2°C and 2°C). Using the model produced in the steps above, as well as the inverted version of the Planck's Law,
pixel values were converted into temperatures. The level of emissivity was fixed at 0.95, which is the emissivity of concrete, according to the ASTER Spectral Library.
3.3 Model Validation
Some of the ground samples were left for the model validation process. Temperature estimates produced by the model were compared with the ground measurements.
4. Results and Discussion
4.1 Thermal Map of the UNI Campus
shows the surface temperature variation map of the campus. This map clearly depicts the hot spots on the campus. Areas covered in vegetation tend to exhibit lower temperatures, while man- made pavements are relatively warm. Overall, places with lower height, tall vegetation or clusters of buildings tend to be warmer than high, open grounds, because they are better shielded from the cooling effect of the wind. Another easily spotted object is a small local stream (Figure 5
, bottom right corner) - due to high thermal capacity of water, its warm stream is clearly outlined on the cool background. This study focused on two main objects of interest, on-campus buildings and underground steam pipelines mainly because of the facility services interests.
4.2 On-campus buildings
In general, rooftops of the on-campus buildings are in good condition with no major leaks visible. A number of buildings in this region have roof temperatures elevated above the average level, as well as other issues. For example, Maucker Union (Figure 6
) has the warmest rooftop. Latham Hall (Figure 7
) has a signature hotspot on its rooftop that can probably be attributed to the moisture, captured under the insulation material [4
]. McColum Science Hall (Figure 8
) hosts a number of chemistry and physics labs that are well-ventilated. The heat exhaust from the ventilation system is unmistakable, along with the overall warm rooftop of this hall.
For all buildings on campus, a so-called halo effect is present. Two surveys [6
] mention possible relationship between the walls temperature and the halo intensity. This effect is attributed to the reflection and re-emission of thermal radiation by the ground surface. Treado & Burch [12
] attempted to derive the temperature of the object using its halo, but no clear relationship was found. Nonetheless, this effect serves the purpose of illustrating the wind-sheltering effect [8
] of the on-campus buildings. The direction and the speed of the wind remained steady throughout the night (330°, 8 m/s). As a result, the areas to the south of the structures are noticeably warmer than their northern counterparts.
As was mentioned before, the halo effect is poorly suited for analysis of wall temperatures. The study by Baraniak and Williams [1
] is an example of a ground-based thermal imaging survey that deals with the building's walls directly. That study demonstrated that a number of significant temperature anomalies can be found by examining the structures from the side view. Because of the sharp turns the plane was performing at the end of each flight line, a number of oblique images of the buildings in the vicinity of the campus were produced. One of the best examples is present in the Figure 9
. The building shown is Cedar Falls High School, located about 1 km to the north of the campus. The high visual contrast between the rooftop and the walls is obvious. No data on the insulation levels of this building was collected at the time of this paper being written, neither were there any ground samples available for that particular location. It should be noticed that factors other than temperature, such as the directional emissivity of the surfaces as well as wind effects contribute significantly to the observed contrast. Nonetheless, it would be useful to investigate the possibility of automated analysis of oblique thermal imagery in the future.
4.3 Steam pipelines
Several papers [2
] illustrate that aerial infrared imagery is convenient and cost-efficient when heat loss from buried pipelines is analyzed. As sh