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Calculations of street level spectral irradiances using the Google Street View
photographic database: Implementation and potential applications on ultraviolet light studies
Roberto Carrasco-Hernandez, Andrew R.D. Smedley &
Ann R. Webb University of Manchester, School of Earth, Atmospheric and Environmental Sciences & Williamson Research Centre for Molecular Environmental Science, University of Manchester, UK.
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The Problem
UV & Spectral measurements
are taken up here
People are exposed to
UV light down here
Photograph: Google Street View.com
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Layout
• Important: Easy Access Modelling Tools
• Image processing approach to reconstruct urban canyon geometries from Google.
• Considerations needed for Canyon Irradiance Models (Total shortwave and spectral irradiances).
• Current state of development and future scope of the project.
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Image Digital Processing Software tools:
Google Street View + Hugin
Snapshots of the Hugin interface: A.-Independent photographs covering the
full upper hemispheric view. B.-The reconstructed hemispheric view
using a fisheye projection.
Set of photographs linked to one single position in the city of Manchester, taken from Street View.
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Image Digital Processing Software tools:
Google Street View + Hugin
Snapshots of the Hugin interface: A.-Independent photographs covering the
full upper hemispheric view. B.-The reconstructed hemispheric view
using a fisheye projection.
SVF = 0.503
SVF = 0.325
SVF = 0.550
A.- Orthographic reconstructed projection; B.- Equidistant reconstructed projection;
C.- Control fisheye photograph, taken with an adaptable phone mini fisheye lens.
Pictures were taken at the Zochonis Canyon, University of Manchester.
SVF: Sky View Factor for each image.
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Total Shortwave Irradiance Calculations: Rayman Software (Matzarakis et al. 2010)
Diagrammatic output results from Rayman.
The software can calculate Sky View Factor (SVF), Global, Direct and Diffuse
Irradiances.------We tested it
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Assumptions and Limitations of the Rayman Model.
• NO Spectral Irradiances
• “Black” Obstructions
• Isotropic Diffuse Irradiance
(NO Circumsolar Irradiance)
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Assumptions and Limitations of the Rayman Model.
• NO Spectral Irradiances
• “Black” Obstructions
• Isotropic Diffuse Irradiance
(NO Circumsolar Irradiance)
This anisotropic distribution is not considered by these models
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First approach to calculate Spectral Irradiances in Urban Canyons
• Assumption 1: Canyon Spectral Irradiance -at any wavelength- is a fraction of Rooftop Spectral Irradiance, at the same wavelength. And it is a function of the SVF.
• Assumption 2: The ratio of Diffuse/Global irradiance is explained by Rayleigh scattering, and is dependent on wavelength, Ozone, turbidity and SZA.
• Assumption 3: Diffuse light is isotropic.
UV
People
0.00E+00
2.00E-01
4.00E-01
6.00E-01
8.00E-01
1.00E+00
280.0 380.0 480.0 580.0 680.0 780.0
10 Deg SZA
20 Deg SZA
30 Deg SZA
40 Deg SZA
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Model:
𝐺𝑢,𝛌 = 𝐺𝑓,𝛌 ∗ 1 − 𝐷𝑖𝑓𝑓𝑢𝑠𝑒/𝐺𝑙𝑜𝑏𝑎𝑙𝛌 ∗ 𝑆𝑏𝑖𝑛 + 𝐺𝑓,𝛌 ∗ 𝐷𝑖𝑓𝑓𝑢𝑠𝑒/𝐺𝑙𝑜𝑏𝑎𝑙𝛌 ∗ 𝑠𝑣𝑓𝑢
• 𝐺𝑢,𝛌 : Global “Canyon” Irradiance in canyon"𝑢", canyon and at wavelength"𝛌”
• 𝐺𝑓,𝛌: Global “Rooftop” Irradiance -also “full viewirradiance”- at wavelength"𝛌”
• 𝐷𝑖𝑓𝑓𝑢𝑠𝑒/𝐺𝑙𝑜𝑏𝑎𝑙𝛌 : Ratio of diffuseto global irradiances at wavelength"𝛌”, accordingto the
equation: 𝐷𝑖𝑓𝑓𝑢𝑠𝑒
𝐺𝑙𝑜𝑏𝑎𝑙=
𝑙𝑜𝑔305𝑛𝑚 1+𝐴𝑂𝐷+ 305𝑛𝑚
(305𝑛𝑚−𝝀)
𝑆𝑍𝐴100 +0.9
1+𝑙𝑜𝑔305𝑛𝑚 1+ 305𝑛𝑚
(305𝑛𝑚−𝝀)𝟐
• 𝑠𝑣𝑓𝑢: Sky View Factor at canyon"𝑢"
First approach to calculate Spectral Irradiances in Urban Canyons
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Model Validations
• Simulated Canyons
SI1 SVF = 0.545
Full SVF = 0.545
SI2 SVF = 0.415
SI3 SVF = 0.42
Diode Array Spectroradiometer
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0
0.2
0.4
0.6
0.8
1
320 420 520 620 720
Obstructed/Full Ratio
Wavelength
Measured Obstructed/Full Ratios for simulated canyons
si1/full
si2/full
si3/full
• Results: Ratios of obstructed global spectral irradiance to full sky view.
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0.8
0.85
0.9
0.95
1
320 420 520 620 720
Comparison Modelled vs. Measured Ratios -Second Simulated Canyon (SI2)-
si2/full
calc si2/full
0.8
0.85
0.9
0.95
1
320 420 520 620 720
Comparison Modelled vs. Measured Ratios -First Simulated Canyon (SI1)-
si1/full
calc si1/full
Comparisons Modelled vs. Measured Ratios
SI1 SVF = 0.545
SI2 SVF = 0.415
5% difference
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• Results: Comparisons Modelled vs. Measured Ratios
0.8
0.85
0.9
0.95
1
320 420 520 620 720
si1/full
si2/full
Measurements
0.8
0.85
0.9
0.95
1
320 420 520 620 720
calc si1/full
calc si2/full
Models
SI1 SVF = 0.545
SI2 SVF = 0.415
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• Results: Comparisons Modelled vs. Measured Ratios
0
0.2
0.4
0.6
320 370 420 470 520 570 620 670 720
Comparison Modelled vs. Measured Ratios -Third Simulated Canyon (SI3)-
si3/full
calc si3/full
It is necessary to consider the anisotropic distribution of diffuse light, as well as the potential effect of reflective surfaces in the canyons.
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Conclusions • The reconstruction and “fisheye” projection of urban geometries is
possible from Google Street View. This has the potential to become a useful tool in solar radiation studies at street level.
• The Rayman software allows the calculation of total-shortwave irradiances in urban canyons from the reconstructed “fisheye” images, but there is a need for spectral irradiance models.
• The spectral model proposed here represented an acceptable approximation, however, -in order to improve the performance of the models at canyons with different orientations- methods that account for diffuse light anisotropy and reflectivity of horizon obstructions are yet to be developed.