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Failure Prediction in Automatically Generated Digital Elevation Models
GOOCH, Michael (M.J.Gooch@lboro.ac.uk) and CHANDLER, Jim, Loughborough University, Department of Civil and Building Engineering, Loughborough, LE11 3TU, U.K.
Key Words: digital photogrammetry, DEM, accuracy, classification, strategy parameters
Developments in digital photogrammetry have provided the ability to automatically generate Digital Elevation Models (DEMs) that are increasingly used by geoscientists (Chandler, in press). Using overlapping imagery, dense grids of coordinates can be collected at high speeds (150 points per second) with a high level of accuracy. The trend towards using PC-based hardware, the widespread use of Geographical Information Systems, and the forthcoming availability of high resolution satellite imagery over the internet at ever lower costs, means that the use of automated digital photogrammetry in elevation modeling is likely to become more widespread. Automation can reduce the need for an in-depth knowledge of the subject, thus rendering the technology an option for more users.
One criticism of this trend towards the automated black-box approach is the common lack of quality control procedures within the software (Cooper 1998), particularly to identify areas of the DEM with a low accuracy. The traditional method for accuracy assessment is through the use of check point data (data collected by an independent means of a higher level of accuracy against which the DEM can be compared); however, in most cases, check point data are not available and the user is recommended to manually check and edit the data using stereo viewing methods, a potentially lengthy process that can negate the obvious speed advantages brought about by automation.
Work at Loughborough on accuracy optimization (Gooch et al., in press) has primarily been carried out using the ERDAS Imagine OrthoMAX digital photogrammetric system. This software uses an area correlation-based algorithm over which the user has a certain amount of control through the use of a set of strategy parameters (Gooch and Chandler, 1998). These control the acceptance and quality control requirements of the points in the DEM, and early research work assessed the effect of altering these parameters on the resulting accuracy of the DEMs. It was found that altering the parameters has the most effect on points where the algorithm has difficulty in estimating the elevation (areas with a low image content, a sudden elevation change, or where movement has occurred between the capture of the two images). More significantly, in areas where accurate height estimates were generated, changing the strategy parameters was found to have little impact on the elevation of the point.
A software data processing model has been developed that uses this phenomena to identify areas where elevations are unreliable and to which the user should pay attention when editing and checking the data. The user simply inputs two DEMs of the area (each generated with a different strategy parameter specification) and the software identifies where height estimates are likely to be unreliable due to (a) the software having interpolated a point in areas with a sudden elevation change, and (b) low image content or
object movement between the two images. It overlays this information on an orthophoto of the area thus enabling a hardcopy visual output to be printed for easy identification by the user. The user can then focus on these areas when manually editing the DEM, reducing the need for check data and to check the whole DEM.
The software model developed from this research will be explained and described in detail in the presentation. Results from tests on different scales of imagery, different types of imagery, and other software packages also will be presented to demonstrate the efficacy and generality of the technique.
Chandler, J.H. (in press). "Effective Application of Automated Digital Photogrammetry for Geomorphological Research," Earth Surface Processes and Landforms.
Cooper, M.A.R. 1998. Datums, Coordinates and Differences in Landform Monitoring, Modelling and Analysis, S.N. Lane, K.S. Richards, and J.H. Chandler, J. Wiley and Sons, Chichester, Cambridge, 1998, pp.1-17.
Gooch, M. J. and J.H. Chandler, 1998. "Optimization of Strategy Parameters Used in Automated Digital Elevation Model Generation," ISPRS International Archives of Photogrammetry and Remote Sensing, 32(2): 88-95.
Gooch, M. J., M.J. Stojic, and J.H. Chandler (in press). "Accuracy Assessment of Digital Elevation Models Generated Using the ERDAS Imagine OrthoMAX Digital Photogrammetric System," Photogrammetric Record.