![]() This is an embedded Microsoft Office presentation, powered by Office Online.įigure 2: Leveling in the Volcanoes National Park. A ground survey should produce point data. Whilst GPS technology now exists as an alternative to leveling, it has not replaced it in areas where high precision and accuracy is required. A ground survey allows the area of interest to be observed at close range, and the survey tailored to record the most relevant information, such as ridges, peaks and troughs. Ground survey is the oldest source of elevation data, traditionally undertaken by leveling and triangulation (see below). The use of control points of known height to calculate elevation across an aerial photograph is called ground control. All elevation measurements are then expressed relative to this standard datum. A standard datum in this context is a ‘baseline’ height, such as mean sea level. Heights can be referenced to a standard datum if the height of a control point in the overlap area is available. Within the area of overlap between the two frames, surface heights can be calculated relative to one another. Figure 1: Cartoon of photogrammetry to produce stereographic images.
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