Characteristics of New Generation of Digital Aerial Cameras

Characteristics of New Generation of Digital Aerial Cameras

John Trinder
School of Surveying and SIS
University of NSW
UNSW SYDNEY 2052 NSW
AUSTRALIA
Email: j.trinder@unsw.edu.au

ABSTRACT
There has been considerable debate recently on the future of film and digital images in photogrammetry. While film images will still be acquired by some organization in the near future, the issue is more likely when, rather than if will the digital technology lead to the demise of the small but currently essential aerial film industry. Given the apparent advantages of digital over film aerial images and the expected reduction in cost of digital cameras in the future, there is a growing acceptance of digital camera technology for aerial photography. This paper will describe some of the characteristics of the new generation of cameras, their advantages and some operational aspects.

THE DEVELOPMENT OF DIGITAL CAMERAS
The development of digital aerial cameras has advanced significantly over the past 4-5 years, and worldwide sales are reportedly growing rapidly with a number of companies entering the market. Modern aerial film cameras have reached a high level of development, with very high spatial resolution, geometric quality, wide angle of coverage, and overall efficiency for map production. Film image is also a very efficient medium for long-term storage of spatial information describing the terrain surface. In order for digital aerial cameras to compete with film aerial cameras they need to be able to acquire high resolution images with similar or better accuracies, have comparable angles of field, be suitable for mapping and orthophoto production, and take advantage of the particular characteristics of digital image acquisition. While the majority of these aims have been largely achieved, the solution to the issue of long-term storage of digital information is still open.

The two approaches to the design of digital aerial cameras are:

Systems based on linear array scanners (pushbroom). Linear array scanners acquire data by scanning the terrain with one or more linear arrays as the aircraft moves over the terrain, in much the same way as a broom sweeps a surface. They normally incorporate at least three sensors, one looking forward, one looking vertically and one backwards to acquire three separate overlapping images of the terrain that can be used for determining elevations. Multispectral images at the same or reduced resolution may also be acquired. An integrated GPS/IMU system is essential for this configuration for the determination of camera position and tilts, because the image acquisition is a continuous process and not frame based.
Systems based on area arrays. These systems may involve single area arrays (usually referred to as medium resolution) or multiple arrays whose images are stitched together to form a larger frame image (referred to as high resolution). The formats of the images may not be square, but their dimensions approach those of frame aerial film cameras. These frame images can be processed using standard digital photogrammetric software. GPS/IMU system will not be essential for its operation, but some components of such a system may be included as an option.

The general characteristics of most of these digital cameras are:

12 bit dynamic range of panchromatic images with ground pixel sizes as small as 5cm
Most acquire multi-spectral images, some with larger pixel sizes than for the panchromatic images, covering wavelengths from blue to NIR
High geometric accuracies
Data storage requirements total many TerraBytes both for the original data and backup

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.

CONCLUSIONS
The emergence of the new generation of digital aerial cameras is a consequence of the new technologies that have become available for digital imaging. A ‘paradigm shift’ in photogrammetric operations does appear to be occurring, due to the availability of multiple overlapping images which lead to high accuracy and more reliable information extraction with greater efficiency. As well, with the acquisition of multispectral images, the automatic extraction of thematic information from images required for digital mapping and GIS data collection should be more easily achievable. A next step in automation in photogrammetry may be the development of procedures for computing orthophotos in real-time during flight. This requires significant software developments and computing power, but there are certainly moves to achieve it.

While the aerial film photography is not yet finished, the future will no doubt see improvements in the quality and capabilities of digital aerial cameras and a reduction in costs. This will lead to their greater acceptance by users and a decline in the use of film images. The timing of these developments is hard to estimate, but it can be expected that the next decade will see a rapid transformation to digital aerial imaging, with much higher throughput and greater economies in the production of map products.

REFERENCES

Hagman F. (2005) ‘Vexcel UntraCam C in Operation’ GIM International, May 2005 pp 30-31.
Yotsumata T., Okagawa M., Fukuzawa Y., Tachibana K., and Sasagawa T. (2004) ‘Investigation for Mapping Accuracy of the Airborne Digital Sensor-ADS40’ Int. Arch. Photogram. & Rem. Sens. Vol 34 Pt 1, pp
http://www.aerodata-surveys.com/EN/sub_1/news.htm
http://www.aerometric.com ‘Professional Golfers’ Association (PGA) goes high tech with digital championship course’, AERO-METRIC Inc., Sheboygan, Wisconsin, USA
http://www.kkc.co.jp/english/ ‘Intergraph Technology enables KKC to Develop True Ortho Technique’, Kokusai Kogyo Co. Ltd.,