Ideal Flightline Calculation

Following these conclusions, it remains to be calculated which exposure interval would be necessary if a free-flying platform, specifically the autopiloted model airplane presented in Chapter 8.5.2, is used for continuous flightline coverage, and if this interval is feasible with the chosen camera. Taking into account that in windy conditions the airplane might not yield a perfectly aligned image series, a 70% forward overlap is considered as minimum.

FIGURE 9-16 Aerial photograph of Gully Belerda taken with a digital compact Nikon Coolpix S600 from an autopiloted model airplane at 400 m height. Photo by C. Claussen, M. Niesen, and JBR, September 2008.

Using Equation 3-5, the image base is calculated as:

B = DW x(1 — PE/100) = 46 m x(1 — 0.7) = 13.8 m

From this image base and the minimum possible ground speed of the airplane—25 km/h or 7 m/s—the required exposure interval (Eq. 3-6) can be determined as:

The results show that this survey design is critical for two reasons:

1. The exposure interval of ~ 2 s between image frames is quite short for a digital camera of this generation. The survey was conducted later with a compact camera (Nikon Coolpix S600) set to 3 s, the fastest possible interval.

2. The base-height ratio resulting from this survey design amounts to 0.22, which is at the lowest limit deemed useful by photogrammetrists with regard to height accuracy. Although the area of interest, the gully, has high relief variations and thus potentially large stereo parallaxes, this base-height ratio is still lower than desirable for the exact height measurements required for monitoring purposes.

The image base in this flightline calculation could be increased if the images were oriented along the gully

(resulting in B = DL x (1 — PE/100) = 20.7 m and thus a base-height ratio of 0.33). This also would result in a lower exposure interval (3 s). However, taking into account the width of the gully and area covered, this fit is too close for granting sufficient coverage of the gully.

The above example of a give-and-take compromise situation is quite typical for SFAP applications. The survey actually was conducted in September 2008 with the mentioned model airplane (but different camera for technical reasons) at various flying heights: the coverage area of the lowest flight corresponded to the yellow footprint in Figure 9-15, and the coverage for the highest flight produced images 530 m by 400 m in extent (Fig. 9-16). For the low flight, stereoscopic coverage could not be achieved along-flight with consecutive images, as foreseen, owing to the exposure interval, but for the high flight, dense stereoscopic overlaps (>85%) resulted.

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