Digital Aerial Photography Flies

Recent advancements in digital aerial photography is making it an inexpensive choice.

by / February 28, 1997 0
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.

FOOTPRINTS
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
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 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. 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.)
GIS is also generally considered as a specific subset of the overall field. According to Understanding GIS (ESRI, Redlands, Calif.) a true GIS system can be used to perform each of the following five functions:

The GIS can answer what exists at a specific location. The location can be described using place name, ZIP code, latitude and longitude or other location systems.
The GIS can find locations satisfying specified conditions (e.g. an undeveloped parcel of land zoned for light industrial, at least 10 acres in size, with railroad access).
The GIS can spot changes in an area over a certain period of time.
The GIS can find patterns. For example, it could test the hypothesis that proximity to PCB-laden transformers is a factor in the incidence of cancer in children.
The GIS could model various scenarios. For example, if 10 inches of rain fell in a certain watershed, where would flooding occur and at what hour.
GPS coordinates

Global Positioning System coordinates -- a system of locating points on or above the earth through triangulation of satellite signals.

Ground Control Points

Point on the surface of the earth with known coordinates as represented by some geographic grid reference system.

Hue

Property of colors ranging from red, yellow, green and blue, determined by the dominant wavelength of light emitted or reflected from the surface.

Hyperspectral

Camera systems able to record many spectral wavelength bands (up to several hundred) extending from ultraviolet to visible, beyond infrared.

Luminance

The intensity of light per unit area of its source.

Multispectral

Camera systems sensitive to more than one spectral wavelength band.

One-Meter Ground Pixel Resolution

The smallest feature that can be discerned (in one pixel) is one meter in length or width.

Orthoimages

Corrected or "straight" images. When images are made of large areas of the earth, they are skewed by the angle of the earth to the camera, the curvature of the earth, surface irregularities, etc. Orthoimages are corrected to remove such effects.

Orthorectification

Correcting an image (see "Orthoimages" above).

Resolution

Fineness of detail that can be distinguished in an image, as on a video display terminal. "Display Resolution" is the density of the pixels that compose an image. "Spatial Resolution" is the smallest possible map feature that can be accurately displayed at a specified map scale.

Saturation

Vividness or intensity of hue.

USGS Quad Maps

Maps produced by the United States Geological Survey showing one quadrangle, or one atlas sheet.

Sources:

URISA GIS Glossary
Understanding GIS, ESRI, Redlands, CA
The American Heritage Dictionary of the English Language





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