To qualify the experience of people in the outdoor public space, it is essential to determine the visibility of some details in the urban landscape. These can be crucial details, such as road signs, pleasant details, such as vegetation, or unpleasant details, such as advertisements. Many features that fall within pedestrians’ visual field in an urban walk are located on buildings. The methodology presented here was originally developed to assess the visual impact of building integrated solar modules. The proposed workflow examines how to quantify the visibility of elements in urban contexts at the different planning scales, which can be used to drive fundamental decisions in terms of outdoor comfort, psychological well-being and cultural heritage protection.
Going for an outdoor walk in our cities to take fresh air, enjoy a panorama, view a monument or even shop and have a drink is a great recreational experience. Some features that come across our field of view, though, can be disturbing or irritating and engender a negative sensation. Imagine you are sitting in a square in front of a town cathedral, and suddenly a large truck stops right in front of you to deliver supplies to a neighbouring bar. After a while, sun reflections from a solar panel installed on a surrounding rooftop hit your eyes; then you stand up and look for the post office sign while walking down a commercial street, but you cannot find it in the jungle of shop brands. You are facing the impact of visual pollution.
VISUAL POLLUTION
Visual pollution is a combined effect of confusion, disorder, and a mix of different objects and graphics in the environment.
Examples are outdoor advertisements, billboards, street furniture, road signs, waste collection points, parked vehicles, hydraulic fixtures and tubes, wires and cables and mobile communication antennas. Especially in urban environments, these items can disturb observers’ attention. Beyond their emotional and psychological influence, some of these features can be harmful or dangerous: lighting and blinking features can induce disturbance and epilepsy seizures; potential glare sources like artificial lighting emitters and highly reflective surfaces like mirrors are distracting for drivers.
visible cells (deciles of times seen)
1 - 2 0
2 - 5 5 - 9 9 - 14 14 - 20 20 - 28 28 - 38 38 - 52 52 - 75 75 - 78 invisible cells cumulative viewshed
Figure 30
Carouge, Geneva, Switzerland. Cumulative viewshed per deciles of « times seen ». Elaboration of CC data from Système d’Information du Territoire à Genève – SITG.
Several visual impacting elements, including the advertisement, can be integrated into building envelopes, or in other words, rooftops and façades. As such, they assume particular relevance in the framework of regenerative design. In particular, solar modules for photovoltaic or thermal energy production are often overlaid on the exposed surface of buildings, increasing their reflectivity and the consequent glare induction risk. Assessing the visibility of these elements from the public space in the design phase may help prevent uncomfortable situations and allow estimation of a reasonable application range that meets the broadest social acceptance by the local context. In this framework, extensive work has been done to elaborate a scale-adaptive methodology to assess visibility in urban areas. The territorial scale and the district level are covered, with increased detail at the group of buildings stage. As such, a specific index was determined at each scale for inclusion in a multi-criteria model. At the broader scale, visual interest and viewshed based indicators are proposed. At the district scale, photometric models and ray-tracing techniques were explored to mimic human vision and identify the perceived areas of building envelopes that can potentially host solar modules.
DIGITAL ELEVATION MODEL
The urban fabric emerges at the scale of urban development, and blocks of buildings can be discerned individually. Their visibility is regulated by geometric factors and reciprocal obstructions, depending on massing articulation, topography and depth of the urban canyon. At the scale of urban development, counting the number of vantage points on the public space that have an unobstructed view of a building envelope fraction seems adequate, instead of investigating the physical perception from each viewpoint, which would be more rigorous. As such, the number of visually connected locations to façades and roofs is computed in a GIS environment. A suitable indicator for this purpose is the
‘cumulative viewshed’ (also known as ‘times seen’), which counts the number of times each building surface is intercepted by a visibility ray coming from several points sampled on the public space [2]. A digital workflow to place the viewpoints with a given spacing distance on the road network and extract the building elevation information from the Digital Elevation Model (DEM) has been set-up in GIS for this scope. In Figure 30, the viewshed map of a district in Geneva, Switzerland shows at a glance which buildings are more exposed to public view and therefore deserve more careful design interventions. Buildings surrounded by open space, squares or river shores can be visible by up to 100 times the viewpoints than those arranged in courtyards or dense parcels.
