A Multi-Sensor System for Airborne Image Capture and Georeferencing

The development and preliminary testing of a fully digital multi-sensor system for airborne remote sensing and geographic information system (GIS) applications is described. This system was developed at The University of Calgary in collaboration with The University of California at Berkeley, with aircraff and logistics support by HJW Inc., California. It integrates a medium class inertial navigation system (INS), two low-cost Global Positioning System (GPS) receivers, and a highresolution digital camera. During aerial image capture, camera exposure stations and INS digital records are time-tagged in real time by GPS. The derived trajectoly parameters describe the rigid body motion of the carrier aircraft. Thus, they are directly related to the parameters of exterior orientation. During post-processing, these parameters are extracted, eliminating the need for ground control for airborne image acquisition applications. Flight tests were performed over a part of the university campus at Berkeley, using a strip photography approach to test the integrated system performance. In this paper, the concept of direct georeferencing of digital images without ground control is presented. System calibration results are then discussed in some detail, and special attention is given to the geometrical analysis of the system imaging component. An improved imaging system is proposed and validated by computer simulations. The potential of the new system for photogrammetric use is then discussed. The major applications of such a system will be in photo ecometrics; the mapping of utility lines, roads, and pipelines; and the generation of digital elevation models for engineering applications. Introduction Over the past five years, the information demand for detailed forest composition and structure has increased markedly. Therefore, it became critically important to develop new remote sensing techniques that allow for the direct measurement of those parameters. Although satellite imagery is available (e.g., SPOT HRV, Landsat TM, etc.), low spatial resolution (typically, 10 to 30 m) is an obstacle to interpreting and extracting information with the required accuracy. Airborne imagery, on the other hand, resolves the low spatial resolution problem of the satellite imagery and yet provides sufficient spectral bands for remote sensing studies (Gong et al., 1998). The main objective of this research is the development of a multi-sensor system for digital image capture and georeferencing for applications in airborne remote sensing as well as large scale digital mapping and geographic information system (GIS) applications. The system is being developed as a joint research project between The University of Calgary and The University of California at Berkeley. Its major application will be in photo ecometrics of various vegetation species in forested areas where ground control is neither available nor needed, and where directly georeferenced digital imagery is acquired to solve the exterior orientation problem. In other words, system requirements are Fully digital data acquisition, and Direct georeferencing of the digitally acquired images without ground control points (GCPS). Because of the advances in digital image acquisition technology, it became feasible to capture aerial or close-range images in fully digital form, which allows immediate computer access to the imagery after the acquisition process. Compared to film-based photos, digitally acquired imagery is advantageous because no time is needed for film development and image scanning. Further, digital image processing and computer vision have been successfully utilized to facilitate automated procedures in digital aerial images such as interior orientation (Kersten and Haering, 1997), relative orientation (Schenk et al., 1991), and the generation of digital elevation models (Krzystek, 1991.) In addition, a number of sophisticated digital photogrammetric workstations are commercially available, e.g., LeicaIHelava (DeVenecia et al., 1996). Among the currently available image acquisition systems, still video digital frame cameras (DFCS), which started to emerge in the mid nineties, are the most convenient systems for aerial applications due to their freezing motion feature during exposure time. To reconstruct a three-dimensional ( 3 ~ ) model from 2D imagery acquired by DFCs, requires, in principle, only the modeling and estimation of the camera interior and exterior orientation parameters. DFCs have been extensively used in close-range applications utilizing self-calibration, network design optimization schemes, and sophisticated image processing techniques for target mensuration. Results have been reported by Beyer (1992), Peipe (1995), Fraser (1997), and Lichti and Chapman (1997). In airborne applications, however, DFCs have not been frequently used due to limitations in geometry, resolution, and data rate. Current pixel resolution of commercial CCD chips is typically 7 to 15 pm, which is almost the same as that of a scanned image from a film image. DFCs performance has been analysed in tests and results have been reported in King et a1. (1994), Mills et al. (1996), and Maas and Kersten (1997). Pixel resolution, on the other hand, is not a major problem for the Department of Geomatics Engineering, The University of Calgary, 2500 University Drive N.W., Calgary, Alberta T2N 1N4, Canada. M.M.R. Mostafa is presently with R&D Airborne Applications, Applanix Corporation, 85 Leek Cr., Richmond Hill, Ontario L4B 3B3, Canada ([email protected]). Photogrammetric Engineering & Remote Sensing Vol. 66, No. 12, December 2000, pp. 1417-1423. 0099-lll2/00/6612-1417$3.0010