Since the early 1990s, advancements in commercial digital aerial photography systems have provided black and white, color and multispectral imagery for a wide range of applications, including environmental and agricultural studies and analysis; corridor monitoring, overviews and base map development for a broad range of GIS applications -- all without film or chemical processing.
Fast turnaround is the hallmark of digital aerial photography. It can provide emergency response agencies with images of disaster areas within 24 hours of a flight. Digital color infrared imagery can supply forestry services with early information on tree diseases, and alert growers to anomalies before crop damage is widespread. In utility corridors, panchromatic (black and white) imagery can reduce the cost of utility corridor studies for regulatory compliance, while providing a legal record of georeferenced data for AM/FM, GIS or CAD systems.
ELECTRONICS OVER CHEMISTRY
Digital aerial photography systems use electronics rather than chemistry to capture, record and process images. In a digital camera, the sensor is a charge-couple device (CCD), a chip embedded with thousands of light-sensitive photosites (pixels) that convert varying wavelengths of light into electrical signals. An analog-to-digital converter then translates the signals into digital data representing image luminance, saturation and hue values. The system records the data on a hard disk, along with GPS coordinates and ancillary mission and system configuration information.
Post-mission processing software provides options for image enhancement, georeferencing, automosaicking, GIS interfacing and analysis. There is no waiting for processing to see if the intended images are there; an onboard monitor provides in-flight viewing of the imagery being recorded, and controls for adjusting focus and exposure.
The footprint, or capture area, of the image produced by a digital camera is determined by the number of pixels in the array, the focal length of the camera and the altitude of the aircraft. The ADC digital aerial photography system from Daedalus Enterprises, for example, has a relatively small footprint created by an array of 2020 pixels horizontally by 2044 vertically. Daedalus scientist Bill Anderson pointed out that it is more efficient to fly long, linear stretches with a small-format camera than one intended for large-scale mapping. "If you were to fly a corridor with a big nine-inch mapping camera," he explained, "you would be spending a lot of money getting data way off to the left and right of the corridor that the customer is not interested in." The larger the footprint, the fewer lines that must be flown to acquire large-area imagery.
The ADAR System 3000 from Positive Systems uses a 3000 x 2000 pixel array that can be rotated with the long side either parallel or perpendicular to the line of flight to accommodate corridor or large-area photography. Depending on application needs, the 3000 can acquire black and white, color, or color infrared digital aerial photos.
Ground resolution for a particular camera is a function of the focal length and altitude of the aircraft. Cost increases with greater resolution, since it requires lower altitudes and therefore more images to cover the same area. Therefore, aerial photography firms provide the least amount of resolution needed to do the job, said Anderson. "We flew agricultural fields in Kansas at one-meter ground pixel resolution for each image -- farmers don't want to look at individual corn cobs, they want to look at anomalies in the field. We flew a railroad corridor for another customer who needed to be able to see all the switches and other infrastructure in the image. So there was a case for six-inch pixel resolution."
Anderson said the term "resolution" is somewhat nebulous. "We have many images with six-inch ground resolution in which you can actually see all the wires being strung between electrical towers, but you know those are not six-inch wires. Other concepts such as 'detection' come into play in talking about