A Breath of Fresher Air
Based on U.S. Environmental Protection Agency (EPA) findings dating back to 1993, the Arizona Department of Environmental Quality (ADEQ) knew it needed to enhance its efforts to protect residents from the potential harmful effects of air pollution. Specific targets of the study included small particles with a diameter of 10 microns or less (PM-10), and harmful gases called volatile organic compounds (VOC). The scientists at ADEQ’s Air Quality Division envisioned using highly sophisticated imaging technology to identify and quantify sources of air pollution for such a study, but until recently, there had been no imagery available with sufficient resolution to test this theory. In 2000, Space Imaging’s (Thornton, Colo.) IKONOS satellite provided the Air Quality Division with the images they needed, making ADEQ the first Arizona agency to use satellite imagery.
With the new availability of this high-resolution satellite imagery, ADEQ could develop a plan to more effectively identify and quantify PM-10s, VOCs and other air pollution sources in the state. Due to the large concentration of airborne dust and dirt particles, the desert area of Douglas, Ariz., and Aqua Prieta, Sonora, Mexico was chosen as the first place to conduct a pilot study.
An Advanced ProcessSpace Imaging, through an Arizona-based reseller, provided both 1-meter black-and-white and 4-meter multispectral IKONOS images of the 33-square-mile Douglas/Aqua Prieta area. However, before the IKONOS data could effectively be used, it had to be merged and indexed with existing ADEQ emission source data. This detail-oriented process was very important since the data had to be logged in a way that facilitated geospatial processing and also ensured the coordination of efforts between field and office workers. Once this task was completed, ADEQ was able to produce digitized polygon maps of the study area.
To create these maps, ADEQ divided the study area into a tiled grid. Each digitized field map had to be capable of producing a paper map large enough for field workers to take notes on, yet small enough for easy handling. They also needed to show as much detail as possible without exceeding the limits of a person’s visual interpretation capabilities. Based on experimentation by the ADEQ staff, the optimum resolution for these maps was determined to be 17 by 21 inches. These maps, which represented approximately 1,000 meters on each side, were then color-coded according to land use and geographic characteristics. This helped workers orient themselves in the field, transmit and send data, and identify areas they might have previously missed.
The main features ADEQ was interested in studying were unpaved roads and parking lots, and any other areas with disturbed topsoil that might contribute to air pollution. Using ArcInfo ARCEDIT from ESRI (Redlands, Calif.), ADEQ was able to digitize these land features directly over the original IKONOS images. Because this was a pilot study, ADEQ wanted to experiment with the effectiveness of different methods of fusing color into the maps, so they acquired both black-and-white and multispectral data. The images were then reduced to 8-bit GeoTIFF files, which are TIFF files that have geographic or cartographic data embedded as tags within them. Using this file format, the geographic data can be used to correctly position images on a map.
The use of the fused 1-meter black-and-white and 4-meter color imagery was found to be extremely important during the digitizing process and significantly increased speed and accuracy in interpreting the data. By experimenting with the color histogram, ADEQ determined that 11-bit data (2,048 shades of gray) increased the color depth of the images significantly more than 8-bit data (256 shades of gray). Due to the extreme sunlight and brightness combined with white or metal roofing in many buildings in the Douglas/Aqua Prieta area, it was difficult to clearly see details in the imagery. However, ADEQ determined that by paying careful attention to collection specifications, the 11-bit data helped them identify details that would be otherwise lost in a color-reduced environment.
Once the digitization was complete, final map books were prepared to help ADEQ staff identify sources of harmful air pollutants, as well as collect soil samples and traffic volume data. Using these maps, ADEQ was also able to track other local sources of air pollution, including businesses that used potential contaminants, such as paint factories, restaurants, auto body shops, dry cleaners, electronics and other manufacturing facilities. Data was collected on fuels used for home heating and cooking, wood burning and agricultural practices. Once these field studies were completed, the ADEQ staff was then able to create a detailed emissions inventory, which will be used in the next phase of the study—computer modeling of the dispersion and movement of air pollution in the Douglas/Agua Prieta area. This computer modeling will allow state officials and policy makers to evaluate which control measures, such as paving unpaved roads, are the most effective for improving air quality in that area.
Satellite Imagery BenefitsBy using remote sensing techniques to collect and map geographic data, ADEQ recognized significant savings, both in cost and time. Before turning to advanced imaging technology to help implement this project, ADEQ was sending workers into the field with paper maps to physically identify and evaluate PM-10 and VOC sources. This was an extremely time-consuming and tedious process and did not contribute to optimal data collection. The short time frame between when the IKONOS images were collected and when ADEQ staff could make field visits using the polygon maps was extremely valuable to the success of this project. Also, maps often vary from place to place and may be outdated or inaccurate. Therefore, it would have taken much longer for ADEQ to identify local scene changes if older base maps were used. Maps created using IKONOS imagery are more accurate and can be updated quickly; identifying new roads, bodies of water, agricultural areas, building footprints and other types of land use. In addition, satellite imagery allows ADEQ to research areas of land that were previously inaccessible to humans. The use of 1-meter imagery is critical for ADEQ’s purposes, as lower-resolution images would not be as effective in identifying smaller specific features of interest.
ADEQ continues to learn from the success of this pilot project and is refining its methods based on the availability of new technologies. In fact, ADEQ is planning to implement the use of high-resolution satellite imagery to study air pollution in other areas of Arizona within the next few years. Software packages are currently being developed that can process satellite images in the field. This will give ADEQ staff the ability to use laptop computers equipped with digital field assessment maps for easy reference, and access to commercial databases. Once they return from the field, ADEQ staff will be able to easily upload the data collected into the ArcInfo program, speeding up the entire data collection and entry process. As this data is processed and more information is learned, ADEQ will be able to share it with other government agencies, such as the EPA, to provide scientists with an accurate model to more effectively study the sources of air pollution.Another method ADEQ had previously used to study air pollution was aerial photography. However, this method was not as effective, both in terms of cost and political implications. It can actually be cost-prohibitive, as it is very expensive to purchase, manipulate and process aerial imagery of non-metropolitan areas. In addition, many different permits are required for airplanes to fly over foreign countries to take photographs. By using high-resolution satellite imagery, ADEQ no longer had to deal with the problems and bureaucracy associated with working with other governments to obtain permission for such activities. After all, IKONOS flies in non-sovereign space and can legally take an image anywhere in the world.