al-
ric
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er,
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‚OWN
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ilts
figure 1: the nave (overview, left) and
the cross column containing the test ob-
jects (more detailed, right)
figure 2: natural stones with different
level of weathering
CLAIMS, CHOICE OF SURFACES AND PHOTO-
GRAMMETRIC TASKS
After an initial inspecting of the
test object six parts on the cross column
were chosen and spread over all directi-
ons to get information about an influence
of the position. Any portion covers an
area about 20*20 cm?. The chosen surfaces
also differ due the level of weathering
(s. fig. 2). This was done because it is
possible that form and quantity vary in
different phases of the weathering pro-
cess.
The curators of monuments restricted
the number of control points per area and
required a respectful treating of the hi-
storical environment. Because a priori no
Statement about the account of the assu-
med loss of surface could be made by the
cooperating mineralogists the range of
accuracy was fixed to 0.1-0.3 mm.
The photogrammetric camera
To apply two photogrammetric cameras
(Wild P32) for the tasks described above
some modifications had been caried out.
The focal length was changed to achieve
an image scale of about 1:8 (i.e. a di-
stance of 60 cm to the stone). It was re-
alised by inserting metal cones. After-
wards tbe depth of focus range was deter-
mined empirically using an optical bench.
In this case the range is very small and
it should be guaranteed that the range of
differences in elevation on the test ob-
ject ( max. 5 cm) is within it and the
desired distance to the object can be re-
alised.
After this modification the two sur-
vey cameras were calibrated using a test-
field (s. fig. 4). An alraedy existing
testfield (metal plate with borings to
insert control points) was modified to
adapt the differences in elevation to the
new conditions. For both cameras in each
case 8 photos were taken for the cali-
bration procedure (s. tab. 1). The con-
trol points were introduced in a bundle
block adjustment with a standard devia-
tion of 0.1 mm in X and Y, 0.2 mm in Z.
The results are summarized in table 2.
