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Aerial Photography

film digital cameras mapping

DAVID MALIN
Anglo-Australian Observatory, RMIT University

DONALD L. LIGHT
Airborne and Space Systems for Mapping and Remote Sensing

Aerial photography has a surprisingly long history and reflects the photographer’s ever-present urge to seek a better vantage point for the camera. The possibility of a bird’s eye view proved irresistible, and in 1858, Gaspard Felix Tournachon (Nadar) took aerial photographs of a French village from a captive balloon, a feat that was repeated in the United States by James Wallace Black. His picture of Boston in 1860 (Figure 1) is the earliest existing aerial photograph.

In the first decades of the 20th century improvements in photographic technology allowed cameras to be carried by rockets, kites, and birds and, by 1909, in aircraft. The military potential of this was obvious and aerial photography was widely used in WWI. It was soon found that vertical views were the most useful but could be difficult to interpret, thus the art-science of photo interpretation was born. In recent years the subject has broadened considerably and is now embraced by the term “remote sensing,” which includes downward-looking observations from earth-orbiting satellites as well as aircraft. This topic is dealt with in a later section, as is multispectral imaging, also pioneered by aerial photographers.

Apart from military surveillance, applications are enormously varied, and include agriculture, archeology, forestry, environmental monitoring, and demographics as well as urban planning, geology, minerals prospecting, and surveillance of all kinds. However, aerial photography is most often used for cartography, where the science of photogrammetry is used to remove the distortions inherent in all photography. This enables accurate dimensions and topography to be derived for mapmaking.

For this, special-purpose, downward-looking, automated mapping cameras were developed that used 9-inch (about 23cm) roll film. Since cartography and surveying use large amounts of film, suppliers made special emulsions, including high-contrast, high-resolution monochrome and color films and infrared-sensitive products to penetrate atmospheric haze. Some of these emulsions were available in smaller formats, and cameras using 70 and 35mm film are also used for aerial photography. Not all such images are intended for mapmaking or for accurate dimensional surveying, so very useful aerial images can be made very conveniently with these smaller formats. Today, film or digital media can be used and operated manually from aircraft or remotely from kites, tethered balloons, or remotely piloted aircraft. Balloons and remotely piloted aircraft can operate at a lower altitude than piloted aircraft, which is a great advantage over residential areas.

For cartography and survey purposes, it is usual to make a series of images along a predetermined flight line at a fixed altitude. The images are made in a sequence so that there is
approximately 30 percent side lap and 60 percent forward overlap. This allows for complete ground coverage and for stereo viewing of overlapping pairs of pictures. Each image may include some ground control points (GCPs) whose positions are accurately known by using global positioning satellite (GPS) measurements. GCPs by GPS measurement are precise to a few centimeters. Using these data points and calibrated cameras, accurate coordinate systems can be developed by photogrammetry for a variety of applications including mapping. Where it is not possible to have GCPs, the camera position, time of exposure, and precise orientation in three dimensions are recorded with the imagery data. This new technology utilizes GPS and inertial measuring units called position and orientation systems (POS) that obviate the need for GCPs.

The traditional, large-format film cameras developed during WW II became the workhorse around the world for mapping and photogrammetric applications. These cameras used 9-inch wide film for black and white, color, or color infrared photography. The most common lenses had focal lengths of 6 inches (152.4mm), with some 12-inch (304.8mm) focal lengths for higher ground resolution. By the late 1980s these cameras and their associated films were a mature technology, and later still often incorporated forward motion compensation, angular motion stabilization platforms, high lens and film resolution with virtually no distortion, and a camera shutter interfaced to the GPS to give a precise position at the moment of the exposure. These gradual improvements brought about significant increases in the overall usefulness and effective resolution of modern mapping cameras which today achieve an area weighted average resolution (AWAR) of 40 to 50 line pairs per mm. The AWAR depends on the film type, atmospheric conditions during exposure, turbulence, etc.

While the film mapping cameras were reaching a mature state, by the late 1980s, solid-state charge-coupled devices (CCDs) began to appear in military aircraft and in the marketplace. CCD detectors are sensitive to wavelengths between0.4 and 1.0 (µm, covering the visible and near-infrared parts of the spectrum and compete directly with film as a focal plane detector for aerial imaging.

The introduction of the digital detector began a slow transition from film to all-digital systems, and by the turn of the century, this change was in full progress. Photogrammetrists and mapmakers now have a choice of film or digital media. Most organizations today are able to scan the film and turn it into digital pixels for use in soft-copy workstations; the transition to the digital era is happening even though many organizations still use their time-proven film cameras. Converting to digital data is essential for those who use geographic information systems (GIS)—a powerful tool in today’s imagery-driven mapping business.

The first consumer digital cameras in the early 1990s had sensors that were typically 12 × 18 mm and produced 1200 × 1600 pixel (about 2 megapixel) images. By 1995 this had grown to a 2 × 3k array (about 6 megapixels), and by the turn of the century professional camera backs for large format cameras offered 4 × 5.5 k sensors (22 megapixels). However, to achieve useful results from normal 9 × 9 inch aerial film, it must be scanned at 2400 ppi to produce a 20,000 × 20,000 ppi image, equivalent to 400 megapixels per frame. Although basic megapixel count is not the only consideration in digital imagery, the manufacture of sensor chips approaching this size for the relatively small aerial and remote sensing market is complex and expensive. In addition, computer hardware and specialized software to analyze it is also costly, which is why the transition to all-digital systems has been slow.

However, digital CCD detectors are comparable to film for spatial resolution and offer many other advantages. Early in the new millennium, the two major film mapping camera manufacturers (Zeiss/Intergraph Imaging and Leica) introduced digital systems. For their digital mapping camera (DMC), Z/I Imaging combines four CCDs to produce one complete 13.8 × 7.7k panchromatic frame camera capable of up to 2 frames per second. The camera also has multispectral capabilities, electronic forward-motion compensation, and a field of view of 74 × 40 degrees with its 120 mm focal length lens.

In contrast, the Leica ADS40 Digital Sensor uses a different approach, with seven linear arrays to sense panchromatic and multispectral bands. The panchromatic band employs linear arrays of 2 × 12,000 pixels staggered by one-half pixel, and all seven bands use one set of optics to provide near-perfect registration of the bands. The field of view for the ADS40 is 62.5 degrees.

Both the DMC and the ADS40 are high-end, multispectral digital cameras of a kind destined to take over the mapping-camera market in a few years. They offer spatial resolution as small as 15 cm, and they are becoming a formidable competitor in the world-wide mapping-camera market. Table 1 tabulates the DMC and the ADS40 along with the other smaller, but capable digital cameras that are available, reflecting the transition in this field. These sensors use CCDs and compatible magnetic oxide semi-conductors (CMOS) and other solid-state detector technologies that promise improved capability in the visible, near IR, short wave IR, and thermal IR ranges for a wide variety of remote sensing applications.

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