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In general, digital photogrammetry is the art of using computers to obtain the measurements of objects in a photograph. It typically involves analyzing one or more existing photographs or videos with photogrammetric software to determine spatial relationships. Although commonly used to create topographical maps, it may also be useful in a variety of industries such as architecture, manufacturing, police investigation, and even plastic surgery.
There are several important variables involved in digital photogrammetry. First, it may be useful for a photogrammetrist to know information about the camera, such as what type of camera took the photograph, the focal length of the lens, or the distance of the camera to the object of interest. A special type of camera, called a metric camera, is often used to take these photographs because it is calibrated to take precision photographs with little lens distortion.
A second variable to consider in digital photogrammetry is the desired dimensions of the measurements. Some projects require only two dimensional (2D) measurements, such as the height of a building or the width of a river. These measurements can be made from a single photograph. If, however, the measurements need to be three dimensional (3D), the process will then typically involve creating a 3D model from two or more photographs.
This process of creating a 3D model is usually called digital stereophotogrammetry. Stereophotogrammetrists usually analyze two or more photographs of the same object, taken from different angles. They typically use the space where the photographs overlap or common points of reference shared by the photographs to create a digital 3D model. The images may then be mapped, pixels by pixel within the model to create measurable space in three dimensions.
A third variable is the type of photographs used. Photogrammetrists usually use either still digital photographs or video. Film photographs and video tape typically must be scanned into a computer in order to be used in digital photogrammetry.
The two main categories of photographs are aerial and close-range. Aerial digital photogrammetry, often used in topographical mapping, begins with digital photographs or video taken from a camera mounted on the bottom of an airplane. The plane often flies over the area in a meandering flight path so it can take overlapping photographs or video of the entire area to get complete coverage.
Close-range, or terrestrial, digital photogrammetry often uses photographs taken from close proximity by hand held cameras or those mounted to a tripod. Close-range photographs can be used to create 3D models, but they are not usually used in topographical mapping. This type of photogrammetry is useful for the 3D modeling of many objects or areas such as buildings, automobile accident scenes, or movie sets.