Imaging satellites employ a complex technology that until recently hasn’t been readily or widely understood. This has put satellite companies at a disadvantage when it comes to capturing the mind share—and market share—of local government agencies involved in mapping projects. Although traditional film-based aerial photography offers many benefits for mapping applications, the advantages of high-resolution satellite imagery shouldn’t be overlooked.

Satellite imagery is sometimes a more practical alternative, and often remarkably complementary, to aerial work. As detailed in the accompanying sections, local governments can benefit from the value of high-resolution digital satellite imagery for mapping and geographic information system (GIS) updates, economic development, environmental protection and preservation, and disaster management and public safety.

Traditional Aerial Orthophotos vs. Satellite Imagery
Digital orthophotography, which achieved widespread acceptance as an urban planning tool in the early 1990s, provides an effective way to compare satellite imagery and traditional film-based aerial imagery. Satellite imagery is collected using a digital sensor rather than a film camera, so a digital product—a digital orthophoto—must be used to accurately compare the two data collection methods.

The film-based aerial camera employs a technology that has been easily understood and used for nearly a century, an advantage that makes it a popular tool among government users. In addition, film resolution is extremely high, providing high-resolution digital orthophotos for accurate mapping applications.

When the original aerial imagery is scanned, however, the process typically doesn’t preserve the film’s original resolution. Additionally, only one type of image—panchromatic, color or color infrared—can be collected per flight, which may necessitate multiple flights. Localized weather patterns, airspace management/security issues and lack of access to international airspace limit the aircraft itself.

Many local government organizations that want to improve mapping efficiencies and streamline processes are exploring the use of high-resolution satellite imagery. Because the imagery is collected digitally, there’s no data loss during the scanning process. Digital imagery also contains more information than scanned film because it provides panchromatic, color and infrared data, and covers a significantly larger ground area than aerial photos. Additionally, satellites aren’t subject to weather conditions or limited by national and international airspace management issues, plus they can revisit target sites every few days, enabling customers to acquire updated imagery more frequently. In many cases, local government agencies want to exploit the complementary advantages of both imagery sources for their mapping needs. Some satellite imagery providers now offer subscription programs that afford government organizations the option of capturing high-resolution aerial orthophotos every three to five years while supplementing their databases with updated 2-foot satellite image collections every one to two years.

 
Publisher’s note: The Nale Report (August 2002) offers a product comparison between DigitalGlobe’s QuickBird 2-foot satellite imagery and 1.5-foot digital aerial orthophoto imagery of Boulder County, Colo. The aerial orthophotos were derived using traditional aerial camera collection, film scanning and photogrammetry. The report indicates that satellite-derived digital orthophotos compare favorably to traditional digital orthophotos. The report can be downloaded from DigitalGlobe’s Web site (www.digitalglobe.com).

Unfortunately, local governments—especially those that serve areas undergoing rapid growth—face the “data vintage” challenge associated with GIS base maps, because their imagery often is outdated by at least a year. As a result, the maps don’t depict new construction, demolished buildings, new transportation routes and updated utility features.

The use of traditional aerial photography to update areas of change is a time-consuming and expensive process that involves a series of steps, including submitting requests for proposals and contracts; flight planning and flight map generation; aircraft tasking; selecting and surveying geodetic control points; and, finally, processing, titling, scanning and storing aerial film. Given these challenges, using satellite imagery for updates can be an attractive alternative.

The 2-foot-pixel resolution satellite imagery supports many of the same types of applications as aerial orthophotography, including parcel mapping, property appraisal, land development, local and regional planning, environmental change and vegetation monitoring, river and stream management, flood hazard mapping, disaster preparation and response, law enforcement, and fire prevention and tracking.

For GIS and map updates, the use of satellites to collect ground data is appealing because it significantly cuts down on image acquisition and processing time. Satellites may be tasked to capture large areas during a specified timeframe, and the digital data may be delivered in formats and projections required for immediate review and analysis by local government mapping and planning professionals. This streamlined process eliminates much of the time and many of the costs normally associated with aerial data collection.

