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Sight from a flight
GIS Development Staff
An aerial photograph presents a "bird's-eye view" of the features on the ground as seen from the air. They are different from ordinary photographs as they present an aerial view of the objects i.e. objects are portrayed from an overhead position. These photographs are also recorded in infra-red wavelengths of light. Although both maps and aerial photos present a "bird's-eye" view of the earth, aerial photographs are NOT maps. Maps are orthogonal representations of the earth's surface, meaning that they are directionally and geometrically accurate (at least within the limitations imposed by projecting a 3-dimensional object onto 2 dimensions). Aerial photos, on the other hand, display a high degree of radial distortion. That is, the topography is distorted, and until corrections are made for the distortion, measurements made from a photograph are not accurate. Nevertheless, aerial photography is a powerful tool for studying the earth's environment.
The history of aerial photography dates back to 1858, when Nadar took photographs of the ground from a rising balloon. During World War I, aerial photography became recognised as an operational military reconnaissance tool and airplanes photographed enemy territories and movements. After the war, the potential use of this technology for civilian purposes was acknowledged and it began to be used for making maps, retrieving information regarding the topography, landform, vegetation and cultural features present on the surface of the earth. Since then, countries have been photographing their areas on a regular basis, both for military and civilian purposes. With the advance in technology, better instruments began to be used for obtaining good quality photographic data. Soon remote sensing satellites came into picture, which could also provide good quality data in a repetitive cycle and the data was available at much cheaper price than their counterparts. Because of this technology, the importance of aerial photography began to be questioned. The scenario is fast changing now. As most of the world has been mapped at scales of 1:50,000 and some even less, the demand for higher resolution photographs is increasing day by day for mapping areas at much smaller scales. This increase corresponds to the rapid increase in industrial and urban development. In this respect aerial photography is proving to be a boon as it can provide us with very high-resolution photographs with ease. Some aspects of aerial photography are covered below.
Catching a view
In aerial photography the photographs are taken in "runs" in the direction of flight in such a way that there is approximately 60% overlap between adjacent photographs. The plane is flown in such a way that there is 30% sidelap between adjacent runs. Aerial photographs are classified on the basis of orientation of the camera axis: Vertical Aerial Photography and Oblique Aerial Photography; on the basis of the lens system: Single lens photography, Trimetrogon (Three lens photography), Continuous strip photography and on the basis of films, filters and equipment: Black and white (panchromatic) photography, colour photography, colour infra-red photography etc. Vertical Aerial Photographs are taken with the camera axis pointing vertically down or nearly so at the time of exposure, whereas Oblique Aerial Photographs are taken with the camera axis inclined with respect to the vertical. In panchromatic photography, the film is exposed through minus blue or yellow colour permitting the recording of wavelengths from only parts of the visible range of the spectrum. This type of photography is most suited to general photo interpretation. Infra-red photography records wavelengths of only red and infra-red part of the visible spectrum and is best suited for forest studies and for water vegetation discrimination. Similarly, colour infra-red photography records spectral colours and infra-red in combination resulting in false colours. This type of photography is helpful in vegetation studies related with discrimination of plants and crops etc.
Geometry of Photography
Photogrammetry is a science in which the aerial photographs serve the basis for the making reliable measurements for preparation of maps, digital elevation models etc. A few features of photogrammetric operations are listed below:
- Helps in determination of horizontal ground distances and angles made from measurements on a photograph.
- Relief displacements and measurement of image parallax help in determination of object heights.
- Helps in preparation of maps in stereoplotters.
- Based on the above, preparation of digital
elevation models and orthophotographs.
Mapping the photograph
Photogrammetric procedures are utilised for the preparation of maps. In the aerial photographs certain ground control points are identified. The precise geographic location of such points is then found out. In ideal case there should be at least three such marked points on ground in each aerial photograph. With the help of such three points, the locations of all the other points are determined. The points identified on the photograph are transferred to a base map of suitable scale and projection. The plotted points on the photograph are pin pricked and positioned in a plotting machine and adjusted for scale, tilt and other distortions until the points on the photograph and the base map coincide. Thus, an accurate model of the terrain is recreated. This whole procedure is carried out on a stereoplotting machine. The created model can be viewed stereoscopically and used to create a planimetric map having no relief distortions. Topographic contours can also be plotted on the map as the height of the features on the model can be easily determined and transferred onto the map.
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