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Photogrammetry

systems cameras images photogrammetric

CLIFF OGLEBY
The University of Melbourne

Photogrammetry is the art, science, and technology of making measurements from photographs. The discipline started in the 1880s, but more recently the advent of digital imaging has dramatically changed the accessibility and application areas of photogrammetry. Originally used for the measurement of buildings, and then adopted for producing maps from aerial photographs, the applications now encompass mapping, architecture, engineering, medicine, forensics, 3D modeling, and the creation of low polygon models of objects suitable for display on the Internet.

The word photogrammetry was coined originally in the 1880s, using words of Greek origin, to describe a process developed for measuring and documenting buildings. Following the advent of hot air balloons, and later, aircraft, the photogrammetric camera was taken aloft and used to make maps. The technology for measuring from photographs has dominated map making for about the last 100 years; the vast majority of small- and medium-scale maps are made using photogrammetry.

The original photogrammetric cameras were large, heavy, and engineered to have precisely known and non-changing, parameters such as the focal length and a flat film plane. Aerial cameras used a vacuum system to hold the film flat, and terrestrial cameras often used glass plates as the photographic medium. The photographs were taken from predetermined locations, and planned so that a significant area of each photograph overlapped the next, thus changing the parallax. Such overlapping images are known as a stereo-pair and this mimics the way humans see in three dimensions. This overlapping area could be viewed three-dimensionally, or in “stereo,” using yet another piece of specialized equipment. Providing some measurements such as scale, tilt, or flying height were known, or able to be derived, the stereo-model could be oriented so that accurate distances and heights could be measured.

While the primary use of photogrammetry up until the 1970s was aerial mapping, there were also applications in architecture, archaeology, medicine, and manufacturing.

Once computers became more readily available, it was possible to calibrate the camera systems rather than rely on high manufacturing tolerances. This ability meant that many cameras not designed for photogrammetry could be used to give accurate and reproducible measurements. The next stage in the evolution of photogrammetry was the development of digital imaging sensors.

Digital imaging has completely changed photogrammetry. Almost any imaging device, from a multi-mega-pixel camera through to a pinhole spy camera can be calibrated so that it can be used for measurement and modeling. The “metric” parameters that were originally engineered into the cameras are now solved, and compensated for, by software.

Photographs no longer need to be taken from predetermined locations, and much of the software available is optimized for convergent photography where the axes of the cameras point toward the object. There are expensive software systems that have application in professional aerial mapping and the production of orthophotographs—photographs free from any distortion—whereas other, inexpensive systems are versatile enough to enable non-skilled users to create 3D models of objects. Some of these systems do not even advertise as photogrammetric software packages, they are marketed as “photographic modeling” systems. The images these packages use can come from image sensors as diverse as consumer digital cameras or earth-orbiting satellites.

The major shift in the analytical approach to photogrammetry came about by considering each image as representing a bundle of rays from the center of the lens system through points on the image to the object photographed. These individual rays could be corrected for distortion in the lens system (for example, consider the barrel distortion common with wide-angle lenses), the focal length of the lens and the geometry of the film, plate, or digital chip. With several images from different stand-points, the bundles of rays would then intersect in space where they would have struck the real object.

There are a variety of sources of additional information and examples of the ever-increasing fields of application. The International Society for Photogrammetry and Remote Sensing (ISPRS) is the international body that organizes conferences and symposia on photogrammetry, and the conference proceedings give the most up-to-date theory and application of measuring from photographs. Examples include applications in medicine, where limbs, bodies, faces, and even viruses are measured and modeled from an image source; architecture and archaeology, where the images are used to derive plans, elevations, and the framework for computer visualization; mechanical, civil, and aerospace engineering, where relative accuracies of 1:200,000 of the maximum dimension can be obtained; and still topographic mapping. Most countries have their own similar organizations, and they are generally members of ISPRS. Another organization that promotes and publishes photogrammetric applications is CIPA Heritage Documentation, a scientific committee of the International Council on Monuments and Sites (ICOMOS), which serves as a liaison body between ICOMOS and the ISPRS. Most of these examples are oriented toward cultural heritage applications, and many examples are available from the Web sites.

Photogrammetry has become a very versatile measurement tool, and developments in the process are continuing. Automated extraction of features from images (for example, the derivation of building roofs and profiles from aerial imagery), image “understanding” where pixels are interpreted to be objects (for example, facial recognition), and the combination of images with other data sources are all current areas of development. One recent innovation has been the production of 3D laser scanners, and although the operation of some of these use the time-of-flight of a laser beam, others use photogrammetric triangulation as the method for measuring the surface relief of objects. Some close-range laser scanners project a laser stripe onto the object, which is imaged by a camera at a known distance and orientation to the laser. Photogrammetric principles and procedures also have application in satellite and radar remote sensing, providing calibration, coordinates, and a measurement capability for satellite-borne imagery.

Measurement from images now forms the basis of the information systems that manage the built and natural environments, systems that oversee manufacturing and processing quality control, forensic analysis, the documentation of cultural heritage, medical diagnosis and treatment, and even exploration of the solar system. It has become a mature, non-contact, and reliable method of measurement and monitoring.

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