Characteristics of New Generation of Digital Aerial Cameras
GENERAL COMPARISONS OF PUSH-BROOM AND AREA ARRAY CAMERAS
While film aerial cameras for many years have been based on consistent square formats, focal lengths and angles of field, there is little consistency in these parameters of the digital cameras. Wide angles of field of film cameras were used on the basis on their economy, since fewer images needed to be handled and observed during the mapping process. The same situation does not apply to digital cameras since most processing is automatic. From published case studies, there does not appear to be a significant increase in the cost of digital aerial photography, even though the angles of coverage of digital cameras are smaller. A general comparison of the two camera configurations can be summarised as follows:
Push-broom Scanners
- The geometry of the complete image is not a perspective projection. Hence, special software is required to process the images.
- The accuracy of the geometry of the image is dependent on the quality of the GPS/IMU positioning system
- Linear arrays are less subject to loss of pixels than area arrays, but this advantage is offset by fact that if bad pixels do occur, only the neighbouring pixels can be used to interpolate lost data
- While linear arrays are claimed to have larger dynamic range and data is less subject to blooming, it still occurs
- Linear arrays in principle are more suited to smaller scale imaging than frame cameras because of the implications of motion of the aircraft. However, manufacturers of linear array systems have recently demonstrated GSD of 5cm
- Most linear array systems enable the acquisition of only 3 images per point along-track, but multiple imaging is possible across-track
Area Array Images
- The images have the same perspective geometry as normal aerial images and hence standard digital photogrammetric software can be used
- No GPS/IMU system is required for the determination of exterior orientation
- Forward motion compensation is possible and hence in principle they are more suited to large scale mapping
- While area arrays may be subject to a larger number of bad pixels than linear arrays, there are many more neighbouring pixels from which to interpolate new pixel values for the erroneous data
- The formats of area arrays are generally smaller than the format of film cameras and hence the B/H (base/height) ratio is smaller: this necessitates the acquisition of multiple redundant images with overlaps of 80% to 90% in the flight direction to achieve B/H ratios similar to those of film cameras; however, the improved image quality will partly or fully offset the effects of the smaller B/H
- Array imaging enables aerial triangulation of multiple redundant frame images leading to high geometric accuracies and high reliability of elevation determination
- If a high quality GPS/IMU system is installed for direct orientation, aerial triangulation may be avoided, and hence it is possible to process the images for elevations and orthophotos immediately on downloading the data
OVERALL ADVANTAGES OF DIGITAL IMAGES
Summarising the advantages of digital images:
- The elimination of degrading effects of film and the improved dynamic range will result in superior quality images, and enable imaging under poorer illumination conditions than required for film; hence more data acquisition will be possible per day and throughout the year, especially in higher latitudes
- The high level of redundancy will lead to the ‘paradigm shift’ in photogrammetry, resulting in a new approach to photogrammetric production
- Achievable geometric accuracies are as good as, or better than for film aerial photogrammetry, even with the reduced B/H
- Processing of images can be done as soon as the aircraft lands and data storage devices are removed, leading to more rapid throughput
- The high level of redundancy will enable near ‘real’ orthophotos, based on the areas on each image around the principal point where relief displacement is a minimum
- Automation of thematic and spatial information extraction will be more robust
- New approaches to project management will result with no additional or reduced operational costs
- New markets should be available for high resolution orthophotos and vector mapping
- New opportunities in remote sensing with high resolution multi-spectral images
OPERATIONAL ASPECTS
Many examples are emerging on various web sites, of applications of the use of these cameras in practice, some of which have been given in the reference list. While the majority of examples are for marketing purposes, and hence do not reflect negative aspects of their operations, or details of their return on investment, their achievements appear to be very significant.
Digital aerial cameras are designed primarily for large scale mapping with pixel sizes as small as 5cm, but they should are also suitable for medium to smaller scales. For aircraft with a ceiling of about 8000m, the maximum GSD will be less than 1m for most cameras, which is of the same magnitude as the GSD for the high resolution satellites. Future digital imaging solutions may therefore involve a combination of digital aerial cameras for large scales and satellite images for the smaller scales.
Yotsumata et al (2004) state that the ADS40 has been shown to have the potential for 1:2,500 scale mapping in Japan and in future it will be applied to 1:1,000 mapping. Map production is said to be faster and more economical than with traditional photogrammetry. In the USA, ADS40 imagery was acquired for orthophoto production over approximately 380,000 square miles (approximately 1 million square km) of land in Texas, Idaho and Louisiana USA, (10 Terrabytes of data), for the USDA Farm Service Agency (FSA) over a 3 month period in 2004.
Hagman (2005) states that Aerodata International Surveys (Belgium) experienced no significant difficulties in introducing the UltraCamD into their operations. Time was required to adapt to new system in flight planning, but it was not significant. Aerodata (Belgium) states that ‘during only three days of perfect weather conditions more than 5000 images were taken and just a few days later the photos passed through quality assurance to enable further processing without any delays’. Quoted accuracies of processing this data have been as high as 2-3 µm on the image, while matching of digital images with larger overlaps for elevation determination was more reliable than for standard aerial photography with 60% overlap.
Kokusai Kogyo (KKC) in Japan has claimed that operations for mapping urban areas with very narrow streets with a Z/I Imaging DMS cost less than half that of film-based camera missions. KKC collected more than 12,000 images in 40 projects in a period of 6 months. They state that the digital camera had the potential to generate inexpensive true orthophotos, based on 80% overlaps along and across the strips. The increased overlap allowed for the extraction of accurate DEMs for ground features as well as vertical structures.