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In the Charter Issue we ran a story about Bryan Logan and EarthData International. In this issue we serve up another slice of the EarthData information: its unique method of creating TrueOrtho. TrueOrtho is an orthometric photogrammetry product that corrects the building lean inherent in all aerial photographs. That is, looking straight down onto an unadjusted aerial photograph, everything in the image appears to lean away as one moves away from the center of the photograph. This is due to the fact that camera lenses produce distortion in the image. The lean problem has been the bane of aerial photogrammetry since the onset of aerial photography. With ground orthos, ground elevations across an entire image are correct, but such is not the case with objects above the ground. For many projects, the primary interests are the ground elevations and objects on the ground such as roads and streets. But what about buildings, bridges and overpasses? Depending on how much money a contracting agency wants to spend, bridges and overpasses can be corrected by hand, but when these objects are moved to their true position, gaps and overlaps are created above the underlying ground. Likewise, when a building in an image is "leaning" it will hide information on the ground that may be of interest to the end-user. Until now, consumers of orthophotographs just had to put up with it. But now, with EarthData's TrueOrtho, an entire image is correct, and every point on an image appears as if the camera were directly above it.

On this second visit to EarthData’s Frederick, Maryland headquarters, I met with proposal director Louis Demargne to learn more about this fascinating technology. Demargne was born in the United States and grew up in Britain. In 1992 he obtained a degree in oceanography from Southampton University in the UK, then went on to receive a Masters in remote sensing from the University of Paris in France in 1993. He then spent a year in Mexico helping the Mexican Institute for Geography, Statistics and Computing (INEGI), produce maps from Spot and Landsat imagery. In 1995, he started with Spot Image, combining Radarsat data with Spot imagery. Five years later, in 2000, Demargne went to work for ISTAR of France, which had developed an image processing tool capable of automatically generating accurate digital elevation models from Spot stereoscopic imagery.

A few years prior to that, the German Space Agency (DLR) had developed a push-broom scanner for a mission to map the planet Mars, a mission which was later abandoned for lack of funds. (This camera was a predecessor to the Leica ADS40 digital camera.) ISTAR realized that data from this type of  sensor could easily be adapted to its processing system. The company worked with DLR to adapt the sensor for use as an airborne imaging sensor to produce much higher resolution images and elevation data than what could be obtained from satellite systems. ISTAR leased one of these sensors from DLR—called the HRSC-AX—and began flying speculative missions over more than 100 cities for the important European telecom markets to develop 25 cm resolution black and white TrueOrthos and digital surface models with a one-meter post spacing.

In October 2000, ISTAR Americas, Inc. (ISTAR) was created in order to address the U.S. telecom market, and to start building a similar archive of high-resolution, “off-the-shelf” orthoimage and DSM products over U.S. cities. Once the telecom market plummeted, however, the company quickly realized that customers in federal, state and local governments needed new data, not archived data and started offering to fly and produce TrueOrthos on contract. Right off the bat, ISTAR’s first customers saw the benefit in having imagery without building lean and with the excellent horizontal accuracy that the system provided (one-foot horizontal accuracy RMSE). Another important benefit was the fact that TrueOrtho offered the possibility of updating existing planimetric maps by heads-up digitizing within the users own GIS environment. Most local governments have limited budgets and appreciate the possibility of updating their maps with in-house resources; the TrueOrtho provided a way to do this.

In the meantime, EarthData Technologies, the research and development arm of the EarthData group, had started working with the Leica ADS40, which was largely based on the DLR’s HRSC design. Searching for  a system that would make digital aerial imagery a more efficient alternative to conventional aerial photography, EarthData began discussions with ISTAR in late 2002. In February 2003 they bought ISTAR America's assets, including an exclusive license to use the ISTAR processing system in North America.

Demargne, who subsequently joined EarthData, explained that the ADS40 is like a satellite sensor installed on board an aircraft. Combined with the ISTAR processing system, it rapidly generates accurate orthoimage products and DEMs. The ADS40 simultaneously collects imagery forward, nadir and backwards of the aircraft's flight line, providing stereoscopic imagery that is then used in the ISTAR processing system to generate an accurate elevation model of the observed terrain, including all above-ground features such as buildings, bridges and vegetation. This elevation model is the basis for the orthorectification of the ADS40 imagery to transform it into a TrueOrtho product.

