ts 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.

According to James W. Sewall Co. Senior Vice President Mark Jadkowski, "A small gas pipeline company may have lines crossing several state boundaries and map coordinate systems. Large companies might operate 30,000 miles of pipeline and manage 15,000 miles of rights of way and associated parcel easements. For companies with extensive corridor infrastructure, the cost of film-based aerial photography is prohibitive."

A joint project in 1990 by the Stennis Space Center, the Sewall aerial photography firm and Algonquin Gas Transmission Company produced the first digital aerial camera system specifically designed for commercial pipeline management and monitoring. Development of the Digital Aerial Remote Monitoring System (DARMS) was funded by NASA's Earth Observation Commercial Applications Program. DARMS has been used successfully on several pipeline projects. An improved second-generation DARMS led to development of the present ADC system by Daedalus Enterprises.

SATELLITE COMPETITION

Mark Jadkowski doubts that one-meter resolution satellite imagery will replace digital aerial photography for such regulatory compliance activities as federally mandated class-location studies for natural gas pipelines. "The minimum resolution you can get away with, and still be able to discriminate land use well enough to do these studies, is about two feet. We've done projects for clients using one-meter resolution, but you end up having to do a lot of field checking to resolve land and building use that cannot be determined well enough from the imagery. I'm not saying we can always do that with one-or two-foot resolution, but the number of question marks increases greatly above that."

COST

The cost of flying imagery is roughly the same for film and digital. "Then there's the cost of creating a georeferenced digital image product," said Jadkowski. "If you start with the image on tape, and create a georeferenced digital image product, it will be about half the cost of creating the same product had you started with a hard-copy photograph or film. You can get into projects where two-thirds of your cost is flying, and one-third is actually processing. You can also get into projects where it is the other way around."

Since digital imagery can be processed in less time and at a lower cost, continued advances in CCD technology and image-processing software will eventually make digital aerial photography the cost-effective choice for most applications.

Bill McGarigle is a freelance writer specializing in communication and information technology. E-mail him at: .

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Terminology

AM/FM

Automated Mapping/Facility Management, a computer-based Land Information System used to produce, edit, archive and correlate Automated Map features with nongraphic facility-related design or operation attributes.

Analog

Refers to a quantity which is continuously variable, rather than one which varies only in discrete steps.

Automosaicking

Producing a single image from multiple overlapping images.

Base Map

Basic reference information onto which other information of a specialized nature is placed. Usually shows the location and extent of natural earth surface features and permanent man-made objects.

CAD

Computer Aided Drafting, a process for interactively creating, modifying and manipulating spatial information.

Digital

Anything in computer-readable format usually stored on magnetic tape or disk. Spatial or attribute data stored in electronic format, digital units, on a hard drive. Discrete numerical representation of information vs. analog representation.

Digital Elevation Models

Digital cartographic representation of a terrain surface or a subsurface feature as defined by a series of three-dimensional coordinate values.

Focal Length

The distance from a lens or mirror to its focal point.

Georeferencing

Connecting map features to actual points on the earth.

GIS

Geographic Information System (GIS): (1.) GIS is an overall term encompassing the entire field of computerized mapping. (2.)