3. DIGITAL STILL CAMERAS: STATUS AND APPLICATIONS
3.2 Advantages
The single characteristic of a digital frame camera that results in several advantages is the digital format of the imagery that it produces. The direct acquisition of digital image data means that processing costs are considerably reduced in relation to photography since film development is eliminated and no scanning is required (Light, 1996; Mills and Newton, 1996; Positive Systems, 1996; King et aI., 1997; Mason, 1997). By eliminating the scanning of photographs the distortions innate in such a process are reduced (Mills and Newton, 1996), and the accuracy potential of the imagery is improved (Maas and Kersten, 1997). Maas and Kersten (1997) found that in close-
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range applications with signalized targets, well-defined edges or good texture on flat surfaces, precision of the order of 1/50 of a pixel (0.2,um) in image space could be regularly achieved.
Eliminating fiJm development and scanning also substantially reduces turnaround time with weekly fly-overs being possible and information being available within two or three days of the flight (Oltman, 1996a). For immediate assessment of the impacts of catastrophic events, resource managers can reduce the turn-around time even further by making use of the digital camera's inherent electronic capability to transmit its digital image from the aircraft to the ground (Light, 1996). Alternatively, the images can be viewed in the cockpit ofthe aircraft (Light, 1996; Positive Systems, 1996; Mason et aI., 1997). This would serve a further purpose of allowing the evaluation of exposure and focus parameters, and would provide verification that the targets are being acquired. This would avoid costly return flights to a site.
Digital format imagery is also more suited to image processing for improvement of radiometric characteristics, such as noise reduction and reduction of brightness variations with view angle, geometrical warping and to quantitative image analysis (King et al., 1994). In particular, a higher signal-to-noise ratio is achievable under conditions oflow scene contrast and low light levels in comparison with film-based systems. This higher ratio is possible due to the ease of digital contrast enhancement (Light, 1996). Such image processing is also easier since having digital images allows easy integration with softcopy' image processing systems (Toth, 1997). Furthermore, by having a digitally based system, different sets and different kinds of data can be integrated, merged, intersected and compared, resulting in wider and more flexible operations within and beyond the field of mapping (Ackermann, 1996).
The higher resolution of Digital Frame Camera (DFC) imagery when compared with video and satellite is another advantage. Unlike standard videography, which can already provide an order of magnitude better resolution than satellite remote sensing, DFC's are not confined to National Television Standards Committee (NTSC) or other television scanning specifications (King et al. ,1994). As a result, sensors have been developed which contain many more photo sites per unit sensor dimension than video sensors (King et aI., 1994), thereby providing a vast improvement
Softcopy refers to the fact that the original hard copy or print of the photograph is not processed, but rather a digital representation or softcopy of the photograph is used
in resolution. DFC's, with their current resolutions, could therefore be considered as a substitute for photography where the needs do not demand the very high quality and resolution of photography. Further advantages of still cameras over analogue video are their greater fleXIbility since no on-board power supply and computer are necessarily required, and their higher mapping accuracy potential since the effects of framegrabbing analogue video images to produce digital imagery are avoided (Mason et aI., 1997).
Being able to control the camera via a computer is a further benefit, since it affords the opportunity to automate the in-flight image acquisition process. Computer-based control also facilitates the performing of some real-time processing of the imagery. This could include correcting for lens distortions and lens vignetting, reducing image noise through use of image filtering, and adjusting for non-uniform radiometric response of the CCD array. King (1995) provides a review of research into these aspects of image processing.
The digital nature of the data acquired also introduces the vast potential for automation of post- flight image processing. Heipke (1995), Ackermann (1996), Schenk (1996), and Toth and Krupnik (1996) all identify the potential of automating photograrnmetric procedures through the use of digital imagery, thereby providing quick response times. The prime examples identified by Ackermann (1996) were aerial triangulation, generation of digital elevation models and production of digital orthophotos. The use of digital still camera imagery for some of these applications will be reviewed later in this chapter.
In terms of the other characteristics of the imagery, King et al. (1997) states that digital cameras produce frame exposures with geometric characteristics as good or better than film and much better than line scanner images. Airborne line scanner imagery typically suffers from line-to-line geometric variability (Monday et aI., 1994; King et aI., 1994), particularly under turbulent flying conditions where roll, pitch and yaw are unstable. This results in severe image reconstruction problems (Light, 1996) and makes photogrammetric analysis and geometric transformations more difficult. Radiometric performance of CCD sensors is also often significantly better when compared to scanned film (King et al., 1994; Maas and Kersten, 1997). The spectral fidelity of the imagery is also excellent, and there are no problems associated with film emulsion batches, temperature and humidity (Positive Systems, 1996).
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There are, however, some disadvantages relating to the use of DFC's which currently F 'r""l"
them from becoming the sensor of choice for all aerial imaging applications, These disad":3'" 8 7,.
are described in the following section.