted to assess the view from the rooftop with respect to the existing land use of the city so as to have a general idea on the vista from the tower.
Different scenarios were explored to arrive at a win-win solution. Visibility analysis techniques and metrics especially Viewshed and Line-of-Sight analyses were used during the course of the exercise.
To start the analysis, an applicable surface model is required. While the Digital Elevation Model (DEM) can be used for this type of analysis, using a more detailed surface like a Digital Surface Model (DSM) with better resolution can provide a more realistic result when it comes to visibility analysis. The height of the buildings and other obstructions such as trees is considered when DSM is used.
For this exercise, the Salzburg 5-meter DSM, accessible through the Salzburg Open Data portal was used.
The first scenario shows the visual impact of the current proposal of the developer. As mentioned, the city wanted to visualize the impact of building a 100-meter tower to its immediate neighborhood, as well as from some popular tourist spots in the city.
To do this, the Viewshed tool in ArcGIS was used to generate a surface showing the areas that would be visible from the tower. With this, we can also see if the tower itself would be visible from the two vantage points that the city defined; Modern Museum and Festung Hohenzsalzburg.
Within the Viewshed tool, there is a parameter called "observer offset". This value indicates a vertical distance to be added to the observer elevation, and in our case, this is equal to the initial proposed height which is 100 meters. I also set a buffer of 3km from the tower as the extent of the viewshed analysis to cover the area of Festung. Depending on the the existing atmospheric condition, farther objects may not be visible within this radius.
We can visualize from the result that the viewshed surface indeed shows that the two vantage points are visible from the top of the tower. This also means that from these two points, the top of the tower at least is also visible if someone looks towards the tower's direction.
Also, because a detailed DSM was used, we can clearly see the outlines of the buildings that would be visible from the tower.
Relying on the result of the viewshed analysis to dismiss the proposal may not be enough so a line-of-sight analysis was also conducted. This way, we can verify if the tower is indeed visible from the two vantage points, as well as the areas within the sight lines where we can see the tower.
To do this, first, the Construct Sight Lines tool was used first to generate the line features that represent sight lines from our observer points to the tower.
In the resulting polyline layer, I added two fields named OFFSETA and OFFSETB. The value in OFFSETA will be added to the observer's elevation, whereas value from OFFSETB will be added to the target. I used 2 meters for OFFSET A, which is the relative observer's height and 100 meters for OFFSETB for the height of the tower.
The result of the Line-of-Sight tool shown here gives us two important points when it comes to the visibility analysis. First, we can visualize the areas where the tower is visible within the constructed sight lines using the "VisCode" field. Second, we can determine if the tower is visible from the two points using the "TarIsVis" field.
Based on these, we can visually see that the tower is visible from the two vantage points (Modern Museum and Festung Hohenzsalzburg). It can also be seen in some areas near the Salzach River and the Kapuzinerberg.
Although this is the case, the city still cannot dismiss the proposal because of the potential income that the investment will contribute to the city. And as mentioned before, our goal is to find a win-win solution for the city and the developer. So instead of dismissing the proposal, one alternative is to check if the visual impact will change if, for example, we proposed a new height for the building.
With this, we can investigate the visual impact if the height of the tower is decreased say for example to 50 meters.
Running the Viewshed analysis again, we can see that the visible area from the new height of 50 meters somehow decreased. The Line of Sight Analysis result also shows that the tower is only visible from the Modern Museum and not from the fortress.
Assuming the the city approved the alternative scenario presented, they decided to amend the development plan by recommending this height to the developer. In addition to the initial visibility analysis, the city is also interested in determining the quality of visible areas from the 50-meter tower.
Using the Salzburg Land Use layer, visual impact score was assigned for each land use classification depending on the perceived visual ranking. For example, construction areas and industrial areas will have a lower visibility score as compared to more desirable views like green spaces, forests, and urban areas. Below are the visual impact scores assigned to each land use class:
A web mapping application was also configured to present the results of the visibility analysis. Using the web app, viewers can interact dynamically with the layers and view some additional information about the map and the layers within. A Swipe Tool was added to easily compare the layers while the Chart Tool can be used to compare the size of the area within each visual impact score category.
