Photogrammetry is the practice of converting photographs of objects or terrain into precise data which can be used to describe the objects or terrain in two or three dimensions. This data can be used in a number of ways, such as making topographical and other maps, creating three dimensional models or diagrams, and plotting the dimensions of geographical features. Such photographs are often, but not always, taken from the air. Close range photogrammetry is done using photographs taken at closer range, often less than 1,000 feet (about 300 meters) and is used for detailed three dimensional renderings and plotting of small-scale features and objects.
The practice of photogrammetry has been in existence since the invention of photography and was first developed in the mid-19th century. Early photogrammetric studies of terrain and objects were bound by the restrictions on camera placement due to the limits of technology. High-altitude aerial photography of large-scale terrain features was impossible until the invention of advanced aircraft near the turn of the 20th century. In 2011, most close range photogrammetry is done from the ground, as many countries prohibit civilian aircraft from flying lower than 1,000 feet (about 300 meters). For this reason, it is often called ground-based photogrammetry.
Ground based or close range photogrammetry is used when actual physical measurements of an object or terrain feature are not possible or when direct contact is undesirable, such as in the case of delicate, small scale geographic features or archaeological sites. In some cases, close range photogrammetry is useful when conditions are hostile or dangerous as well, such as near volcanic features or under exposure to radiation. Other possible applications include features or structures that are physically difficult to measure accurately, such as cell-phone towers or rock outcroppings.
The conversion of photographs to precise measurements and dimensions for objects and features requires fairly simple calculations, if any precise dimension of the feature or object in the photographs is known. It can, however, involve more complex formulas and algorithms as well. In some cases, photographs taken from different angles can help in refining the derived dimensions and measurements further. Close range photogrammetry can sometimes provide data that is unavailable or much more exact than that which is possible through aerial photogrammetry, due to the perspective and the proximity to the subject.
Software programs are now available that can perform a number of photogrammetric functions automatically. Basic digital imaging allows for quick conversion into three-dimensional models and scaled representation of geographic features or archaeological sites. These tools have greatly advanced the usefulness of photogrammetry in recent decades. One fairly recent application of close range photogrammetry is in analyzing photos of vehicles damaged in collisions. Photogrammetric techniques allow experts to determine the degree of damage and to use this data to determine the relative direction of travel as well as the speed of the vehicles involved, information that can be useful when the photographs may be the only record of the damage.