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AUTOMATIC TIE POINT EXTRACTION IN AERIAL TRIANGULATION 
Eija Honkavaara and Anton Hggholen 
Finnish Geodetic Institute 
Department of Photogrammetry and Remote Sensing 
Geodeetinrinne 2 
FIN-02430 Masala 
Finland 
e-mail. Eija.Honkavaara@fgi.fi & Anton.Hogholen @fgi.fi 
Commission III - Theory and Algorithms 
KEY WORDS: tie point extraction, aerial triangulation, adjustment, accuracy, automation, block 
ABSTRACT 
A new conceptual division of the automatic tie point measurement process into tasks is presented. An important feature in this 
approach is that attention is paid to the accuracy questions and the treatment of problems in tie point extraction. A system for 
automatic tie point measurement, which is under development at the Finnish Geodetic Institute, is outlined. 
An empirical investigation on the number, distribution and completeness of the tie point observations was carried out. The OEEPE 
test block Forssa with 30 um pixel size was used as test data. The investigated factors affected especially the height accuracy. The 
accuracy of the block improved with an increasing number of observations, but only up to a certain limit. The 5x5 distribution of tie 
point areas gave only slightly better accuracy than the 3x3 distribution. The accuracy of the block deteriorated with decreasing 
completeness of the observations. The RMS errors in the check points were in the best case, when using automatic tie point 
observations: X: 1.8 cm, Y: 2.3 cm and Z: 3.7 cm. The results are promising. There is reason to believe that the accuracy can be 
further improved. 
1. INTRODUCTION 
During the last years, automation of the tie point measurement 
process has been a popular research topic among 
photogrammetrists. Quite a number of approaches with high 
automation level have been developed. 
The common idea of the systems seems to be as follows. First, 
the areas where tie point extraction will take place are defined. 
Corresponding points are measured in these areas using image 
matching. Because the quality of the matched points is 
unknown and may be poor, a huge number of tie points is 
measured to achieve good accuracy. Interactive measurement is 
used in problematic cases, which arise because of failures in 
image matching. 
There are questions concerning this approach. First, the level of 
automation is questionable, because the rate of failures is non- 
deterministic. Second, there are also uncertainty concerning the 
accuracy and the reliability of a block. Previous results, see 
(Honkavaara and Hggholen, 1995), show that the accuracy of 
the block does not improve infinitely with an increasing number 
of observations. 
To handle these questions, the following conceptual division of 
the tie point measurement process into tasks is suggested: 
Tie point area definition. 
Corresponding point definition. 
Block adjustment and point selection. 
Quality control. 
Treating the unsuccessful tie point areas. 
. Process flow. 
These tasks are not necessarily separable, but may be combined. 
QAM S ae 
The research activities on automatic tie peint measurement have 
been concentrated mainly on tasks 1 and 2. Anyway, it is clear, 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B3. Vienna 1996 
that in order to make reliable, accurate and fully automatic tie 
point measurements all of the tasks should be dealt with. 
The tasks in the tie point extraction process are described and 
the system for tie point measurement being under development 
at the Finnish Geodetic Institute (FGI) is shortly outlined in 
Section 2 (more thorough description is given in (Honkavaara 
and Hggholen 1995)). Empirical results about the effect of the 
number, distribution and completeness of the tie points are 
presented in Section 3. 
2. AUTOMATIC TIE POINT MEASUREMENT 
2.1 Tasks in tie point measurement 
2.1.1 Tie point area definition 
It is usually sufficient to extract tie points in a limited number 
of distinct, homologously distributed locations, so called tie 
point areas. For stability reasons it is important to select tie 
point areas so that there exists a maximal number of 
overlapping images. For instance, in a typical photogrammetric 
block with 6096 forward overlap and 20-3096 side overlap, 6- 
fold overlaps are quite frequent. The tie point area distribution 
is discussed in Section 2.2.1. 
The paradox on defining tie point areas is that the exterior 
orientation of the images as well as the topography should be 
known. There exist different methods for tie point area 
definition with varying complexity, see overview in (Fórstner 
1995). The most simple methods use only approximate values 
of the exterior orientations and assume smooth terrain. The 
more sophisticated methods define the overlap areas by using 
progressively refined image fingerprints (defined by using 
orientation parameters and terrain heights determined during the 
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