What is Photogrammetry? Remote sensing is the process of determining information about an object or situation from afar; that is, without being in direct physical contact with it. One of the earliest forms of remote sensing is the science of photogrammetry, which combines three Greek word roots that mean “light” (photo), “drawing” (gram) and “measurement” (metry). In plain terms, photogrammetry is the ability to determine measurements through the use of photography. Two Types of Photogrammetry The two basic classifications of photogrammetry are “aerial” and “close-range.” As its name clearly states, aerial photogrammetry is achieved by placing a camera in an airplane and taking pictures of the ground, generally in a straight-down manner. The camera’s shutter activates every few seconds as the aircraft proceeds in a straight line. In the early days of this process, film cameras were used. Today, the digital camera is much preferred, mainly because little additional processing is required to manipulate the collected images into something useful. Whether film or digital, each frame captured by the camera overlaps the next frame. These adjacent images are then “stitched together,” matching them edge-to-edge in order to come up with a complete picture of the target area. Close-range photogrammetry involves either a hand-held camera or one mounted on a tripod. As with its aerial cousin, individual images overlap and are stitched together to create a three-dimensional picture of the object in question. While aerial photogrammetry is used primarily for terrain mapping, close-range photogrammetry is used to capture images of building facades, vehicles, or any other physical representation that can be seen from ground level. 3D Imagery One valuable application for photogrammetry is in the creation of three-dimensional models. This process, sometimes called stereo-photogrammetry, involves the combining of two photographs taken of the same object or land feature from slightly different angles. The old stereopticon, a popular device back in the late 1800s, used a similar process to give people the feeling they were seeing something in 3D. If taken from the air, data captured and processed through photogrammetric means can help produce a Digital Elevation Model (DEM), which provides the viewer with 3D photographic evidence of the ground and the objects on it—trees, buildings, natural formations, and so on. Photogrammetry can be combined with other remote sensing methods, such as LiDAR (Light Detection and Ranging) to create amazingly lifelike images from the air. These digitized data prove useful to engineers, archaeologists, geologists, mining experts, city planners, and anyone else needing a highly precise visual map of an area. The Surveyor and Photogrammetry Photogrammetry is useful to a surveyor for several reasons. First, it allows the collection of visual data over a much larger area than one can easily survey in a reasonable amount of time. Second, the photographs offer a 3D representation of items and landmarks that may have changed over time, and possibly no longer exist as they once did. Surveyors are oftentimes called upon to show that a specific object was in a particular location in respect to another object. While a simple photograph may suffice, the ability to provide this sort of evidence in 3D imagery is incredibly powerful. Also, photogrammetry and its representation in digital space allows for very precise measurement. Far more than a basic photograph, a photogrammetric image is an x–y–z model that has the added benefit of being fixed to a particular time and place. The digital processing of such imagery adds in GPS measurements to ensure the highest possible accuracy.
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What is Photogrammetry?
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