Abstract
The Sky View Factor (SVF) is a dimension-reduced representation of urban form and one of the major variables in radiation models that estimate outdoor thermal comfort. Common ways of retrieving SVFs in urban environments include capturing fisheye photographs or creating a digital 3D city or elevation model of the environment. Such techniques have previously been limited due to a lack of imagery or lack of full scale detailed models of urban areas. We developed a web based tool that automatically generates synthetic hemispherical fisheye views from Google Earth at arbitrary spatial resolution and calculates the corresponding SVFs through equiangular projection. SVF results were validated using Google Maps Street View and compared to results from other SVF calculation tools. We generated 5-meter resolution SVF maps for two neighborhoods in Phoenix, Arizona to illustrate fine-scale variations of intra-urban horizon limitations due to urban form and vegetation. To demonstrate the utility of our synthetic fisheye approach for heat stress applications, we automated a radiation model to generate outdoor thermal comfort maps for Arizona State University’s Tempe campus for a hot summer day using synthetic fisheye photos and on-site meteorological data. Model output was tested against mobile transect measurements of the six-directional radiant flux density. Based on the thermal comfort maps, we implemented a pedestrian routing algorithm that is optimized for distance and thermal comfort preferences. Our synthetic fisheye approach can help planners assess urban design and tree planting strategies to maximize thermal comfort outcomes and can support heat hazard mitigation in urban areas.
Highlights
Heat is the leading cause of weather-related mortality in the U.S and poses a significant threat to pub-Urban Planning, 2017, Volume 2, Issue 1, Pages 19–30 lic health (National Oceanic and Atmospheric Administration [NOAA], 2015)
To assess the accuracy of the Google Earth fisheye images compared to real world photos, we generated 18,367 fisheye images from Google Street View panoramas across the Phoenix metropolitan area (Figure 2) using the Google Street View Application Programming Interface (API)
We evaluated the accuracy of the Sky View Factor (SVF) calculations by computing the SVFs of 527 randomly selected Google Earth fisheye images in the Phoenix metropolitan area using our implementation of Chapman et al (2001), the RayMan Pro model v2.1 by Matzarakis et al (2007, 2010), the SkyViewFactor-Calculator v1.1 by Holmer et al (2001), and the unweighted, naive approach of counting pixels
Summary
Heat is the leading cause of weather-related mortality in the U.S and poses a significant threat to pub-Urban Planning, 2017, Volume 2, Issue 1, Pages 19–30 lic health (National Oceanic and Atmospheric Administration [NOAA], 2015). Dense urban forms can create local cool islands during the day and are effective at mitigating heat in hot desert environments where water is scarce (Middel, Häb, Brazel, Martin, & Guhathakurta, 2014; Pearlmutter, Bitan, & Berliner, 1999). Heat is most commonly expressed as air temperature, but temperature alone is not a comprehensive indicator of outdoor thermal comfort or stress. Radiation is one of the most important variables affecting thermal comfort; perceived thermal comfort can vary several degrees in the shade and sun (Mayer & Höppe, 1987; Middel, Selover, Hagen, & Chhetri, 2016). Thermal conditions in urban areas vary widely due to complex shading patterns from buildings and trees that determine solar access at the pedestrian level. Air temperature maps fail to accurately represent the significant variation of thermal conditions in built environments
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