The ISTAR processing system produces seamless orthoimage mosaics and a digital surface model (DSM). Since the ADS40 acquires data in four separate spectral bands (red, green, blue, and near-infrared), the orthoimages can be produced in either black and white, natural color, or color infrared. Final product accuracy is dependent on the quality of the airborne GPS and the inertial measurement data. Hence, controlling flight parameters during acquisition in order to eliminate IMU heading fatigue, and aircraft distance from the GPS base station, is crucial. The system requires less ground control overall. Generally five ground control points per area acquired in a single flight are used, regardless of the size of that area.

The highly iterative process of the TrueOrtho system looks at all the various stereoscopic views from the flight line strips and correlates data on a pixel-by-pixel basis to extract the elevation of the surface. Additional processing features include image "dodging" (a type of radiometric processing of the imagery) as well as feature extraction from ADS40 digital stereo pairs. This allows the collection of planimetric details such as ditches, fences and breaklines that are difficult to interpret from a 2D image. Stereo-compilation also allows the extraction of accurate two-foot contours. The company has also applied its know-how from processing lidar data to the processing of ISTAR-derived DSMs in order to automatically produce bare-ground DEMs.

A procedure called quick look processing is applied to the data prior to production to generate a reduced-resolution, geo-rectified mosaic of all the acquired data. This allows for the checking of anomalies in the data such as gaps in the flight lines. It also enables the determination o f radiometric prior to full-resolution production. Today, EarthData is applying this unique technology all over the country. Last year the company completed projects that included 88 counties in the statewide mapping effort for Nebraska, and for 28 counties in North Carolina following the Hurricane Isabel disaster. Both of these programs were sponsored by federal agencies such as the USGS and the USDA. The Nebraska project was part of the USDA-NAIP program, which involves 1-2m imagery of entire states, flown every year, for monitoring crop health and stage of growth.

Another rapid response effort involved mapping the areas affected by last fall's southern California wildfires . The imminent danger of mudslides from denuded hillsides resulted in EarthData winning a contract to develop orthoimagery and DEMs of the affected areas. EarthData has started the year 2004 with a project to deliver Digital Orthophoto Quads (DOQ) covering the entire state of Florida for the USGS and local water management entities. EarthData is now considering how the ISTAR process might be used to process imagery from conventional frame-based cameras, as well as satellite imagery. Their experience with ISTAR stands as successful example of integrating a proven technology with new processes and systems to achieve even greater production efficiencies.

Next: A look at EarthData's radar mapping system, GeoSAR.

Marc Cheves is editor of the magazine.

Sidebar:
When compared to a conventional film/scanning approach, the ADS40/ISTAR digital system offers:
• Increased horizontal accuracy--Using the multiple stereoscopic views, the system correlates the data so that each pixel location is measured multiple times. This results in a much higher level of horizontal accuracy when compared to traditional film/scanning.
• Superior image quality--The detail of the digital orthophotography produced by the ADS40 system is extremely sharp and far more detailed than conventional photographicallyderived aerial image products. Also, there are no artifacts such as lint, dust or scratches on the images as is often the case with scanned film.
• Ability to "see into shadows"--ADS40 imagery is collected in 12-bit format as opposed to 8-bit format. 8-bit data can contain up to 256 levels of gray, whereas 12-bit data contains 4,096 levels. This provides a much finer level of radiometric detail for users to work with in order to extract information inside shadows or saturated areas.
• Multiple products--The ADS40 digital sensor acquires aerial imagery at a greater dynamic range than comparable film cameras, and collects multispectral data through red, green, blue and near-infrared spectral bands. A customer can order digital orthoimagery in color infrared at a fraction of the cost of the total project, and the color infrared rendition will precisely match the panchromatic and true color versions.
• Automated production for rapid completion--The all-digital processing of ADS40 data occurs in a parallel processing environment, which reduces production times.

A 4.027Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE