Chapter 6 Infrared Thermographic Analysis of the Urban Environment and its Individual
6.5 Urban landscape component analysis
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different position to its footprint by an amount depending upon the height of the building and the distance the building is from the image nadir. Since the building layer is used to calculate zonal statistics of the thermal image, it is important for the building layer to be aligned with the correct pixels of the thermal image. To correct for this difference, the building footprint for each individual building was moved and altered to correspond with the roof of the building.)
An important effect of the observed topographic displacement is that many taller buildings have their façade visible in the thermal image, allowing their temperatures, as viewed from above, to be measured.
Roads are a major component of the urban area, which, in Adelaide’s CBD, are made from bitumen. These roads have surface characteristics that are known to enhance the UHI effect, being dark coloured, and rough, and hence with a low albedo (Akbari et al, 2001). Six different roads throughout the study area were used. Each initially was comprised of many road segments containing different unique identification numbers. The roads segments were merged together to form one polygon for the whole road area, which was then used in applying zonal statistics to determine the mean road temperature.
Footpaths surrounding the associated building were used, with any obstructions of the footpath removed with Editor in ArcMap (ESRI, 2011).
Zonal statistics were used to calculate the mean temperature for each building, façade, footpath and road. Using the zonal statistics tool, the mean temperature for each polygon from the building, façade, footpath and road layers based on the temperature data in the thermal image, were calculated. Where the building layer included several polygons for one individual building, the unique building number (UBN) was used to identify separate buildings.
Vegetation cools an area through evapotranspiration and shading of the ground during the day (Akbari, 2001). Therefore, trees were excluded when obtaining the mean temperature for footpath areas as they would reduce the mean temperature for a footpath segment. A five meter buffer around each tree point was created and excluded from the analysis as well. Also, some trees that were not represented in the layer, but were identified through aerial imagery, were manually excluded.
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Table 6.1 shows the mean temperature of roofs, facades and footpaths for 25 selected buildings in the CBD. Buildings that were not tall enough or were obstructed by other buildings, have no temperature recorded for the façade. There is no clear relationship between building roof temperature and façade temperature, indicating the highly varied nature of construction materials used on buildings and different levels of exposure to the sun. Façade temperatures are obviously best observed from ground level (presented later in this chapter), but they are included here as part of the analysis of the city thermal image.
Table 6.1 Building location, building height (m), and mean temperature (°C) for roof of building, façade and associated footpath area. (N/A: data not available)
Location Building Mean Temperature (°C) height
(m) Troof Tfacade Tpath 91 King William Street 130.36 25.3 26.4 28.7
19 Grenfell St 70.14 25.9 24.3 25.1
33 King William St 69.34 23.9 23 N/A
1 King William St 68.02 24.8 25 27.6
67-79 Waymouth St 67.95 24.1 25.9 27.9
8 Grote St 63 25.8 22.4 20.7
22 King William Street 60.23 24 24 24.9
431 King William St 58.38 22.7 24.5 25.8
250 Victoria Square 54.4 12.3 N/A 18.8
25 Pirie St 51.46 24.5 N/A 24.9
136 North Terrace 50.74 25.1 24.5 25.8
81 Flinders St 50.24 24.4 24.9 27
60 Waymouth St 48.89 23.2 21.4 24.9
11 Hindmarsh Square 45.53 23.1 16.8 N/A
144 North Terrace 45.12 23.2 27.3 N/A
215 South Terrace 42.1 23.3 23.3 24.1
100 Pirie St 40.38 22.2 19.8 21.3
51 Pirie St 33.37 22.7 N/A 24.9
169 Pirie St 28.26 19.9 18 23.8
84 Flinders St 15.23 20.6 N/A 21.5
10 Waymouth St 14.89 24 N/A 24.8
76 Light Square 13.49 15.2 N/A 21.2
18 Grote St 12.66 10.8 N/A 20.7
142 South Terrace 10.03 19.1 N/A N/A
213 South Terrace 9.65 14.3 N/A N/A
Using the data in Table 6.1, correlations between the various parameters were examined. Footpath temperatures show a weak correlation (R2=0.49) with building roof temperatures. The correlation is slightly higher against facade temperatures (R2=0.55). The scatter could come from a number of sources. Some footpath temperatures could not be obtained for some locations due to obstructions from trees and buildings. The orientation of
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the footpaths and their position relative to surrounding urban geometry would have a significant effect on these results. However, it is evident that 85% of footpath areas had a greater mean temperature than the associated building roof, which is consistent with the canyon effect of higher temperatures at ground level posited in section 6.3.
The relationship between roof temperatures and building height is shown in Figure 6.9. This shows a weak positive relationship between this temperature and building height.
However, building radiative roof temperature, as recorded from above, is highly dependent on the roofing materials used in any individual building and their reflective properties, especially in relation to night time sky temperatures. This, together with proximity to open space, may explain the weak correlation observed.
Figure 6.9 Relatonship between roof temperature (°C) and building height (m)
From air temperatures measured in the CBD network (described in chapter 2) at the time of aerial thermal imaging, air temperature contours were obtained. These contours were used to define zones, with half degree Celsius ranges, within which data on the associated mean height and number of buildings, and buildings per ha were extracted. Table 6.2 shows the results. The temperature zone above 28°C has the highest average building height of 40.0 meters. The average temperature decreases as height decreases; by an average building height of 12.1 meters the temperature has dropped to the 26 – 26.5°C
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range. The average building height remains stable between 12 and 11 meters until the last temperature zone of 23.5 - 24°C.
The number of buildings per ha is highest in the 23.5 – 24°C temperature zone, which is the outermost zone in the CBD. The higher temperature zones in the city centre have a lower ratio of buildings to area, with the temperature zone 27.5 - 28°C having 6 buildings per ha, which is indicative of the larger buildings in these areas of the CBD. These results show that higher average building heights are associated with higher ambient air temperatures.
Table 6.2 Temperature zones and areas, number of buildings, average building height and buildings per ha
Zone Temperature
(°C) Zone Area (ha)
No. of Buildings
Avg Bldg
Height (m) Buildings / ha
Above 28 1.22 13 40.0 11
27.5 - 28 13.67 87 23.0 6
27 – 27.5 18.11 170 17.0 9
26.5 - 27 25.31 193 14.5 8
26 – 26.5 37.65 599 12.1 16
25.5 - 26 39.81 480 11.1 12
25 – 25.5 40.05 526 11.1 13
24.5 - 25 41.93 584 11.0 14
24 – 24.5 42.96 656 12.2 15
23.5 - 24 54.81 1022 11.1 19