Thermal radiation is not perceived by the human eye, which complicates the process of hot spot detection. Thermal remote sensing makes thermal radiation visible, which makes it an excellent solution to the problem of locating hot spots. Thermal infrared sensors can be handheld or fixed on a platform such as an airplane, a satellite or a car. Each of these platforms has its own advantages and disadvantages.
Ground level surveys are very flexible and can easily be performed on demand. They are, however, prohibitively time-consuming when dealing with a large study region [1
]. Another problem is that the results of the ground survey cannot be easily integrated into modern GIS systems. Airborne and satellite-based surveys, on the other hand, provide data that can be easily georeferenced, imported into any GIS system and prepared for automated analysis as described by MacKay [2
Satellite systems have a number of advantages: they offer vast coverage, collect imagery at regular intervals and at lower cost. On the other hand, satellite imagery is not available on request because the orbital path is not easily adjustable. Also, the combination of limited pixel dwell-time and low level of signal in the infrared part of the spectrum only allows for spatial resolution of 60m or worse (60m for Landsat, Band #6 and 90m for Aster, Bands #10 to 14, according to NASA specifications). Finally, sun-synchronous satellites (such as Aster and Landsat) are not suitable for precise heat loss analysis, because it is the heat coming from inside the buildings that is of interest, not the re-emitted heat coming from the sun.
Aerial imagery can be acquired on request (at any time of day and night) and with high spatial resolution because of the much lower altitude of flight. Several papers have demonstrated that aerial imagery can produce temperature measurements accurate to within 0.6-1.7°C [3
]. A number of drawbacks, such as long deployment time (the plane needs to travel to the place of survey) and relatively high cost (costs of $200,000 for bigger projects are not unusual [6
]) may become a barrier for smaller projects. However, the advantages of aerial imagery make it ideal for on-demand high- resolution thermal infrared surveys.
The goal of this project was to obtain aerial thermal infrared imagery for the campus of the University of the Northern Iowa and assess its usefulness from the position of temperature anomalies detection. High spatial resolution of the imagery, fully digital image-processing framework and an attempt to perform GIS-based analysis would differentiate this survey from other papers on aerial- based infrared thermography. Analog, low-resolution infrared sensors and the lack of GIS integration significantly limit the potential of the thermal infrared research.
2. Data Collection
2.1 Study Area
The study area for this survey was the Campus of the University of Northern Iowa (UNI) in Cedar Falls, Iowa, USA (Figure 1
). Despite its small area (about 2 km sq.), the UNI campus represents a wide variety of objects of interest, including flat rooftops, sloped rooftops, underground pipelines, heat exhausts and other infrastructure. Insufficient heat insulation of dormitories, on-campus apartments, classrooms and laboratories may cause an increased financial burden on UNI students. Therefore, such a study should be highly important for all campus residents.
2.2 Aerial Data
Aerial data was collected by the private contractor, AITScan (a division of Stockton Infrared Thermographic Services, located in Randleman, North Carolina) on April 04, 2007. The survey was performed between 11PM and 12AM to ensure maximum thermal contrast between the objects of interest (buildings, steam pipelines) and their surroundings [6
]. AITScan used the thermal infrared camera “Phoenix-Mid” manufactured by FLIR Systems. This camera has a resolution of 640 x 512 pixels with a FOV of 14.6°. With the average flying height of 600 m above the ground level, ground resolution is approximately 0.29 m for the whole dataset. This camera has a radiometric resolution 