Ortho-imagery: Geometric Accuracy Assessment23/06/2005 |
| Towards Product Specification for Urban Areas |
| Anomalies such as building-lean result from the processing of traditional orthorectified imagery and this limits its use in urban areas. However, the arrival of digital sensors has meant fully orthorectified products coming onto the market. The authors test the positional accuracy of fully orthorectified imagery for urban areas and suggest a product specification. |
| Wim Devos and Simon Kay, Joint Research Centre of the European Commission, Italy |
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Orthophoto production used to be mostly limited to medium and small-scale applications, and even in the 1990s this market remained so underdeveloped that line-map restitution from stereo imagery was cheaper than producing large-scale orthophotos. Orthorectification transforms an image from a perspective projection to an orthogonal one; the image takes on the geometric characteristics of a map. Traditional orthophoto products are obtained by rectifying aerial photos using coarse ground elevation models. For built-up areas the results are poor because objects above ground level, such as rooftops, are not positioned correctly and buildings typically appear to lean. In fully orthorectified products these effects are removed by using greatly overlapping imagery and a dense Digital Surface Model (DSM). The resulting image shows real near-vertical views for every position.
Results The Figures show residuals for Gent and Mol as a function of elevation above ground level. Zero elevations refer to manholes, non-zero elevations to building corners. No straightforward relation between elevations and residuals appears in either figure. However, in Figure 5 the spread for building points is much higher than for manholes. It is thus feasible to distinguish ground-level performance from elevated data performance. Figure 6 presents distributions of ground and elevation classes; although the median values for Gent confirm the trend identified in Mol their distributions do not give evidence of significant class distinctions. This might, however, reflect difficulties in identifying manholes in the black-and-white HRSC image; over the small test area used there was little contrast with the street pavement. Analysis Accuracy is proportional to ground sampling distance (pixel size) for ground-level data. For the three test areas ground-level accuracy is better than one pixel RMSE2D, whilst the elevated-data accuracy ranges from 1.5 to 2 pixels RMSE2D, although the values are unrelated to building height. The achievable accuracy is much better than the target specification and better than the ‘rule of thumb’ of two pixels used for estimating accuracy in traditional orthophotos. As compared with independent survey data the images thus prove to be well suited for urban applications. The similar values for elevated data for Mol and Gent, 23.7cm and 24.6cm respectively, may indicate that accuracy does not depend only upon pixel size but also on DSM resolution, 100cm for both. An alternative explanation for the similarity lies in the photo-mapping and photo-restitution process. Building points in the Gent image (pixel size 25cm) could be well identified, whilst in Mol (pixel size 16cm) discrepancies may be introduced by measuring façade top (possible overhangs) rather than façade base. But the particular production processes used make it hard to generalise on actual cause here. The visual differences in Figures 2c and 3c are not reflected in the quantitative results. Indeed, the ADS40 provided a high-quality DSM approximating very well to the building contours. The HRSC DSM appears much more blurred. Nevertheless, for both sensors correspondence between rectified image and reference survey data is high (Figures 2a and 3a). Figures 2c and 3c show also that the DSM ‘object’ is wider than the actual building, so that the façade top in the DSM may match a position on the ‘extended’ building roof. During TrueOrtho production pixels at the border of a building could have been retrieved from a vertical view input pixel; this would reduce the effect of building height on horizontal shift such as observed. But shift may also depend upon DSM quality and as such would be minimal for features at ground level. Concluding Remarks The better the required accuracy, the denser the DSM. This has an impact on the technology used to acquire the DSM and thus on the cost of production. However, the completely automatic removal of anomalies is wishful thinking. Therefore what is needed as a key condition is the introduction of a widely accepted standard that specifies criteria under which an orthorectified image may bear the prefix ‘fully’. Although the basic principles have been known for over a decade such a standard has not yet been established. The scientific literature provides few clues for deciding acceptable levels of anomalies and on determining costs to limit them. We therefore feel, based on the empirical results of our tests, that the time is right to put forward a provisional product specification as given in the textbox. We welcome any exchange of ideas to extend the debate on this emerging issue. Acknowledgements Thanks are due to ISTAR, Vito, Netmanagement NV and OC GIS Vlaanderen. Further Reading
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| Biography of the Author(s) Ir. Wim Devos and Dr Simon Kay are scientific officers at the JRC Agriculture and Fisheries Unit. Mr Devos holds a MSc in Agronomy from the KULeuven and was charged with establishing Flanders’ Large-Scale Mapping Program (GRB, OC-GIS Vlaanderen) before being seconded to the JRC. Dr Kay holds a MSc in Remote Sensing and a PhD in Geography from University College London. Both authors are involved in operational benchmarking projects related to the use of geomatics technology in European Union-policy implementation for agriculture and cadastre. |
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| Archive |
| Archive > June 2005, Volume 19, Issue 6 > Ortho-imagery: Geometric Accuracy Assessment |
