The practice of converting photographs of objects or terrain into precise data that can be used to describe the objects or terrain in two or three dimensions is known as photogrammetry. This information can be used to create topographical and other maps, three-dimensional models or diagrams, and plot the dimensions of geographical features, among other things. Photographs like these are frequently, but not always, taken from the air. Close range photogrammetry is used to create detailed three-dimensional renderings and plotting of small-scale features and objects using photographs taken at a closer range, usually less than 1,000 feet (about 300 meters).
Photogrammetry is a technique that has existed since the invention of photography and was first developed in the mid-nineteenth century. Due to technological limitations, early photogrammetric studies of terrain and objects were limited in camera placement. Until the invention of advanced aircraft near the turn of the twentieth century, high-altitude aerial photography of large-scale terrain features was impossible. Because many countries prohibit civilian aircraft from flying below 1,000 feet, most close range photogrammetry is done from the ground in 2011. (about 300 meters). As a result, it is frequently referred to as ground-based photogrammetry. x000D_
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, ground-based or close-range photogrammetry is used. Close range photogrammetry can be useful in some situations where the conditions are hostile or dangerous, such as near volcanic features or when exposed to radiation. Other features or structures that are physically difficult to measure accurately, such as cell phone towers or rock outcroppings, could be used.
If any precise dimension of the feature or object in the photographs is known, converting photographs to precise measurements and dimensions for objects and features requires relatively simple calculations. It can, however, also include more complicated formulas and algorithms. Photographs taken from various angles can sometimes aid in further refining the derived dimensions and measurements. Due to the perspective and proximity to the subject, close range photogrammetry can sometimes provide data that is unavailable or much more precise than that which is possible with aerial photogrammetry.
There are now software programs that can perform a variety of photogrammetric functions automatically. Basic digital imaging allows for quick conversion of geographic features or archaeological sites into three-dimensional models and scaled representations. In recent decades, these tools have greatly increased the utility of photogrammetry. Close range photogrammetry has recently been used to analyze photos of vehicles that have been damaged in collisions. Experts can use photogrammetric techniques to determine the extent of damage and use this information to determine the relative direction of travel as well as the speed of the vehicles involved, which is useful when photographs are the only record of the damage.