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In every part of the world the rate of map revision is alarmingly low when compared to the rate of change of many human influenced surface features. There is therefore a requirement to regularly gather up-to-date information about surface features and to incorporate changes in maps both quickly and effectively before it becomes history. When an aerial photograph is taken, it contains errors one category of which is caused by the aircraft taking the photograph being tilted. This aerial photograph by itself is of little commercial value. Accurate maps cannot be created from it, its use in GIS (Geographical Information System) is limited and correct metric information about ground features cannot be extracted. It requires orientation in order to attain accurate real world information about the location that it represents. This orientation is a time consuming process that does not yield a definitive product. The traditional way to rectify an aerial photograph involves expensive fieldwork in the form of establishing suitable ground control points, which slows the production process down greatly. There now exists a substitute to the information gathered by such field surveys, and it is contained in products created from aerial photography- Orthophotos and Digital Terrain Models (DTMs) where they exist. Other methods of correcting such errors include the use of GPS/INS systems on board aircraft. However, whereas GPS is used on almost every flight taking aerial photography,a combined GPS/INS system is not, as the INS part is still very expensive. Through the use of DTMs and Orthophotos this research shows that the orientation of aerial photographs is possible and can obtain reasonable results compared to GPS coordinates for a set of checkpoints within the project area. X, Y and Z values of control points were extracted using DTMs and Orthophotos, which traditionally would have to be determined by surveyors in the field or by aerial triangulation which also requires ground control. The position, size and local accuracy of different sized Orthophotos and DTMs were examined in order to find which combination produced the most accurate results. Conclusions and future recommendations could then be reached.
O'Cionnaith, F.: The Exterior Orientation of Aerial Imagery using Existing Orthoimagery and Terrain Models – a Performance Test. Masters Thesis. Dublin Institute of Technology, 2004.