Michael Cramer
camera air stations are interpolated into the 50Hz trajectory solution. In order to relate orientation and camera module,
the spatial and rotational offsets between the different sensor components are applied. To obtain optimal misalignment
calibration two sets of misalignment angles were estimated from all imagery of each image scale, separately. The
detailed description of the test flight, the data processing, the misalignment calibration and the internal accuracy checks
of the different GPS/inertial trajectory solutions depending on the varying baseline lengths can be found in Cramer
(1999),
3.3 External quality of GPS/inertial exterior orientations
The comparison between the orientation parameters obtained from AT and the directly measured GPS/inertial
orientations gives a first estimation of the expected accuracy potential. Since the theoretical accuracy of the perspective
centre coordinates from AT is scale dependent and two different misalignment corrections are applied, the accuracy
checks are done for each image scale, separately. In Table 3 the accuracy (RMS) calculated from all differences at 72
and 32 camera stations for the 1:13000 and 1:6000 imagery is given for four different GPS/inertial trajectory solutions
with varying distances to the test site. The results show remarkable consistency. Although the baseline length differs
from 25-380km distance, the horizontal accuracy of the coordinates of the perspective centres is within 15-20cm and
10-15cm for the 1:13000 and 1:6000 images. Focussing on the attitude differences, the results are even more consistent.
The RMS is within the 10arc sec level for , and about 15-20arc sec for — . Nevertheless, some systematic errors
are clearly visible in the vertical components of the perspective centres. The vertical accuracy is significant worse
compared to the horizontal values. For the 1:6000 images the RMS is between 10-20cm, for the 1:13000 imagery the
variations are between 20-50cm. Although the vertical RMS values are slightly different dependent on the GPS/inertial
trajectory solution, the ratio RMSeoo, vs. RMS 13000 is almost constant, except for the 380km basis which performs little
different due to the very long baseline distance and the influence of e.g. uncorrected atmospheric effects. The mean
ratio of 0.46 corresponds exactly to the ratio between the two image scales. This systematic is most likely due to errors
in the photogrammetric reference positions. As pointed out in Section 2.1 the estimated orientations are quite sensible
on the used parameters in AT. Remaining systematic effects are directly projected into the estimated orientation
parameters. In this case, the vertical offset might be due to any scale depend errors influencing the vertical component
of the estimated camera stations. Most easily such an offset can be explained by erroneous focal length used in the
bundle adjustment. In
our case the scale T[GpS/inertial | Image scale | RMS EO Position [m] — | RMS EO Attitude [10 ^ deg]
dependent systematic solution m, X; Yo Zo
is modeled improving
the used focal length S2 13000 18.1 17.5 30.8 3.2 3.0 5.9
by a factor of about 25 km 6000 9.9 77 13.5 3.0 2.0 3.9
20 m. Applying this F 13000 14.5 16.9 48.1 3.2 3.0 6.1
correction io the 120 km 6000 10.4 6.9 22 2 3.0 2.0 4.1
interior orientation B 13000 16.0 16.9 36.9 3.2 3.1 6.1
parameters of the 230 km 6000 17.9 6.7 18.2 3.1 2.0 4.1
camera, the vertical H 13000 14.8 16.9 23.9 32 3.0 6.1
accuracy is improved 380 km 6000 13.5 8.6 6.1 3.0 2.0 4.4
to the level of the
horizontal RMS Table 3, Accuracy of GPS/inertial exterior orientation parameters compared to AT
values.
3.4 Direct georeferencing
To estimate the accuracy of direct georeferencing, known object points are re-determined from image coordinate
measurements and compared to their given reference coordinates. Since observations from image space as well as the
directly measured orientations from GPS/inertial are necessary for this accuracy investigation, the resulting differences
in object space represent the overall accuracy of direct georeferencing for the sensor system consisting of the imaging
part and the orientation module. Additionally, this accuracy check gives more reliable results since the check point
coordinates provide truly independent reference values, which are not affected by remaining systematic errors like in
the estimated camera stations described before. To check the quality of direct georeferencing, the influence of varying
image overlap (number of image rays that can be used for the point determination in object space) and the influence of
different baseline length are treated separately. The quality checks were done using the 1:13000 imagery since the
influence of orientation errors on object point determination is correlated with the flying height and therefore any
remaining errors in the GPS/inertial orientations are more clearly visible for smaller image scales from larger flying
heights.
202 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B3. Amsterdam 2000.
