Advances in Cameras and GPS Make Aerial Photography a Cost-effective Sub-meter
Map Source
By Stewart Walker

Years of expectation preceding the launches of planned high-resolution imaging satellites have focused attention on the enormous market for highly accurate and detailed mapping services. Although satellites have grabbed the headlines, it has been the aerial survey industry that has cultivated the market for orthophoto maps, both as end products and as GIS layers in numerous applications.
    While the GeoTechnology industry has waited for 1-meter (m) satellites, the aerial photography business has quietly grown in size, expertise, and cost-effectiveness. Customers are receiving better quality photographic products more quickly and less expensively than ever, prompting the industry to take notice of the many advantages of aircraft- abilities to fly below clouds, revisit a site several times a day, and acquire black-and-white, color, or false color imagery at any resolution. Moreover, recent developments have underlined the reliable nature of aircraft technology, which is not plagued by launch and communications problems, to which satellites are apparently still prone.
    Both aircraft and satellites enjoy respective benefits that will make them complementary data sources for many applications in the 1-5m resolution range. However, due to the recent significant technological improvements in airborne photography, aerial surveying will continue to dominate mapping projects requiring resolution and accuracy at the sub-meter level.
    Many of the advances that make consistent and accurate sub-meter aerial photography a reality relate to improvements in cameras and the introduction of GPS. And more importantly, aerial survey firms have assembled these hardware and software elements into integrated systems that have refined air photo collection from start to finish.
    One firm that has consistently led the industry in incorporating these technologies and developing new techniques is Keystone Aerial Surveys, Inc., of Philadelphia, Pennsylvania. A close examination of this company reveals how GPS and other technologies-when properly applied-make aerial surveying a cost-effective source of highly accurate and detailed map data with minimal need for ground control.

Introducing Keystone
In business since 1965, Keystone operates ten aircraft (including a Learjet, two Cessna Conquests, and seven Cessna 320s), conducts about 65 percent of its work for private industry, and the remaining 35 percent for government clients. It performs the bulk of its commercial work using 6-inch focal-length lenses to acquire photos at 1:12,000, 1:6,000, and 1:2,400 scales for a variety of topographic, cadastral, and engineering applications.
    To keep up with the demand for timely, high-quality aerial photography, Keystone frequently surveys much of the area within range of its home airport on speculation. The firm knows there will be buyers for the photography soon after it is acquired.
    Accuracy is critical to Keystone. For Keystone, the accuracy of its finished product begins with its camera systems. Fifteen years ago, owner Gill Mallinckrodt converted his entire operation to Wild aerial camera systems (later renamed Leica), favoring their high quality Swiss optics and durable drive mechanisms. "Wild was really committed to the aerial survey industry and built its systems with the aerial photographer in mind," says Mallinckrodt.
    Currently, Keystone aircraft are outfitted with Leica RC30 cameras from LH Systems, LLC, with built-in forward motion compensation, gyro-stabilized camera mount and light-weight film cassettes for quick film reloading.
    These state-of-the-art cameras come delivered with external data interfaces to allow for direct communication among camera, flight management system, navigation units, and GPS receivers-all important elements in Keystone's pursuit of accuracy.
    Quick turnaround is another Keystone trademark. The firm maintains its own processing laboratory on site and has equipped many of its aircraft with long-range fuel tanks so they can usually return film to the homebase for processing each day.

