image resolution. There is nothing inherently limiting about the amount of resolution possible with digital imagery; you can have complete control over the focal length and flying height. With a helicopter, there is no reason why you couldn't get one-inch or less resolution."

Daedalus expects to have its panchromatic ADC system on the market in 1997, and a multispectral version soon after.

GEOREFERENCING

Digital imagery intended for a GIS vector data set must be referenced to a map coordinate system. In areas with minimum terrain relief, where overview is more important than horizontal pixel location, processing software can tie the image to map-based coordinates by registering it to overlaid GPS files, USGS quad maps or existing orthoimages. A geographic coordinate is assigned to each pixel, although not with the precision of digital orthoimagery. Automated mosaicking software combines individual, overlapping digital images into a single composite image, considerably reducing the cost, time and effort once associated with mosaicking.

Developing a precision base map from digital imagery requires ground control points and digital elevation models -- basically, the same complex, orthorectification process used for film. Digital orthorectification corrects each image for displacement, tilt, curvature and terrain relief.

SCANNING VS. FRAME CAPTURE

Digital images are formed either by scanning the array one line at a time (at the speed of light, of course), or by capturing an entire frame at once as film cameras do. Hyperspectral or multispectral scanning systems -- used almost exclusively in satellite remote sensing -- can provide an enormous amount of spectral information about a subject. An airborne hyperspectral scanner, such as the Compact Airborne Spectrographic Imager from ITRES Research Ltd., in Calgary, Alberta, provides up to 256 bands of spectral information. Such sophisticated systems are used mainly in research and exploration. Multispectral scanning systems with fewer bands are more economical for day-to-day applications.

The ADAR System 5500 is a four-band, frame-based capture system capable of producing color and color infrared images simultaneously at 0.5 to three-meters-per-pixel ground resolution. This multispectral system has a wide range of applications -- capturing imagery to determine the health and vigor of vegetation, diseases in trees and forests, anomalies in agricultural fields; assessing the scope of pollution in lakes, drainage systems and coastal outflows; and identifying and mapping different types of plants in wetland habitats. Although not able to define spectral bands as narrowly as scanning systems do, frame-based technology is usually more tolerant of turbulence, and its data less complex to process.

CORRIDOR MONITORING

Digital aerial photography has proved to be an efficient method of acquiring data for mandated survey studies of corridors -- pipelines, power transmission lines, railroads, highways, viaducts, rivers and coastlines. Power companies, for example, are concerned with sag in the wires, hot transformers and vegetation encroachment. Railways must do classification studies based on wetlands areas. If there is a derailment in relation to a wetland, the company is required to classify bridges and other infrastructure relating to that wetland. Pipeline companies must comply with regulations affecting the management of lines carrying natural gas, crude or refined products.

Regulatory compliance can involve mapping, inspection and rehabilitation, facility inventories, environmental reporting and public safety notification programs. Gas pipeline companies are required to conduct annual class-location studies to determine the density of residential buildings within 660 feet of their underground natural gas lines. The Department of Transportation uses the density figures to set pipeline pressure levels for purposes of public safety. As residential building density increases, line pressure must be reduced or, in some cases, the line itself strengthened or removed.

Natural and man-made features within and adjacent to pipeline corridors are continually changing, requiring periodic aerial surveys to satisfy both routine infrastructure management and regulatory compliance. Companies with extensive corridor utilities may find digital aerial photography the most cost-effective method of documenting their rights of ways.