6. SYSTEM VERIFICATION
6.4 Image Spatial Accuracy Assessment
6.4.6 Bundle block adjustment results
The results of the bundle block adjustment using the manually defined image tie points are presented and discussed in the following section.
approximately 55m South East from the position where it had been captured in the field, as shown in Figure 30.
20 0 20 40 60 80 100 120 140 160 180 Meters
i~~~~~ __ ~~~~ __ ~~~ __ ~~~~ __ ~~~
Figure 30: Incorrect measurement ofGCP number 1.
By incorrectly measuring this point, Orthobase had calculated large tip, roll and yaw angles for the image in order for the GCP position measured in Orthobase to spatially match the coordinates of the actual point captured with the GPS in the field. Figure 31 shows image 4-13 and the position of GCP number 1. It is evident from the non-rectangular shape of the image that the software has had to stretch the North-West corner of the image to make the measured GCP position match the GPS coordinates of the actual point captured.
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--
Figure 31: Image 4-13 showing the distortion due to the incorrect measurement of OCP 1.
It was confirmed with Craigie (2000) that the incorrect measurement ofGCP number 1 would not have affected the calculation of the exterior orientation parameters for the remaining images or the overall results of the bundle block adjustment.
From Table 15 it is also evident that the tip angles are high for all images. Again, this was unexpected based on the good conditions under which the test flight was flown. However, the tip angles did not fluctuate dramatically from one photo to the next and were all positive or negative6 for a particular flight line. This indicated that there was possibly a constant error in these results.
On further analysis it was concluded that the error had been introduced by the manner in which the camera had been mounted in the aircraft. The camera had been mounted flush with floor of the
6
The positive or negative values of Omega, Phi and Kappa are as a result of the rotation coordinate system used. Omega is a positive rotation around the X-axis, phi is a positive rotation around the Y-axis, and kappa is a positive rotation around the Z-axis. X is the primary axis, indicating that the first rotation in forming the rotation matrix is around the X-axis. Rotation follows the right-hand rule, which says that when the thumb of the right hand points in the positive direction of an axis, the curled fingers point in the direction of positive rotation for that axis.
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aircraft and as a result the camera CCD chip's x, y and z axes were not orthogonal to the x-y-z axes of the aircraft when in flight. Therefore, even if the aircraft had been flown so that its x and y axes were parallel to the x and y axes of the reference coordinate system for all the images, the camera's axes would be rotated relative to the reference coordinate system. If the aircraft was flown from South to North and its x and y axes were orthogonal to the reference coordinate system, then mounting the camera flush with the aircraft floor would have introduced a positive rotation about the X-axis, in other words, a positive tip angle. Furthermore, it was likely that the aircraft would fly with its nose pointing slightly upward, thereby introducing a further positive rotation about the X axis when a flight line was flown from South to North, as flight lines 4 and 6 were. A negative rotation about the X-axis would be introduced for flight lines flown from North to South, as flight line 5 was. The results tabulated in Table 15 substantiate this hypothesis.
If the absolute average tip angle of9.01 degrees was assumed to be the constant error introduced by mounting the camera in this manner, then rotation about the X-axis due to turbulence (excluding image 4-13) was less than 5 degrees for all images. This magnitude of tip was considered to be reasonable and would not significantly reduce the accuracy of the orthophotographs produced.
Excluding image 4-13, the roll and yaw angles were considered to be correct and acceptable, although the variation in the yaw was in excess of 2 degrees for four pairs of the images. The implications of a large variation in the yaw angle between consecutive images were discussed previously.
To establish the spatial accuracy ofthe imagery, the cadastral boundaries ofPietermaritzburg were obtained from the City Engineers Department of the Pietermaritzburg City Council. These cadastral boundaries were assumed to be 100% accurate and were overIayed on the digital orthophotographs produced as shown in Figures 32 through 35. The colour orthophotographs have been displayed as greyscale images in Figures 32 to 35 in order for the cadastral boundaries to be clearly visible. A visual estimation of the images' accuracies was hence made.
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Figure 32: Example 1 of cadastral boundaries overlayed on digital camera orthophotographs.
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Analysis of the digital camera orthophotos with the Pietermaritzburg cadastral boundary data overlayed indicated that the orthophotos were planirnetrically accurate to an average of 5 to 10m.
The maximum error encountered was 14m within the demarcated circular area 'a' in Figure 32.
Based on the limited number oftie points used in the bundle block adjustment and the non-metric nature ofthe digital camera, these results were considered to be good. Imagery ofthis planimetric spatial accuracy would be suitable for many resource management applications, such as land use mapping, mapping of informal settlements, farm and area resource assessments, alien vegetation mapping and management, and forest fire damage assessment. If these images were used as base maps in a GIS for the mapping of land use and other data, the resulting GIS coverage would certainly be considerably more accurate than the 20 to 50m accuracy achievable with MBB' s video based system. The timeliness of producing the end product would also be increased as discussed previously.
These results confirm the potential of using digital camera imagery for mapping and management applications where small areas are involved and a moderate spatial accuracy of 5 to 10m is required. Further improvements to the in-flight system, such as the incorporation of a tip roll sensor, and further experimentation with the use of digital camera images in Orthobase or similar software could produce even more favourable results.
l34