visual amplitude
low visual amplitude medium visual amplitude high visual amplitude
0 [logMAR]
3.94 [logMAR]
Figure 31
Hollande district, Geneva, Switzerland. Visual amplitude index of the building envelopes. Elaboration on CC data from Système d’Information du Territoire à Genève – SITG. Background photo from Google Earth ©
The assessment procedure becomes more detailed when zooming in to the building level. Occlusion from vegetation and urban furniture (lamps, benches, signs …) generates non-uniform shades of visibility of envelope surfaces: different tilts and volume intersections with chimneys, terraces, etc. affect the global perception of a surface fraction beyond the discrete attribution of
‘visible’/ ‘invisible’ from a given set of locations. Physical factors such as visual acuity and contrast should be taken into account for each envelope surface feature made in the previous stage. As such, visibility rays are projected from a grid of scattered pedestrian eye-level positions to building surfaces, as a lighthouse would do with its spotlight [3],[4]. Visual stimulus is quantified as the solid angle produced by a target surface, representing a possible solar module array, on the spherical visual field of each viewpoint about its perceptual threshold. This ratio can be converted to a metric everyone is familiar with; the LogMAR visual acuity measure: it is issued from the typical test of ‘reading the characters’ performed by an optometrist. The entire calculation procedure is carried out in Grasshopper for Rhino by combining vector tracing components and by decomposing the target surfaces in triangular meshes.
THE VISIBILITY INDEX
Figure 31 shows the visibility index, here referred to as ‘visual amplitude’ (VA), calculated on a 1.50 m resolution mesh on building surfaces of the ‘Hollande’ district in Geneva, Switzerland.
Contributions from a set of viewpoints are averaged to get a mean visual amplitude index per mesh face. It can be noticed how flat roofs are non-visible, especially from narrow streets, and façades are more visible than tilted roof pitches. The methods mentioned above are useful to quantify visibility on an objective basis and can be matched with solar energy generation potential for planning purposes. Other possible practical applications include the optimal placement of outdoor advertisements, the quantification of vegetation presence within a view and the establishment of maximal brightness thresholds for luminous signs. As for pedestrian comfort, combining the absence of glary and polluting luminous sources with the presence of natural elements in the visual field can enhance the ‘biophilic’ connection with the environment as well as increase the outdoor psychological well- being. Sky view and ‘visual openness’ should be evaluated at this scale to preserve the meteorological influence on human chronobiologic cycles. Moreover, a sufficiently wide depth of view is linked with the human instinct of safety and the opportunity to escape the area, in this case, a dense urban space.
REFERENCES
[1] P. Florio, M. C. Munari Probst, A. Schüler, and J.-L. Scartezzini, ‘Visual prominence vs architectural sensitivity of solar applications in existing urban areas: an experience with web-shared photos.,’ in CISBAT 2017, 2017.
[2] P. Florio, M. C. Munari Probst, A. Schüler, C. Roecker, and J.-L. Scartezzini, ‘Assessing visibility in multi-scale urban planning: A contribution to a method enhancing social acceptability of solar energy in cities,’ Sol. Energy, 2018.
[3] P. Florio, C. Roecker, M. C. Munari Probst, and J.-L. Scartezzini, ‘Visibility of Building Exposed Surfaces for the Potential Application of Solar Panels: A Photometric Model,’ in Eurographics Workshop on Urban Data Modelling and Visualisation, 2016.
[4] P. Florio, S. Coccolo, A. T. D. Perera, and J.-L. Scartezzini, ‘Matching visual impact, solar energy production potential and energy system optimisation for enhanced solar integration.
An experience with a novel pre-design tool,’ in PLEA - Smart and Healthy within the 2-degree Limit, 2018.