POSTERIORI RELIABILITY IN BOTH CASES ADOPTING DATA-SNOOPING AND 
ROBUST METHODS 
Y.D. Huang 
Civil Engineering Department 
Northern Jiaotong University 
Beijing, P.R. China 
ABSTRACT 
The concept of reliability is generalized so that it can apply both 
to a photogrammetric system and a photogrammetric result, not only 
in the case adopting data-snooping but also in that adopting robust 
methods. A measure of result reliability named posteriori 
reliability is proposed for assessing residual gross and systematic 
errors in concrete photogrammetric results. The expressions of the 
measure for both cases of adopting data-snooping and robust methods 
are given. 
1. INTRODUCTION 
When delivering a photogrammetric result, we are also asked to hand 
over an assessment of the quality of the result. Because of the 
unavoidable presence of gross and systematic errors in the result, 
not only a measure of precision but also a measure of reliability 
is required to assess the result, the latter evaluating the gross 
and systematic errors. However, the existing measure of reliability 
is not appropriate for that purpose, although it may be very 
suitable for being used to evaluate a system in the design stage of 
photogrammetry. That is because it only reflects the ability of a 
photogrammetric system to detect gross and systematic errors, but 
not the magnitude of the errors themselves in a concrete result. 
This paper will give a measure of reliability for that purpose. It 
contains the information offered by observations about the concrete 
magnitude of gross and systematic errors, So can be more accurate 
in assessing concrete results than the existing measure. 
Besides, the existing theory of reliability only applies to the 
case adopting data-snooping technique, but not to the case using 
robust methods. While the robust methods are now recieving more 
and more attention and research. They have been found to have 
many advantages on the theoretical plane, and some experience in 
their application has been acquired. They are tending to wider 
and wider application. So it is in urgent need to develop the 
existing theory of reliability so that it can apply to the case 
using robust methods. The paper will Show how to evaluate the 
reliability of result when robust methods are utilized. Some ideas 
about how to translate the existing theory of reliability wholly to 
the cases using robust methods will also be seen in the paper. 
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