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Wed, Nov 24, 2004

Precision Georeferenced Imagery (Part One)

GeoVantage Displays Evolutionary Technology, Revolutionary Concept

By Senior ANN Correspondent Kevin R.C. "Hognose" O'Brien

Take one airplane, four Sony surveillance cameras, some cheap photographic filters, a large helping of software wizardry made possible by Moore's Law, and prepare over New England Yankee ingenuity as interpreted by a team from all round the world. What do you get?

Precision georeferenced imagery -- in very near real time. That's the promise of GeoVantage, Inc. of Swampscott, Mass, which is delivering aerial photos that are extremely accurate and that have location and terrain data embedded right in them, for the benefit of customers in a variety of industries including forestry, agriculture, urban planning and environmental management.

Aero-News was there at General Aviation Services at John Mountain Field in Beverly, Mass, (KBVY), when GeoVantage held an open house and demonstration of its advanced technology, on "GIS Day": November 17, 2004.

GeoVantage's technology is evolutionary: they used COTS (commercial off the shelf software), generic hardware, and common electronic components to the maximum extent possible, and only wrote their own stuff where it was needed to glue these disparate pieces together. They also don't break new ground in the idea of using overhead aerial imagery to examine things on the ground: that's why the world's armies all got air-minded in 1914. But their concept is revolutionary in several ways: in the way they have automated the system, simplified the process, squeezed cost out and -- most vitally
-- shortened the loop between the desire for imagery and the delivery of accurate, precision imagery that can be used in geographic information systems (GIS). Nobody ever had a really good, really accurate system that can be flown in a light single-engine plane and produce accurate imagery in near-real time. Precision imagery, fast, has been the domain of massively expensive, classified DOD programs -- not delivered via general aviation to commercial end users.

The Airplane End

GeoVantage's Andy Lee was there with his yellow and white Cessna Cardinal. Andy's Cardinal is fairly stock, except... except for the small window in the bottom of the baggage compartment. It's an STC'd camera window. But the camera in it is unlike any you have ever seen.

Instead of one lens, it has four. "Regular Sony surveillance cameras,"
Andy explains with a wry grin. New England Yankees still think frugality is a virtue. Each lens has a filter on it: one is near-infrared, the other three are Red, Blue, and Green. The software will combine the images to provide color or IR images, each of which has its uses. The four cameras are connected by FireWire (IEEE 1393) to a hub, and into a computer that's placed in the back seat of the Cardinal, running custom software on Windows XP. A look at the camera with its sleek cover off reveals that it is pretty much a breadboard project. Hey... is that duct tape?

"No," Andy deadpans. "We call that 'NASA Tape.'"

As anyone who's ever done aerial photography knows, a given focal length at a given altitude will yield a specific scale. Most GeoVantage missions are flown at 8000 ft. which produces a resolution of one meter per pixel or 4000 feet, which produces 0.5m/pixel images, and other resolutions are available in similar proportion. The resolution required depends on the intended use of the images, balanced with the cost of the air mission -- naturally you only cover half as wide a swath at the lower altitude/higher resolution, so you have to fly more.

The camera is only part of the system, of course. Anyone can take aerial imagery, and even scaled aerial imagery is pretty straightforward. But the georeferencing is supplied by GPS.

Differential GPS is used, as in other very-high-precision applications such as surveying; in DGPS a ground station at a surveyed, fixed location receives and measures GPS deviations and transmits corrections. This is the same principle (although not the exact same technology) used in aviation's WAAS.

The GPS data not only records the information about the movement of the plane, it compares it to a plot that was established during preflight planning. To make a course plot, GeoVantage can use as little as the GPS grids for the four corners of the intended image area. Once coordinates for the area of interest are recorded, GeoVantage establishes a course plot. The course plot establishes the intended course, including turns and straight legs, that the pilot must fly in order to achieve the necessary coverage. To ensure complete coverage, there's a lot of overlap; only the 20% in the center of each image that makes up the mosaic will be unique to that exposure.

The course plot is part of the flight plan that is loaded into a removable hard drive, and thence into the airplane. The pilot flies the mission as plotted -- an indicator called the Steering Bar sits atop the dash, and shows the pilot the extent that he has deviated from the programmed course, almost like a localizer needle. If he gets too far off the course, or gets out of the parameters for which the software can correct the camera automagically shuts down.

At all times the computer unit in the aircraft is recording not only the images but also the movement of the aircraft on its removable hard drive. The GPS is only one source of data about the aircraft movement -- an Inertial Measurement Unit or IMU also tracks the plane's flight.

The resulting computer file contains differential GPS location and speed information, but also data about the airplane's pitch, yaw, and roll, sampled 125 times a second. This will be used to correct for aircraft movement when the image is stitched and georeferenced.

All this capability is amazing, but Andy set it up in the Cardinal in a few minutes: the equipment is all portable, and the only modification to the aircraft is the camera window, commonly done by STC or 337. The roomy Cardinal with its yawning doors makes it easy to load, and to use, the system, but the lightweight, portable system is easily adapted to any aircraft that has an STC'd photo door or window. The greatest amount of the wizardry takes place without direct human interaction.

The camera automatically turns itself on and off, and sets exposure automatically. The challenge to the pilot is primarily to fly the plotted course with the greatest possible precision and not to exceed the parameters that would abort photography and require a start-over.

To Be Continued...


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