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Tab.4 RMSE at the given 25 points 
  
  
  
  
  
Blunder threshold | No. of points | Root mean square errors (RMSE, m) 
in resection For resection X Y 7 
1.8 85 0.878 0.938 0.0667 
1.90 262 0.732 0.833 0.0381 
2.00 454 0.735 0.7035 0.0594 
  
  
  
  
  
  
  
Tab.5 RMSE at manually measured points 
  
  
  
  
  
Blunder threshold | No. of points | Root mean square errors (RMSE,m) | No. points for 
in resection For resection | X Y Z Statistics 
1.8 o0 85 1.138 1.177 0.170 61 
1.9 o0 262 1.071 1.158 0.231 60 
2.000 454 1.028 1.076 0.196 58 
  
  
  
  
  
  
  
  
Another evaluation is done by using the manual measurements as check points. After exterior orientation parameters are 
obtained through automatic image matching, mono-image intersection is performed for all check points to calculate 
their ground coordinates, which are then compared with the ones derived from measurements on the orthoimage. The 
RMSE is listed in Tab.5. Since the measurement is done manually on the screen in mono mode, its precision is expected 
no better than 1 pixel and several blunders have been detected and deleted. Considering this fact, the RMSE for 
planimetric coordinates listed in Tab.5 actually has two components, errors caused by space resection and by 
measurement errors on check points, namely 
si ei ” 
(RMSE) “XY = S resection tS check points (5) 
Taking the average of all (RMSE )'xr for X and Y in Tab.5 and counting S poi, as 0.8m (1 pixel), the actual 
affection of space resection on planimetric coordinates on the ground is then estimated as oresection = (1.23 — 0.82) = 
0.77m (< 1 pixel). It should be noted that this value is also an accuracy estimation for the new updated orthoimage. As 
the elevation of ground points is virtually determined via DTM interpolation, its accuracy (RMSE); is solely dependent 
on terrain slope and the planimetric accuracy, namely 
(RMSE), = A(F, + F,’) (RMSE), (6) 
where Fy and Fy are terrain slopes at X and Y directions respectively. For flat area as covered by this test image, the 
slope angle is less than 5 degree, therefore the elevation accuracy is estimated better than /.474*tan5 #J1.23 = 
0.14m or 0.003% flying height. 
5 CONCLUDING REMARKS 
An approach for automatic exterior image orientation is proposed based on DTM and orthoimage in current GIS 
database. The approach uses automatic feature extraction and matching within image patches in aerial image and 
orthoimage. Large amount of feature points (ca.2000) is included in the bundle adjustment, which may reach a precision 
of 1/3 pixel for orientation parameters due to the large redundancy. To evaluate the quality of this approach, a novel 
mono-image intersection algorithm is developed. Analyses and comparison based on results from manual measurements 
indicate that the accuracy of planimetric coordinates of ground points, determined by mono-image intersection, is better 
than 1 pixel. This is also the accuracy estimation for the new updated orthoimage. The accuracy of elevation is solely 
dependent on DTM interpolation. Further research will be focused on linear features detection and using them along 
with point features in GIS to automate the exterior orientation and orthoimage update procedure. 
  
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B3. Amsterdam 2000. 835