Michael Cramer 
combined as test blocks. Again, the improved focal length of the camera was used for intersection. The obtained results 
are given in Table 5. Similar to the accuracy from the comparison at the camera air stations the accuracy on the ground 
is quite consistent for all baseline solutions. There is almost no dependency on base station distance visible. The mean 
accuracy obtained from all baselines is about 14.1cm and 10.6cm (east), 10.1cm and 12.8cm (north) and 22.8cm and 
19.3cm (vertical) for the first and second block configuration, respectively. Although these results are quite promising 
especially for the long baselines, they might be different for different test conditions and have to be re-confirmed by 
additional investigations. 
3.5 Combined GPS/inertial AT 
The accuracy tests have shown the potential of direct georeferencing using the Applanix high-end integrated 
GPS/inertial system in a standard photogrammetric environment. Assuming an image block with very strong block 
geometry due to high image overlaps the accuracy of object determination is close to the accuracy of photogrammetric 
point determination. From Kraus (1994) the theoretical accuracy of object point determination from AT is expected to 
be within x y= 5em for horizontal and z= 10cm for vertical components. Nevertheless, the proper system calibration 
is an issue of major importance and is absolutely necessary for highest accuracy and reliability requirements. In this 
context system calibration means determination of the spatial shifts and misorientations between the sensor components 
(misalignment between IMU and camera), the interior orientation of the camera and any remaining systematic effects 
from image space. 
To investigate the influence of system calibration a final test is performed where the results of direct georeferencing are 
directly compared to the reference accuracy from traditional indirect image orientation using standard AT. For this 
purpose the sub block consisting of two east-west strips (scale 1:13000) with standard overlap was chosen again (Table 
5). The GPS/inertial orientation parameters are obtained from the 25km baseline result originally. To simulate 
production environments where the system calibration (e.g. misalignment correction) is performed using a calibration 
site which is different to the aspired block area, the calibration angles from all 72 images are applied. In the first step 
the standard AT is calculated using 9 well distributed ground control points (CoP). Applying additional self-calibration 
parameters (radial lens distortion and decentering distortion) the 9 a posteriori is about 4.2 m and the empirical 
maximum deviations from 122 check points (ChP) did not exceed 22cm and 35cm for the horizontal and vertical 
coordinates, respectively (Table 6). Comparing this accuracy level to the object point accuracy from direct 
georeferencing without any ground control (DG, Version 2) the results are worse. Since the GPS/inertial exterior 
orientations are used as fixed values and assumed to be error-free, the y of about 10 m indicates remaining tensions 
between the image observations and the orientation parameters. The non-optimal determined system calibration for the 
misalignment and the image distortions provoke the errors in object space. The only way to overcome these systematic 
errors and to determine the calibration terms optimally is to re-introduce AT with additional self-calibration 
functionality again. To perform this combined GPS/inertial AT approach a bundle adjustment program developed at the 
Institute for Photogrammetry (ifp) was used, where the directly measured exterior orientations are introduced as very 
high accurate observations of the camera air stations. In this particular case their standard deviation is assumed to be 
5cm and 0.001deg for position and attitude, respectively. Since there is enough information from image space (130 tie 
points available) additional self-calibration terms are estimated. In Version 3 the calibration parameters proposed by 
Brown (1971) (see Section 2.1) are introduced, which increases the accuracy in height and north, but slightly decreases 
the accuracy in the east component. For the second run of combined GPS/inertial AT (Version 4) the Brown self- 
calibration terms are supplemented with three additional offset angles to correct for the non-optimal misalignment 
between inertial and camera coordinate frame. This step improves the east component. Now the RMS values are about 
5cm and 9cm for the horizontal and 13cm 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
for the height component. There are still Approach | CoP | ChP 9 ERMS Obiect Caordinates Nu 
some differences to the AT accuracy [ m] | East North | Vertical 
(Version 1), especially in the north 1 AT 9 122 | 4.24 4.5 6.3 12.1 
coordinate. This is mainly due to the fact, 2 DG 0 131 | 10.80 8.8 11.9 17.8 
that for this combined GPS/inertial AT 3 1 DG+AT 1 130 | 461 13.4 9.5 13.3 
approach only one control point is used, [4] (yy; = > 
which is located in the centre of the block. patie |.) 130 [yd 0 $2 92 55 
Therefore, extrapolation to the borders of Table 6, Combined GPS/inertial AT (14 images, 2 parallel strips, 
the block is necessary. standard photogrammetric overlap conditions) 
  
204 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B3. Amsterdam 2000.