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C. Vincent Tao 
  
PCI software supports several sensor models including the rational function model. The results computed using the PCI 
RFM module are also given in Table 2. Both the root mean square (RMS) of the residuals and the maximum errors at 
control points and checkpoints in image are listed. Based on the analysis of these results, the following observations are 
obtained: 
= Both the rational function models and the polynomial model (i.e., p2=p4=1 case) can reach the accuracy around 1 
pixel. 
= In general, the cases with rational components are slightly better than the polynomial models. 
= The fitting accuracy of the cases with four-degree is very close to those with six-degree. It is understandable since 
the aerial photogrammetry data set may not involve high order distortions. 
= The accuracy of the direct solution method is comparable with that of the iterative solution method when the 
number of control points (i.e., 50 or more) are enough. The iterative solution is often slightly better than the direct 
solution when only a few more control points than the minimum number of control points required are available. 
5 CONCLUSIONS 
An overview of sensor models are given in this paper. These models are the collinearity equations based differential 
rectification model, the polynomial model, the projective transform model, the extended direct linear transform model 
and the rational function model. The remarks on their properties, and advantages and disadvantages are also discussed. 
It can be observed that the RFM is essentially a generic form of the all described models. 
Unlike the physical sensor models, the rational function model needs no knowledge specific to each type of imaging 
sensor physics, such as orbit parameters and platform orientation parameters. It has been noticed that rigorous models 
are not always available, especially for images from commercial stateliness (e.g., IKONOS), where the rigorous sensor 
model is hidden to the end user. Therefore, the RFM becomes an alternative sensor model to image rectification. 
For aerial optical images (i.e., frame sensor type), the test results show that both the rational function model and the 
polynomial model can reach reasonably good accuracy. Because the fitting accuracy of the cases with four-degree REM 
is almost the same with those with six-degree RFM, high order forms are often not necessary. The iterative solution 
method to RFM provides a better accuracy than the direct solution method, but the direct solution method is usually 
adequate when enough control points are available. 
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