4. Case Study
A streopair of aerial photography covering National Central University (NCU) is used for case studying. The photo scale of the stereopair is 1:9,000 and the base-height-ratio is 0.55. Both photos were scanned at 12.5 mm to achieve a nominal ground resolution of 11.25cm. The DBM for orthoimage generation is shown in figure 6. A final orthoimage is shown in figure 7, which was resampled at 50cm ground resolution with 1792 x 1792 pixels. In which, the relief displacements of building were corrected according to a given DBM


Figure 6: DBM for NCU Campus

Figure 7: The generated orthoimages
A simulated bird-view image for NCU campus is shown in figure 8.a picture taken from a helicopter, i.e. figure 9, was used for visual comparison. Because the test image were acquired 5year before the helicopter one , some buildings do not appear in the simultedi image. Except for that difference, we got a high reliable orthoimage.


Figure 8: Simulated bird-view image

Figure 9: A picture taken from helicopter
Finally, for error analysis, we used leica SD2000 analytic stereo plotter to digitize 170 corners from 48 major buildings as a reference date set. Under the condition with 107cm of altitude error, we got planimetric error of 45cm as shows in table . that is to say, the orthoimage we generated achieve accuracy better than one pixel comparing to 50cm ground pixel spacing.

Table 1, statistics of error analytic (units: cm)

	X	Y	Z
Standard error	45.5	45.2	107.2
Mean	37.5	21.8	-34.5

5. Conclusion
In this paper we present a new scheme to generate orthoimage provided that a raster-based DBM is available. The proposed scheme composes (1) correction of the relief displacement for buildings, (2) recovering of the information within the hidden areas, and (3) stitching the filled-in gray value with the master image data to reduce the discontinuity effect. From the visual comparison for similarity assessment we got a high reliable result. In the error analysis, we manually digitized 170 corners from 48 major building in the study area for validation. An accuracy of 45cm planimetric error was achieved. Comparing to 50cm of pixel spacing, the accuracy for the orthoimage is better than one pixel.

6. Reference

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