Preparing for Flight
Keystone clients usually provide the firm with flight plans containing specific coordinates for photographic center points. Technicians digitize the flight line and generate a bar code containing the photo centers and flight line end points. Before the flight, these bar codes are scanned on to PCMCIA memory cards containing aerial navigation data that the pilot slips into the GPS-based flight management system (FMS) in the aircraft.
    For geodetic survey flights requiring sub-meter accuracy, Keystone uses three GPS units; two in the aircraft and one base station on the ground for differential post-processing. For these jobs, the aerial photographer initializes the on-board geodetic GPS for 20 minutes at a stationary position within 10m of the base station. This GPS is usually set up on a surveyed control point at an airport near the project area.
    "Initialization establishes a baseline between the control point and the geodetic GPS unit in the airplane," says veteran Keystone pilot Ron Slonaker. "This creates a starting point for the survey, which ultimately enhances overall accuracy."
    For projects close to its home field at Northeast Philadelphia Airport, the firm operates the base station there. But when the project site is too far away to maintain accuracy, the pilot flies the extra GPS unit and sets it up at a closer airport. As a rule of thumb, one millimeter in error is introduced for every one kilometer of distance between the base station and the aircraft.

Collecting Center Points
The firm uses Trimble 4000SSI receivers as its geodetic units on the ground and in the air. These are dual-frequency, carrier-phase receivers. The navigation unit on the aircraft is a Trimble 2000A.
    Although numerous off-the-shelf FMS packages are available, Keystone developed its own onboard system. The navigation GPS is attached to the FMS and provides the pilot with visual screen displays that guide him to flight lines and end points. In addition, the GPS is sequenced to the RC30 camera, giving it a shutter firing command as the aircraft passes over the predetermined photo center point.
    At each acquisition point, the accuracy of the Leica camera becomes of critical importance to the mission. Taking into account the shutter speed and motion of the aircraft, the RC30 camera emits a pulse at the precise millisecond the camera is at the center of the exposure. This pulse is transmitted directly to the geodetic GPS unit which records the time and position of the center point.
    On the ground, Keystone performs postprocessing on the GPS data with the GrafNav/GrafNet software package from Way Point Consulting, Inc., of Calgary, Alberta. This software correlates the center point coordinates and time tags acquired by the airborne GPS unit with data collected by the base station unit to interpolate photo center point locations that are accurate to within a few centimeters.
    "These precise position interpolations are used for controlling the photographs during photogrammetric processing," says Slonaker.
    Collection of GPS control in the air has dramatically reduced the cost of high-accuracy aerial photography. The geodetic GPS minimizes the need for control points on the ground, an expensive process involving paneling and surveying by field crews. When geodetic GPS is in use on the aircraft, the number of surveyed control points required on the ground is reduced by up to 90%.
    Not only does this make aerial photography a better value for many end users, it also expands the scope of potential project areas. In many remote areas, ground control is difficult to collect because of either political or natural barriers. But with airborne GPS, aerial photography can be relied upon to make accurate maps even in the most remote or hostile locations.

Creating a Finished Product
When the aircraft returns from flight, the pilot once again parks next to the base station for about 20 minutes. This closes out the survey with a final synchronization of the two GPS units. It also completes the sequence of data collection that recorded exactly where the aircraft went during the flight.
    Once the flying is completed, Keystone performs its own black-and-white processing on premises or contracts out for color work. Here again, years of experience in providing clients with high quality work and fast turnaround have resulted in an optimized workflow for processing, printing, quality assurance, packaging, and shipping. Film photography will never have the instantaneity of digital imagery, but the argument that it is handicapped by long delays between commissioning the photography and receiving the deliverables is simply no longer true. Keystone's typical deliverable is a set of diapositives and a computer disk. The floppy contains the center point coordinates that have already been processed and corrected for accuracy by Keystone technicians.
    Keystone is constantly upgrading its capabilities. Since 1997 it has been operating an LH Systems DSW300 scanner, enabling it to offer its clients digital, as well as hardcopy deliverables. Mallinckrodt is intrigued with several new technologies now being introduced. Among these are airborne laser scanning and digital photo acquisition. The firm has already contacted LH Systems to acquire one of its new line of airborne sensors which will further enhance the quality and economics of aerial photography in the next few years.

About the Author:
Stewart Walker is director of Marketing at LH Systems, LLC, in San Diego, California. He may be reached at [email protected]