Photogrammetry

Photogrammetry is a technical field that combines elements of art and science. Its practitioners blend photographic images with data recorded and measured from electromagnetic energy and other sources. The results offer information about objects and environmental features that could not be obtained from a single source.

Photogrammetry often involves aerial or satellite photographs and provides detailed images used for surveying, mapping, or creating electronic models. Photogrammetry is especially useful for creating images of large areas and for analyzing areas that would be dangerous for direct study by human surveyors.

Background

Photographic technology did not exist until the early part of the nineteenth century, but the concepts involved in photogrammetry go back centuries. The ancient Greeks and Egyptians created art and building plans that made use of the ideas of perspective, or a viewpoint that includes accurate representations of height, width, depth, and relative closeness of objects, as well as projective geometry, or the study of the properties of objects as they appear.

Projective geometry differs from basic geometry. For example, basic geometry views a square as a flat object with four equal sides and corners formed by ninety-degree angles. Projective geometry examines the properties of the square from the perspective of a particular point, such as one corner. From this point of view, the square appears distorted, with some sides appearing longer than others and the corners not appearing as right angles.

The great artist and inventor Leonardo da Vinci (1452-1519) described the concept of photogrammetry in 1480 without naming it as such. A number of da Vinci's contemporaries in both the artistic and scientific worlds continued to research the visual and mathematical components of what would later become known as photogrammetry. By 1759, Swiss mathematician Johan Lambert (1728-1777) had determined a way to calculate the specific point from which an image was created by working backward from the image's perspective.

The invention of photography in the early 1800s provided new ways to apply the idea of object perspective. In 1840, Dominique Francois Jean Arago (1786-1853), a French geodesist (a scientist who measures and monitors the planet to determine the exact location of specific points) first practiced photogrammetry using early photographs known as daguerreotypes. In 1883, Guido Hauck (1845-1905) and his associates established the relationship between photogrammetry and projective geometry. Prussian architect Albrecht Meydenbauer (1834-1921) coined the term "photogrammetry" in 1867.

Results and Uses

The photogrammetry process produces three results: photographic products, computational results, and maps. A photographic product is a photograph that has been created by enhancing and creating a composite of one or more original photographs. The original image is printed and made into a diapositive, or image reproduced on glass or a clear film, such as a slide. The photo may also be rectified, or converted to a horizontal image, which makes an aerial view look as if it were taken from ground level. When the rectified image is further enhanced to remove distortion resulting from camera orientation and adjusted to accurately reflect the distances between objects within it, it is called an orthoimage. Orthoimages can be further manipulated to allow viewing from various angles. Because they are digital, they can easily be shared and used in different software applications.

Photogrammetry can also be used to calculate, plot, and compare two or more points, such as areas of terrain covered by a highway construction project. The computational results gained through photogrammetry allow planners to determine elevations and distances. For example, planners can project the amount of dirt and rock to remove, the ending length of a bridge or road, and other details needed to accurately estimate projections.

The most common use of photogrammetry is the creation of maps. By combining and manipulating photos, the photogrammeter can create a topographical map that shows geographical features from different angles, digital terrain maps that depict elevations and other details, or special-use maps that focus on one specific aspect of an area, such as a waterway or roadway.

Tools and Process

Early photogrammeters often relied on simple cameras, hot air balloons, and manual calculation methods to generate their results. By the time the U.S. Geological Survey began using photogrammetry for topographical mapping in 1904, special survey cameras were available to capture panoramic views.

The development of air flight improved the ability to take aerial photos. Additionally, photogrammeters developed special tools for their trade, such as the stereocomparator devised by Carl Pulfrich (1858-1929) to compare two points. Frederick Vivian Thompson (1880-1917) created a stereoplotter that could plot contours from a photo taken on land.

Initially, photogrammeters followed a simple process. For example, they might take aerial photos from a hot air balloon and then perform calculations manually. By the 1960s, however, photogrammeters began applying more advanced technologies and analytical techniques to their study.

Contemporary photogrammeters have access to photographs from planes and satellites, and the advantage of digital photography, which improves resolution and allows for easier image manipulation and sharing. Specialists have access to computer assisted drafting (CAD) technology and digital terrain modeling (DTM) to create three-dimensional images. They can use software to digitally rectify images and produce orthoimages from multiple photos. They can use light detecting and ranging (LIDAR) equipment to scan the ground to detect and produce features not easily visible by the eye, such as depressions in large grassy areas.

Photogrammeters can employ remote sensing to capture information based on the color bands generated by satellite imagery to help detect and define details of a terrain by color, brightness, or other characteristics. They can create realistic three-dimensional representations of objects or terrain, even when it is difficult or dangerous for human observers. Photogrammetry can be used to study or to help plan expeditions to far-off parts of Earth, such as Antarctica, and to other celestial bodies in the solar system.

Bibliography

Duraiswami, Ramani. "Projective Geometry." University of Maryland Institute of Advanced Computer Studies, University of Maryland. Web. 3 Mar 2016. www.umiacs.umd.edu/~ramani/cmsc828d/ProjectiveGeometry.pdf. Accessed 22 Nov. 2024.

Pillai, Anil Narendran. "A Brief Introduction to Photogrammetry and Remote Sensing." GIS Lounge. 12 July 2015. Web. 3 Mar 2016. www.gislounge.com/a-brief-introduction-to-photogrammetry-and-remote-sensing/. Accessed 22 Nov. 2024.

"What Is Photogrammetry?" Engineering.com, 30 May 2024, www.engineering.com/what-is-photogrammetry/. Accessed 22 Nov. 2024.