can be minimised if the inner orientation to the 
analogue imagery is done using digital images. In 
this case, the only significant instabilities are those 
that occur while the analogue image is being 
digitised. This was basically the philosophy 
adopted while digitising for the investigation to be 
described. The average observed drift of the 
calibration translations during the digitisation of 
one photograph was observed to be 0.5 |im. Two 
values out of a total of thirty-two were observed at 
over the 1 pm level, the maximum being 1.4 pm. 
These figures relate to an average time period of 
thirty minutes. 
particularly experienced operator - in two 
afternoons sessions. The coordinates were not 
corrected for any distortions since these could be 
added later if required. 
4.4 Digital Measurement of the Block 
4.4.1 Outline of the Procedure 
The digital measurements were performed as an 
interactive procedure, divided into two major parts. 
In the first part, images of the signalised points are 
acquired; the second part is the off-line matching of 
these images to obtain the image coordinates. 
4. Investigation into Aerial Triangulation 
using Digital Point Positioning 
4.1 The Aim of the Investigation 
In this investigation, the S9AP with CCD cameras 
will be used to relieve a human operator of the 
point measurement task. Signalised points in a 
photogrammetric test-block are positioned using 
image matching and the calibration data as 
described above. The investigation should shed 
light onto two aspects: firstly how the accuracy of 
digital and manual measurements compare, and 
secondly where the potential problem areas would 
be in a semi-automatic procedure. 
4.2 The Test Area “Heinzenberg” 
The data used for the investigation is taken from a 
test-block flown in August 1986 in an area known 
as Heinzenberg in eastern Switzerland. A sub 
block of the original was used; the relevant details 
are: 
Area covered: 
Ground height: 
Flying height: 
Camera: 
Film: 
Photo scale: 
Overlap: 
Block size: 
6.0 x 6.0 km 
650 m to 2150 m 
4000 m 
Wild RC10, 15/4 UAG 
Kodak Panatomic-X 
1 : 15,000 average 
60% / 60% 
4x4 
The original film negatives were used; on average 
there were 17 signalised points per image, with a 
maximum of 25. The original block was measured 
on the AC1 at the ETH in Zürich; the results of the 
subsequent adjustments have been published (Grün 
and Runge, 1987). 
4.3 Manual Measurement of the Block 
To get a meaningful comparison with manual 
measurements, it is necessary to have datasets 
which are in all other respects equivalent. Hence 
the block was firstly observed on the S9AP using 
the Phototriangulation Measurement software of 
System-9. This was done by the author - not a 
4.4.2 Digitisation of the Signalised Points 
The image digitisation was done in a session-wise 
fashion, whereby three separate sessions, usually 
corresponding to a half-day, were required for the 
16 photographs. Only the right-hand stage of the 
S9AP was used. A session included frequent 
calibrations of the CCD camera so that the results 
relating to the stability could be obtained. The 
instrument calibration was checked before and 
after each session. 
For a single aerial photograph, images of the four 
fiducial mark images were first digitised, followed 
by the images of the signalised points. The images 
were stored on disk together with the stage 
coordinates of the camera position. The time 
required for digitisation of an image with, say, 20 
signalised points was around half an hour, but this 
was aided by a manual inner orientation and 
knowledge of the image coordinates already. A 
complete session of four photographs, including all 
calibrations, was around three hours. 
Image digitisation was done interactively with the 
Maxvideo system and using software developed in 
Zürich. For each image, the gain and offset levels 
of the video signal digitisation were set manually, 
so that, by visual inspection, the point of interest 
(fiducial mark or signalised point) had optimum 
contrast. This optimum depends on the density 
levels in the aerial photography and hence also on 
the S9AP illumination. In a few cases, it was also 
necessary to change this. This all has important 
implications for the potential of an automatic 
procedure. 
4.4.3 Point Positioning by Template Matching 
Within the context of the collaborative project, the 
constrained least squares image matching (LSM) 
algorithm has been rewritten in the language C on 
Sun Workstations. The algorithm is well known 
and well documented in the literature (eg Grün and 
BalLsavias, 1988); it is not the intention to elaborate 
upon it here. For this investigation, the algorithm 
was used in its unconstrained two-dimensional 
mode, known as template matching as the one 
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