Skyline analysis: An urban environment design aid

An urban environment has complex spatial compositions—dense collections of physical structures such as buildings, trees, and street furniture, as well as urban open spaces such as parks or a public city square (plaza). You can better analyze urban spaces and enclosed spatial areas using skylines to help bring awareness to these special places within a city.

The 3D Analyst Visibility toolset contains a suite of skyline tools that can be applied to many situations. They allow you to visually compare the many ways you can determine how much sky an observer point can see and the possible obstructions for a particular viewpoint. In this workflow you will see how to use the tools for preserving an oceanfront view that is important to a city center tower.

A popular tourist attraction and city landmark tower located in the center of an urban plaza has top-floor views of the ocean. Visitors dine in the revolving top-floor restaurant, allowing all customers a view of the nearby waterfront. This top floor will be used as the observer location in the workflow. Planners and developers are now faced with a proposal for a new high-rise right on the waterfront. This high-rise would obstruct part of the view for not only the restaurant, but also the nearby hotels and waterfront apartments. Building height limits are often a concern when planning urban spaces and can greatly affect residents, businesses, tourism, and real estate.

This workflow will compare before and after results of the impact the new building would have on the available view from the tower. The Skyline tool will be used to create an urban silhouette line around the observation point. A skyline extent volume will be generated using the Skyline Barrier tool. Lastly, you will make a graphical comparison of the the sky visibility using Skyline Graph, which represents angles from the observer point to each of the vertices on the skyline. All of these are different depictions of the same information but help to visualize and tell the story for the amount of visible skyline.

Data preparation

Typically for urban skyline analysis the minimum data requirements include an observer point, urban building data, and optionally terrain or surface information. The location of the observer is also critical to know. If your observer is on the ground in the middle of a downtown core, surrounded by buildings, the surface doesn't influence the skyline unless you have mountains intermingled with the buildings and surrounding area. This is partly why the Skyline tool has a default virtual surface. Generally, when looking up from a downtown core to see the sky, you don't see any terrain, so it isn't a required data type to perform the analysis. Conversely, if you are not in a city but rather standing on a ridge or inside a valley, your skyline is quite different, so a supplied surface and elevation values may be more relevant. ArcScene is the application best suited for this localized analysis.

Skyline analysis

The Skyline tool will allow you to delineate the urban skyline as seen from the observer point on top of the tower if the observer were to turn around and view the city from 360 degrees. This output skyline will show the boundary between the building tops and the sky. Basically, it detects the first point where you would see sky from where you are standing and turning around to make a circle of your view.

In this example, the question of interest is whether the view of the water will be obstructed if another high-rise is built along this prime oceanfront. You will compare the skyline impact of the new buildings before and after.

The Skyline tool uses the input observer point (the center of the top of the tower) and the surrounding buildings layer as the factors to compare against. Once the skyline result (skyline1) is added to the view, adjust the line color and width for visual impact.

The resulting skyline radiates out from the observation site on top of the tower. Imagine sitting in the rotating restaurant on top of the tower: you can now clearly see that you would have an unobstructed view of the oceanfront (toward the top right of the scene). This is what most customers would desire. As you turn clockwise, you can see how your view connects with many building tops before the skyline begins.

Note:

• You only need to use the optional Segmented Skyline parameter if you know you want to use Skyline Barrier to generate separate multipatch volumes for each building. This is best suited for fewer buildings.

Skyline barrier analysis

A skyline barrier resembles a canopy-like coverage showing the visible extent. It is constructed by connecting view lines emanating from a central observer point to surrounding skylines. The output depicts the visible range for the observer and can be displayed as a skin-like planar surface, or a volume-like one, by checking the Closed option, in which case a skirt-like feature is built by extruding downward from the skyline. It is used to determine whether features, such as buildings, violate the barrier by protruding up through, it altering the skyline.

To create the barrier, open the Skyline Barrier tool and use the observation point and the skyline result from the previous step.