Because satellites orbit Earth predictably, a local government agency can be assured a satellite will be in position to collect imagery on a scheduled basis—monthly, quarterly, semi-annually or annually—and can predict when imagery can be acquired to update an existing data set.

Using new high-resolution satellite imagery, updated information about land changes can be easily loaded to a GIS and overlaid with existing base map features such as property parcels, street centerlines, railroads, major above-ground utility features, hydrography and new building structures. The imagery’s high resolution lets users digitize features at a scale of 1:2,400, provided the GIS base data are of the same accuracy. Using heads-up digitizing, local government GIS employees or contractors can spend less time and money updating maps.

From a data-sharing perspective, satellite imagery can ease the burdens of multidepartmental agencies with different projects, but a similar need for updated GIS data. According to Larry Asaro, city manager for the city of Prescott, Ariz., satellite imagery addresses his need for accurate data in a GIS.

“Our GIS is shared by different departments that rely on up-to-date parcel maps,” he says. “These maps are the cornerstone of our data pool—underneath the maps are data layers containing information about vegetation, traffic, water, street centerlines, road surface quality, etc. Satellite imagery ensures that our underlying data is as accurate as possible. When new images are captured, their digital nature allows easy change detection and quick database updates.”
 

As the U.S. economy has evolved from one based mainly on agriculture and heavy manufacturing to one of services and information, local governments face challenges associated with regional U.S. competition as well as foreign competition to retain existing industry and attract new jobs. In the new economic environment, satellite imagery can help provide information for promoting economic development and planning growth.

By providing easy-to-understand information about the present condition of a local government’s infrastructure and tax structure, as well as the type, size and location of land available for development, satellite imagery can be used to plan the redevelopment of existing properties and to develop new ones. Local and regional planners can use the imagery to plan new and expanded transportation lines and utility infrastructure.

By combining vector GIS data with submeter satellite imagery as a visual backdrop, cities and counties can attract developers, home builders and engineers with 3-D “urban models” that let them “fly through” realistic visualizations of downtown areas. Other technologies, including light detection and ranging (LiDAR), can be incorporated to generate digital terrain models and determine the correct heights of buildings and trees. In addition, digital photos can be used to create textures and bring buildings, grass, concrete and asphalt to life.

Local governments in areas of high population growth face many challenges associated with an increased tax base; accelerated transportation requirements; and increased sewer, water and power utility needs. In addition, high-growth areas often contend with heightened pollution, flooding and the loss of natural features. Similarly, areas experiencing population shrinkage face a decreased tax base, a crumbling transportation and utility infrastructure, and potential environmental damage caused by abandoned factories and other structures.

All of these challenges represent changes that local governments must manage. Furthermore, governments must minimize the environmental impact of these changes and take actions to preserve a high quality of life for their citizens. Satellite imagery provides local governments with a tool to monitor, over time, the condition of the natural environment and to ensure that local ordinances involving building codes, pervious and impervious surfaces for water management, environmental management and aesthetics can be enforced as efficiently and effectively as possible.

The Comprehensive Resource Management Plan (CRMP) of the Mississippi Department of Marine Resources (DMR) aims to manage the effects of growth and development on the environment and balance coastal resource protection with economic expansion “to promote sustainable development, or ‘smart’ growth,” according to Tina Shumate, CRMP bureau director for DMR.

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“Digital satellite imagery lets us quickly identify areas appropriate for preservation and those that are more suitable for development,” says Shumate.

Alaska’s Department of Natural Resources Division of Forestry (DoF) has turned to high-resolution satellite imagery and GIS technologies for wildfire protection planning. The orthorectified satellite imagery is used to map vegetation and create a database of fuel models, which allows DoF to identify fire-prone areas for fuels treatment and fire pre-attack planning, and to predict fire spread with fire behavior software so threatened subdivisions can be evacuated in time.

According to Lee, the satellite imagery also will support flood assessment and control measures, as well as hazardous material spill responses. In addition, satellite imagery can help emergency managers perform a “plume analysis” when a chemical is released in a populated region. As a result, emergency personnel can quickly identify and evacuate structures at risk, determine affected methods and modes of transportation, and quickly re-route goods and services as necessary.

 

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