Ground Control

For all applications that involve measuring and mapping, ground control is necessary for georeferencing and geometric correction of the images (see Chapter 3). Ground control points (GCPs) are features that appear on the photographs and whose locations in a reference system are known (Warner et al., 1996; Chandler, 1999; Wolf and Dewitt, 2000). Given the typically large scales of SFAP images and small sizes of the covered areas, GCPs are usually small but well-defined natural features or pre-marked artificial features whose coordinates have to be determined in the field using total stations or global positioning system (GPS).

The geometric accuracy of a map derived from the photographs would depend strongly on the accuracy and precision with which GCPs were measured in the reference system and can be identified in the image. Therefore, care has to be taken that the quality of ground control matches the purpose of the survey and resolution of the images. Establishing ground control can be a time-consuming and costly part of the survey and must not be underestimated.

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