The default planar output is shown here:

Add some transparency to the skyline barrier by using the Layer Transparency button on the 3D Effects toolbar:

Rerunning the tool to create the closed volume would look like this, with the buildings turned off to reveal the skirt:

Graph analysis

While visual analyses of a skyline and skyline barrier are necessary, people may want quantitative numbers, as well as graphical charts, to aid a 3D analysis. Creating a skyline graph is a way to help with the process.

Open the Skyline Graph tool, and type the observer point in the Input Observer Point Features field and the skyline in the Input Line Features field. The output creates a table, graph, or both, so you will need to supply these output names as indicated: skyline_table1 and skyline_graph1. Click OK to create the table and polar graph.

The output table is automatically added to the Contents window and is used the same as any stand-alone table or attribute table.

The polar chart automatically opens after running the tool and shows the skyline profile, which is based on the table values.

The numbers listed to the right of the polar graph are the zenith angles for each skyline sector.

To change the symbology of this polar graph so that the sky area outline is more realistic, right-click in the graph and click Advanced Properties. On the Editing dialog box, click Series > Polar from the left column. Select a sky-blue color for the outline and use Pattern for the formatting option to fill the sky color with a solid, semitransparent matching blue.

Now the central sky area is painted with a light blue color, resembling the blue sky.

To understand the polar circle, imagine lying on your back and looking up at the sky. The white, uncolored areas around the ring of the circle are where the buildings are extruding into sky space. The taller the building, the farther into the center of the circle it extends. You can still see the unobstructed view in the top of the graph. It's a polar chart resembling a fish-eye photo that can be used to depict the degree of closeness based on a viewer center point.

The Skyline Graph tool also outputs other useful values. If you check the tool execution results (go to the Geoprocessing menu and select Results, if it's not open), you can find two pairs of values. One is the skyline exposure percentage above the base vertical angle, which can be set in the optional Base Visibility Angle parameter, and the other is the minimum and maximum vertical angles. Here are the current results of the skyline availability:

Additional output values from Skyline Graph geoprocessing results

Percent of sky visible above a base vertical angle of 0.000000 degrees is 95.254876.

The minimum and maximum vertical angles are -6.560444 and 13.617137 degrees.

The graphical and textual tool execution results are based on a vantage point on top of the tower. A semi-circular graph type was used—West (-90 degrees) clockwise to East (90 degrees)—because we're only interested in the north facing waterfront view towards the ocean.

New skyline and results comparison

To compare what the observers would see from the restaurant looking out to the ocean view with a proposed coastal high-rise, add that building layer to the scene and run the tools again.

You can clearly see the new oceanfront building sticking up into the skyline from the graph. Also imagine if your observer point is instead a hotel or apartment building on the water's edge and this building is added in front of or adjacent to your front-facing room. Your skyline could be entirely blocked.

Additional output values from second Skyline Graph geoprocessing results

Percent of sky visible above a base vertical angle of 0.000000 degrees is 94.277842.

The minimum and maximum vertical angles are -6.560444 and 17.496400 degrees.

Comparing with the previous Skyline graph result, th skyline visibility is about 1% reduced, and the maximum vertical angle is about 4 degrees large due to the presence of the proposed building. For an observer closer to the shoreline, the percentage change would be much greater. This is why it is important to consider the location of the observer.

Summary

So far you've calculated a skyline, a skyline barrier, and a skyline graph. This gives you qualitative and quantitative measures on how exposed an urban area is and how a proposed building would obstruct the ocean view from the top of the tower. You can now assemble what you've found in a simple report to show the difference between the original, unobstructed view and the one with the proposed building blocking the view.

From this project, you get a visual sense of how open this urban area is and what the view would be like if a building were to be built at the site. The sky open area is close to 95 percent above the horizon from where the observer stands. The maximum vertical angle is approximately 17.3, and the minimum is about -6.5 degrees. The added building would block the waterfront view significantly for neighboring existing buildings. As for the observation tower, it would be largely enclosed by tall buildings except for the north-facing direction. The spatial perception to visitors of the tower would be potentially negatively affected by the change in visible skyline.