 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 of 14 bits and operates in the wavelength range of 3.0 - 5.0 μm. Cooling is provided by the on-board Stirling closed cycle cooler.
Imagery is provided by AITScan in two formats, JPEG and SAF (Standard Archive Format). The JPEG file format in not capable of storing original 14 bit data (maximum bit depth is 8 bit for the gray channel). SAF files, on the other hand, contain full 14 bit camera output (stored in 16 bit form for convenience), as well as camera calibration coefficients that are used to convert pixel values to temperature and radiance. The SAF file is, in essence, an archive, that stores a time series of imagery in the single-file form. This survey produced 24 SAF files, corresponding to the 24 flight lines. With each flight line consisting of approximately 250 separate images, the resulting dataset contains about 6,000 rasters.
2.3 Ground Data
In order to provide additional control over the results of the survey, ground-level temperature data was collected. A handheld infrared thermometer (“MT4 Minitemp” manufactured by RayTek) was used for ground-level temperature sampling. According to the manufacturer, the accuracy of “±2%, or ±1.7°C, whichever is greater,” can be achieved. Previous to taking any measurements, this thermometer was tested and calibrated in a laboratory setup, as illustrated by Figure 2
A cup of cold water (around 1°C) was set up on a heater. A magnetic mixer at the bottom of the cup was constantly rotated by the magnetic field created by heater. The heater was turned on and as the temperature rose, simultaneous measurements were taken by the infrared and calibrated alcohol thermometer. Measurements were taken until the water reached 21°C. Then, the difference between two sets of temperatures was calculated. The differences were normally distributed with a mean of 0.4°C and standard deviation of 0.16. This test demonstrated that in the temperature range of 5°C to 20°C the accuracy was approximately ±0.3°C.
A number of random temperature measurements were taken around the campus, using