Analysis of the ground sample distance on large photogrammetric surveys
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- Felipe-García, B., Hernández-López, D. & Lerma, J.L. Appl Geomat (2012) 4: 231. doi:10.1007/s12518-012-0084-2
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One of the most important parameters when formalising the specifications for a photogrammetric project is the ground sample distance (GSD). While there are a variety of methods for the computation of the mean GSD for an aerial image, the aim of this research paper is to propose a method for the computation of the GSD that balances both precision and speed of computation. The rigorous computation of the mean GSD on an image requires the evaluation of the ground projection for each pixel over the digital terrain model. Three large photogrammetric projects within the Spanish National Plan for Aerial Orthophotography (PNOA) are evaluated to determine the degree of fulfilment of the required GSDs. The statistical distribution of the GSD is analysed to confirm its (lack of) normality. The rigorous approach to compute the mean GSD was presented, tested and compared with ten approximate solutions based on regular patterns of GSDs. The results achieved with the mean GSD value based on two object points with maximum and minimum ground elevations per image are also presented as well as the base/height ratios achieved on the three large PNOA projects. It is empirically demonstrated that the computation of the mean GSD based on a pattern of 3 × 3 GSDs is recommended. An error better than 1 cm is achieved for 99.8 % of the images, independent of the topography of the mission.