GCP Coordinate Measurement

Given the large image scales for the SFAP emphasized in this book, coordinates for the GCPs usually have to be determined by terrestrial survey with a total station (Chandler, 1999), providing a measurement precision of ±10 mm. Only expensive differential GPS offers similar precisions which can correspond to the high pixel resolution of SFAP imagery. Unlike GPS survey, however, terrestrial survey with a total station for measurement within a particular reference system requires pre-existing trigonometrical points with known coordinates—a feature that is bound to be absent at many SFAP sites. In this case, an arbitrary local coordinate system has to be used, which can be transposed later by offsetting and rotating the local system in order to adjust it to the desired reference system (see Chapter 11.2). However, some sort of link to the latter is still required, e.g., two points in the local system that also have been measured by GPS and which are visible in other, georeferenced, remote-sensing images like those provided by Google Earth.

If a global or national reference system is not required, using a self-defined local system throughout also can have advantages. This approach enables the surveyor to align the axes of the system to match with the site of interest, e.g., line up the y-axis with the direction of a river bed or oblong test plot. Apart from aesthetic aspects—largely empty maps with diagonal stretches of stitched images just do not look as pleasing as a perfect-fit layout—an optimal alignment of study area and coordinate system also saves much file size in raster data.

When setting up the total station for measurement in the field, it is advisable to choose a spot for the tripod that has a clear view of all GCPs. This is best checked, if in doubt, with the help of the person holding the reflector staff. If the GCP distribution does not permit a single measurement station and a local coordinate system is used, at least two points measured during the first round need to be measured again in the second; this would enable combining the two measurement sets into the same coordinate system by a mathematical transposition.

The authors can report overwhelmingly positive experiences with most local populations during SFAP activities in various countries around the world. The ascent of blimps and kites is a spectacle that often attracts an audience (Fig. 9-13), and most people are interested and supportive

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