Full text: Proceedings, XXth congress (Part 6)

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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B6. Istanbul 2004 
  
  
  
  
  
  
  
  
  
  
  
RMSE Number of GCPs 
l 3 6 10 
| Horizontal 0.959 0.979 0.992 0.951 
Vertical 1.319 1.263 1.284 1.302 
  
Table 2. Accuracy obtained from different number of GCPs. 
It can be seen that there is no significant difference in the 
RMSE with varying numbers of GCPs. In all cases it can be 
seen that IKONOS imagery with bias-corrected RPCs produces 
RMS error of around 1m in planimetry and 1.3m in height. The 
approach yielded accuracy surpassing specifications for the 
much more expensive Pro and Precision products. The 
accuracy obtained is around the same as Precision Plus, which 
is the most accurate product provided by Space Imaging. 
Further investigation was undertaken by fixing the number of 
GCPs at three and varying their location, as shown in Figure 5. 
Three different cases were investigated: i) all points at the 
center, ii) all points at different corners, and iii) all points in the 
same corner. From table 3, case III produces less desirable 
results as compared with the other two, but such a GCP 
arrangement may be necessary in particular cases such as 
mapping over the national border where GCPs can only be 
established on one side. 
Figure 5. Three different configurations of GCPs. 
  
  
  
  
  
  
  
  
RMSE Case I Case II Case III 
Horizontal 0.951 0.902 1.247 
Vertical 1.302 1.229 1.305 
  
Table 3. Accuracy obtained from the cases in Figure 5. 
S5. ALTERNATIVE SENSOR MODEL FOR RPC 
In instances where RPCs are not provided, eg with standard 
IKONOS Geo image products, further alternative orientation 
models need to be considered. Once such model is based on 
affine projection (Fraser et al, 2003). The general form of the 
model describing transformation from 3D object space (X, Y, Z) 
to 2D space (x, y) for a given point / within an image is given as 
3 
Yymds X4 de; d Z; d, $ 
The affine model can be considered as a special case of the 
more general rational function model where the polynomial 
degree is one and the denominator is also one. The coefficients 
Az and As can be interpreted as translation parameters. The A, 
A» As, As, Ag and A, coefficients contain combined effects of 
rotation and scale. Despite its simplicity, which may raise a 
  
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degree of suspicion, recent research by Fraser et al (2002) has 
shown that this model can produce high geopositioning 
accuracy comparable to that produced via RPCs for IKONOS 
Geo imagery.. 
The attractive feature of this approach is that no RPCs need to 
be purchased and this leads to an even more economical means 
of acquiring and using HRSI. The tradeoff is that more GCPs 
are required. As can be seen in (3), at least four GCPs are 
required versus one in the case of bias-correction described in 
Section 2. However, in the case of mapping remote areas, the 
major portion of cost of the ground measurement is to get there. 
Since GPS measurement techniques are now very efficient, 
establishing a few more control points does not necessarily 
generate substantially more cost. Another favourable feature of 
this approach, especially when using Barista software, is that 
whenever RPCs are required they can easily be generated. 
Three different sets of 6 GCPs with 75 check points were tested 
with the affine model and all yielded almost identical results. 
The planimetric accuracy was just above 1m while vertical was 
again around 1.3m. These results are effectively the same as 
obtained from the bias-correction method in Table 2. 
6. CONCLUDING REMARKS 
This paper has highlighted that lowest-cost HRSI products can 
meet large-scale mapping needs of developing countries. The 
experimental results clearly demonstrate the capability of HRSI 
for large-scale mapping. In metric terms, a 1:10,000 scale 
topographic map can be derived from IKONOS imagery and 
even up to a 1:5,000 scale line map can also be produced. In the 
test reported, however, the resulting RMS errors were only just 
below the threshold and thus could be considered a little too 
close to allowable tolerance limits for 1:5,000 mapping. 
Apart from metric quality, factors such as whether the desired 
features at the required scale can be clearly identified on the 
source images need to be taken into consideration. Though this 
topic was not addressed in the current research, it is worthwhile 
to note that this user requirement is no less important than the 
metric accuracy. It should also be emphasised that results 
shown in this paper are from the use of only one stereo-pair. 
Further evaluation of both metric and non-metric aspects of 
HRSI is warranted, particularly on the larger blocks of 
overlapping imagery that are often the case in real-world 
mapping projects. 
ACKNOWLEDGEMENTS 
This research has been supported by a grant from the Geo- 
Information and Space Technology Development Agency 
(GISTDA) of Thailand. In particular, the authors would like to 
thank Dr. Suvit Vibulsresth, GISTDA director, and his staff 
including Dr. Chaowalit Silphatong and Dr.Pakorn'Apaphan for 
the acquisition of the IKONOS stereo pairs and for other 
assistance throughout the project. The support of Dr.Wicha 
Jiwalai, GISTDA chairman, during the initial project phase is 
also much appreciated. 
REFERENCES 
Ager, T.P., 2003. Evaluation of the Geometric Accuracy of 
IKONOS imagery. SPIE 2003 AeroSense Conference, Orlando, 
Florida. 
 
	        
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