Over the past decade digital aerial images have become the basemap de rigueur for a geographic information system (GIS). To be useful, these images are transformed into digital orthophotos, that is, digital photo images with the scale and accuracy properties of a map. These images have value due to the richness of feature content and their utility when overlain with other information.
Digital cameras now in use
Generally, digital orthophotos have highest utility when they are freshest. However, the typical production process often results in product delivery nine months to a yeear or longer after the initial imagery acquisition. This time delay between is about to shorten due to increased use of digital cameras and ever increasing computing power that speeds the production process.
Relatively recently, two international manufacturers, Leica and Z/I Imaging, have each introduced an extremely high-resolution, high-precision digital mapping camera. These cameras capture digital images from a moving aircraft, but different from the traditional cameras, do not use film. Images are digitally collected by sensors reading the intensity of reflected light into an array of thousands of sensors. Depending on aircraft flying height, each cell in the array can represent ground areas as small as 3″x 3″.
The camera is part of a suite of instruments carried in the airccraft including airborne GPS, inertial measuring devices, and high capacity computer storage. With the elimination of film developing and scanning, less need for ground-control collection, and high-speed computer processing, the time required for ortho production can be cut significantly.
Digital is faster; is it cheaper?
Currently about a half dozen American mapping firms (one in Wisconsin) have purchased digital camera configurations. Ultimately these firms and others to follow will be offering orthophoto production on a much shortened schedule than is typical today. Some companies are promising product delivery within 45-60 days of the flight date.
The cost of production may not be reduced, however. In fact, costs may increase due to the high investment in the digital camera and the associated equipment. A typical camera configuration runs between $1 and $2-million for a complete setup. This high initial capital cost, plus the sophisticated skills and extensive training required to use these integrated systems will together probably not allow for significant cost reductions.
Get value from earlier work
One way to carve major chunks from orthophoto project production time (and cost) is to recycle the geometric underpinnings of an earlier project. As long as accuracy requirements can be met, a digital elevation data set can be used from one project to the next. This is now fairly common as part of second- or third-generation orthophoto projects, especially where the new work does not involve substantially higher resolution than before.
A similar approach uses an older orthophoto image as the geometric base to which new images are warped. An image-matching protocol fits the new image to the geometry of the earlier image. Of course, the result cant be expected to carry any higher accuracy than the earlier image.
Multiple image types from one acquisition
One particularly interesting feature of digital cameras is that they can collect black and white, and color images simultaneously. Although the internal sensing technology varies among the several camera manufacturers, most have similar imaging capabilities. For many customers, the ability to acquire black-and-white, color and/or multi-spectral images during the same flight opens up entirely new and exciting possibilities. Multiple imaging was generally not common or possible with film cameras unless multiple cameras were mounted in a aircraft.
Satellite data may be an option
Although various types of satellite data has been available for mapping use over the past 30 years, satellite data has never become widely used for base map applications. However, satellite data has certainly been used widely for specialized applications such as natural resources and for studies over broad areas. Wider use of satellite data has been limited due to a variety of factors including low resolution (lack of visible detail), high cost, and licensing requirements which often restricts certain uses and redistribution to others.
Over the past five years the federal government has allowed a number of firms to launch satellites capable of collecting high resolution images, and to market to most anyone worldwide. Currently, two commercial satellites collect black-and-white images at 1-meter resolution (SpaceImaging and Orbimage) and another (DigitalGlobe) collects at approximately 2/3 meter (about 2 feet). Over the next three years even-higher-resolution commercial-satellite images will likely be launched. Companies have applied to the US Department of Commerce seeking permission to acquire half-meter and quarter-meter images.
The licensing issue
Perhaps the biggest barrier to use of commercial satellite imagery by the state- and local-government sector has been the licensing restrictions placed on the data. The satellite companies, to protect their multi-million dollar investment in the satellites and data collection equipment, have imposed restrictions on the sale and redistribution of their data.
Recently, several of the largest users of satellite imagery, particularly the National Imaging and Mapping Agency (NINA) have committed to buying large quantities of commercial data rather than completely relying on government data sources. This purchase commitment has financially stabilized the commercial satellite imaging sector somewhat with the result that companies now appear willing to revisit some of their more restrictive licensing polices for others. In fact one firm is looking at policies of unrestricted redistribution coupled with commitments to volume purchases over a specified period of time.
Where will the trend lead?
The bottom line is that digital aerial and space-based imaging is here and becoming more common every day. Traditional cameras and film still dominate the market, but for how much longer?