Photo Control Survey
At the moment of exposure, each aerial photograph is slightly tilted in relation to the earth’s surface because the airborne platform to which the camera is attached (aircraft, helicopter) is not perfectly steady during flight. The result is that in order to obtain accurate measurements, the tilt of the photographs at the moment of exposure needs to be known. One of the processes commonly used is to survey on the ground points visible and identifiable on the photos. These points, (photo control) are then identified on the photos and the surveyed coordinates used to fit the images to their real world location. For each mapping project, a minimum amount of photo control points need to be established, the determinant factor being mapping area extent and nominal photo scale.
A registered land surveyor normally does this phase of the work with an expected survey accuracy meeting second order standards.
The advent of the GPS brought about the development of new techniques that have reduced substantially the number of photo control to be field surveyed. Airborne GPS (ABGPS) is one of the techniques by which the accurate coordinates of the camera position is space is recorded at the moment the image is exposed. This information can then be used to generate coordinates of suitable image points for mapping purposes.
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