If a mesh is projected onto an object by a projector having the 
same optical characteristics as camera employed, the 
photogram of the object with the projected mesh (raster- 
photogram)and the pseudo-photogram constitute the equivalent 
of a stereoscopic pair. Actually each projected point has an 
image on the raster-photogram (fig. 3) and an image on the 
pseudo-photogram (fig. 2). It is therefore possible to obtain the 
coordinates of the points projected on the object. This 
restitution is analytical and requires a specifically designed 
program which reconstructs the model from the plate 
coordinates of each point, measured on the raster-photogram in 
monocular observation, and from the coordinates of the same 
point on the pseudo-photogram. 
  
Figure 3 Portion of a Raster-photogram 
The accuracy of the monocular reading is directly related to the 
sharpness of the lines of the mesh. In fact this "sharpness" 
should be correlated to the device employed for the monoscopic 
measurements. The pseudo-photogram employed, which was 
already existing as it had been previously used for another 
survey, has the following characteristics: 
Two types of vertical lines, 
(pseudologarithmic) spacing: 
a. Principal lines, 0.08 mm thick, equally spaced 10 mm 
apart and identified with marks every 20 x 20 mm 
b. Secondary lines, 0.04 mm thick, of variable spacing. 
Starting from the centre, the sequence of intervals is as 
follows: 
S(1)=1+1x(+0.05;+0.04;+0.03;+0.02;+0,01;-0,01;- 
0,02;-0,03;-0,04;-0,05) 
for the first centimetre, 
S(n)=1+1x n x(0,05-1) 
for the ith space of the nth centimetre. 
Two types of horizontal lines: 
having variable 
24 
a. Principal lines, 0.08 mm thick, equally spaced 10 mm 
apart and identified with marks every 20 x 20 mm 
b. Secondary lines, 0.04 mm thick, equally spaced 1 mm 
apart. 
The plan for the raster survey is shown in fig. 4; the altar to 
camera distance is approximately 2.30 m for 3 of the sides of 
the altar and 2.60 m for the West side. The distances were 
conditioned by the presence of steps around the altar which 
made it impossible to place an object the size of the projector 
any closer. The projector/camera base about was 400 mm. We 
tried to make normal exposures with a horizontal base parallel 
to the altar and the axes of the camera and of the projector 
normal to the base. 
The bas-relief is so detailed that a greater photogram scale, 
that is a shorter camera to object distance, would have been 
appropriate. 
The projected points of the mesh are separated, on the altar, by 
a horizontal and vertical distance of approximately 2 cm. As 
can be observed in fig. 3, since the surface relief of the altar is 
fairly small, the points are rarely not visible; it is also 
uncommon for the background to hinder the points' visibility 
because of it's darkness. 
More than 3000 points were projected on the smallest side of 
the altar, the North one. 
3.2 Second Phase 
As mentioned previously, the second phase, which has only 
just begun, aimed at plotting points inside the mesh. For this 
purpose two stereo raster-photograms were necessary. Actually 
the 2 photographs were taken with the two cameras positioned 
symmetrically to the middle of the altar while the position of 
the metric projector was central to the altar. In fact, adhesive 
targets were placed on the edges of each side of the altar: these 
targets were connected to an external topographic network and 
made it possible to check the scale of the model and to relate 
the surveys of the four sides of the altar to the same reference 
system. The two stereoscopic raster-photograms made it 
possible to perform the restitution of the points which were 
inside the mesh Figure 4 shows the arrangement used. 
4. TAKING PHOTOGRAM 
Problems arose due to the reflections produced by the golden 
or silver surface of the leafs, which were only partially solved. 
Tests regarding lighting position were made in order to reduce 
reflections. Several exposures were made with films of varying 
sensitivity. PEY64 film was employed, generally with a 
diaphragm setting of 11 and exposure times varying from 3 to 6 
minutes. All exposures had to be made at night because the 
Abbey was being used during the day. The film was assured to 
be unshrinkable but in fact was found to exhibit some 
deformation in time. 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B5. Vienna 1996 
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