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.

The Problem

UV & Spectral measurements

are taken up here

People are exposed to

UV light down here

Photograph: Google Street View.com

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.

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.

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.

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

Assumptions and Limitations of the Rayman Model.

• NO Spectral Irradiances

• “Black” Obstructions

• Isotropic Diffuse Irradiance

(NO Circumsolar Irradiance)

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

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

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

Model Validations

• Simulated Canyons

SI1 SVF = 0.545

Full SVF = 0.545

SI2 SVF = 0.415

SI3 SVF = 0.42

Diode Array Spectroradiometer

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.

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

• 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

• 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.

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.