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Title
New perspectives to save cultural heritage
Author
Altan, M. Orhan

CI PA 2003 XIX th International Symposium, 30 September - 04 October, 2003, Antalya, Turkey
BASTA and documented in Kalisperakis & Tzakos (2001), has
been primarily intended as an educational tool. It functions to
tally within a commercial CAD environment, exploiting its tools
to allow convenient image measurement as well as editing.
Figure 2. The five images of the ‘Tsopotos’ residence.
The program solves the bundle without control (‘relative solu
tion’); it offers options for both full and partial self-calibration
(camera constant, principal point and/or radial lens distortion);
it may handle control coordinates as observations; further, it can
accept any of the three geodetic coordinates of control points as
known and the others as unknowns, a very useful feature in the
context of architectural photogrammetry (see 4.2). For both pro
grams the same 92 tie points were used, but image coordinates
were measured independently within the two environments. An
average of 3.7 intersecting rays per object point formed a strong
bundle configuration. The image at the bottom of Fig. 2 has a
larger scale. Adjustments for all 5 images but also for the re
maining 4 have been carried out to check for any possible diffe
rences due to this particular image (Rova, 2003). 3
3. ADJUSTMENTS WITHOUT CONTROL
As no particularly reliable control coordinates were at hand (see
4.1 below), the first step was to solve the bundle without geode
tic control, i.e. in arbitrary systems, properly scaled in the case
of BASTA whose results were considered as reference data re
garding PhotoModeler. Outcome of the adjustments, which re
lied on the assumed ‘nominal’ values for inner orientation para
meters c = 80 mm and x<, = y 0 = 0, is 92 tie point coordinates in
the two systems of BASTA and of PhotoModeler (B and PM, re
spectively). Point sets represent shape reconstruction and can be
compared to each other by means of a 3D similarity transforma
tion (Rova, 2003). The overall standard error (ct s ) of this trans
formation in all three axes describes the ‘closeness’ of results
from the two programs.
Other measures of precision are also available. For BASTA, this
is the standard error of image coordinates (g 0 ) and, further, the
overall standard deviation (ct t ) of tie points, resulting from the
variance-covariance matrix of unknowns in the bundle adjust
ment. On the other hand, PhotoModeler produces a ‘tightness’
value (t) for every tie point, representing the maximum distance
among intersecting rays as percentage of the largest object di
mension. Furthermore, PhotoModeler can perform what it calls
a ‘self-calibration’ (from now on PMS), meaning a small ‘inter
nal’ adjustment of camera parameters to ‘optimize’ the solution;
however, users do not actually see it. Results from the two pro
grams are concentrated in Table 1. In Table 2 results from the
3D similarity transformations are given.
Table 1
Precision estimations from 92 tie points
B
PM
PMS
CT 0 (Jim)
18.3
4 images
0> T (cm)
1.3
t (cm)
3.8
4.0
CT 0 (ftm)
19.2
5 images
G T (cm)
1.5
t (cm)
3.3
3.2
Table 2. Accuracy g s (cm) of the
3D similarity transformations
B-PM
B- PMS
4 images
2.0
1.9
5 images
2.0
1.9
Image residuals (represented as G 0 = 1.3 pixel) and tie point pre
cision G T are acceptable if the poor quality of object points, due
to building decay, is taken into account. ‘Tightness’ values from
PhotoModeler (being larger than G T by a factor 2 to 3) appear to
be grossly comparable to Ctt since they refer to the largest devia
tions among rays. Finally, the similarity measures g s in Table 2
between point sets confirm that reconstructed shapes are practi
cally equivalent, within the precision of the two methods.
4. SELF-CALIBRATING ADJUSTMENTS
Besides defining the object system, the use of control points can
also allow a self-calibrating approach to optimize reconstruction
(and, additionally, provide information about camera geometry).
In the present case, separate old elevation drawings prepared by
students at the School of Architecture of NTUA were available.
A few control points, unfavourably distributed but sufficient for
self-calibration, were extracted graphically. Being of unknown
accuracy, however, the plots were regarded as unreliable (a fact
also established in the experiments). Under these circumstances,
the next tests were essentially intended to check the camera cali
bration processes rather than the accuracy of space coordinates.
4.1 Use of control points
PhotoModeler does not allow recovery of lens distortion (but it
accepts existing data about it). After some experimenting, it was
concluded that, for 5 images, camera constant (c) and principal
point (x 0 , y 0 ) could be determined from 7 control points (at least
5/image). For 4 images, 6 points were required on all images (5
points/image did not suffice for the principal point); tests were
made with 7 control points, too. The same control was also used
for self-calibration in BASTA (both with and without estimation
of radial lens distortion). Being theoretically in the same object