Stereoscopic and Large Area Coverage

Stereoscopic coverage of a study area is necessary for photogrammetric analysis of terrain heights and orthophoto correction, but even if no such advanced analysis is intended, stereophotos may help enormously with photointerpretation (see Chapter 10). There are two possibilities for achieving stereoscopic coverage: in-flight simultaneous image acquisition with two cameras or along-flight consecutive image acquisition with a single camera. There are advantages and disadvantages associated with both stereoscopy methods.

• In-flight simultaneous image acquisition is achieved with two cameras mounted at a fixed distance, e.g. a twin-camera boom. This has the advantage that the stereo images always have the same scale and relative alignment. The amount of image overlap and stereoscopic parallax, however, differ with flying height, as the photo-grammetric base (see Chapter 3.3) is predetermined. In order to achieve accurate stereoscopic measurements, the base-height ratio should be at least 0.25 (Warner et al., 1996). For the stereo boom presented earlier (see Fig. 7-8), this would result in optimal flying heights below 4 m! Although much smaller base-height ratios are sufficient for 3D viewing and photointerpretation, the stereo-boom solution is not recommended if photo-grammetric measurements are intended.

Another difficulty is that few SFAP platforms are suitable for supporting a stereo-boom. For kites, the boom represents a considerable challenge as an entanglement with the kite line and Picavet lines become more likely, and stronger microservos would be required for pan and tilt of the cameras. Also, movement and vibration of the boom may impair image quality. Stable constructions require a considerably higher weight-bearing capacity. In case of a blimp, this capacity may be at hand but, nevertheless, the maneuverability is restricted as well by the boom, making take-off and landing operations more vulnerable as compared to flights with only one camera.

• Along-flight consecutive images can achieve better base-height ratios as their distance can be controlled by the operator during the flight. Depending on the platform, however, it may be difficult to ensure image pairs with the same flying height and scale and with regular alignment and spacing (see Fig. 3-10). In this case, more is definitely better: the more images are taken, the better is the choice for selecting those images that provide optimal stereo coverage for analysis.

Tips for navigating individual platforms for stereo-coverage are presented in Chapter 8. Similar considerations apply to non-stereoscopic coverage of larger areas with multiple images for creating seamless image mosaics.

Generally, covering an area with a regular pattern of overlapping images is easier with free-flying platforms, which can be navigated more easily along straight lines than can tethered platforms. The exposure calculations for stereo-survey flightlines in the following are useful for planning SFAP missions with free-flying aircraft with near-steady speed. An ideal flightline pattern, as in Figure 3-8, requires a sophisticated combination of flying height, focal length, image extent, aircraft velocity, and exposure interval.

The first difficulty, therefore, is where to start with the calculations—which of these variables are dependent and which are independent? With a given aircraft and camera, the most easily adaptable variable probably would be the flying height. But the flying height directly controls image extent and GSD, and their predefinition is normally of primary concern. This leaves the exposure interval as potentially the most independent variable, suggesting the following sequence of calculations:

• Calculate image scale S, image ground cover A, GSD, and flying height Hg as shown earlier (Eqs. 3-1 to 3-4).

• Decide for the orientation of the rectangular image format with respect to the flight direction.

• Calculate the length of base B for the desired forward overlap PE (Eq. 3-5).

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