HIGH PRECISION AERIAL TRIANGULATION WITH POINT TRANSFER BY DIGITAL 
IMAGE CORRELATION 
F. Ackermann, W. Schneider 
Stuttgart University, Stuttgart, FRG 
Abstract 
It is well established that digital image correlation is capable of 
very high precision for parallax measurement. Application to point 
transfer can push standard aerial triangulation into the high precision 
range. Two examples of controlled blocks with digital point transfer by 
image correlation are presented. The verified results of about 4 yum in 
plan demonstrate the high accuracy capabilities of the method. Similar 
results have been obtainable up to now only with signalized points. 
1. | INTRODUCTION 
1.1 It is well known that the precision level of aerial triangulation, 
as indicated by the g, estimates of block adjustments,is primarily de- 
termined by the transfer precision of tie points, 1*). The method most 
frequently applied is stereoscopic transfer and artificial marking of 
tie points. Mono-comparator measurements in particular require previous 
transfer and marking of all tie points. It is known from experience 
that the precision of point transfer in practice is mostly > 10 um, 
very often > 20 um, although values < 10 um can be reached with modern 
point transfer equipment. The practical limit with aerial photographs 
seems to be about 7 yum, /1/. 
All results from high precision aerial triangulation on the other hand 
show that the noise level of aerial triangulation can be as low as 3 ym 
or less, provided that point transfer errors and all major systematic 
errors (instrument and image errors) are avoided or sufficiently sup- 
pressed. This is particularly the case when all tie and control points 
are signalized, apart from using high precision measuring instruments 
and block adjustment with additional parameters. Signalized points re- 
quire no point transfer and hence have no transfer errors. However, 
aerial triangulation with signalized tie points is restricted to spe- 
cial cases. Aerial triangulation for mapping purposes is almost always 
based on transfer and marking of tie points, for practical and economic 
reasons. 
It is therefore of primary importance to improve the precision of point 
transfer, in order to improve the precision of aerial triangulation. 
This is at present the most essential remaining step for increasing the 
practical performance of aerial triangulation, after the implementation 
of methods for the correction of systematic image errors and for blun- 
der detection. Pushing practical aerial triangulation also for mapping 
application to a precision level as high as possible is not at all an 
academic exercise only. It will have great economic implications as 
higher precision will immediately reduce control requirements. 
1* The precision level is to be distinguished from the absolute accura- 
cy of adjusted blocks which depends in addition on overlap and con- 
trol, i. e. on the propagation of errors related to the geometric 
configuration of blocks. 
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