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what is photogrammetry ?
The term photogrammetry means the process of measuring images on a photograph. In a more comprehensive sense, photogrammetry includes photographing an object, measuring the image of the object on the processed photographs and reducing the measurements to some useful form such as a topographic map. Modern photogrammetry also employs systems which acquire data through other than conventional photographic systems, for example, radar imaging, radiant electromagnetic energy detection and x-ray imaging.
Photogrammetry embraces two broad categories of practice. The first is the classical category in which quantitative measurements are done and from which ground elevations and positions, areas, distances and volumes could be measured or from which plan metric and topographic maps can be drawn. This category is often referred to as metrical photogrammetry. The second category is photo interpretation which photographs are evaluated in a qualitative manner in order. Photo interpretation is used for detecting water pollution, classifying soils, evaluation timber stands, identifying crops and in military uses. Because much of the effort in this category employs non photographic as wall as photographic systems, it is referred to by the general name remote sensing. The major portion of remote sensing, however, includes the interpretation of conventional aerial photographs.
A further breakdown of metrical photogrammetry can be made to reflect the method of measuring and reducing the data obtained from photographs. Problems in photogrammetry can be solved mainly by pure mathematical modeling using simple but highly precise x and y coordinates of image points. The instrumentations for measuring these coordinates is conceptually very simple. This method of solution is called analytical photogrammetry ( sometimes called computational or numerical photogrammetry ).
Aerial photogrammetry is the branch of photogrammetry where in photograph of the terrain in a certain zone is taken in an order by a precision camera fixed in an aircraft flying over the zone. Terrestrial photogrammetry is the branch of photogrammetry where in the photographs are taken from fixed position on the ground or near it. The camera axis is horizontal or nearly so.