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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