are produced with a common CAD system.
3.5 Simple Measurements
Since technical equipment of various kind is readily
available in the field, simple hand measurements are used
only for small objects of very simply geometry such as
stone blocks. Hand drawings of stone sculptures of
consideral artistic and geometric quality (figures 6 and 7)
are created by a Chinese expert.
3.6 Close Range Photogrammetry
Stone sculptures carved from limestone decorate all
Tang mausoleums. Considering that they have been
standing in the open air for more than thousand years,
many of them are in a comparatively good condition. All
sculptures are photographed by the archaeologists. Close
range photogrammetry is applied to selected figures only.
Three methods are used. A complete digital object model
(DOM) is derived for some objects. Vector drawings
showing one or more views are used in other cases. For
less important objects, simple stereo models are photo-
graphed; the films are stored to make later plotting
possible, if desired.
Figure 5: Documentation of a stone sculpture.
Digital object models (DOM) give a full representation
of an object. The data could be used to reconstruct or
duplicate the complete object. Most stereo plotter software
packages have modules to facilitate measurements.
Nevertheless, the measurements are time-consuming
and very boring. When complete sculptures are docu-
mented, the DOM has to be put together from several
views of the object. The idea to derive DOMs for
three-dimensional closed objects by the way of automatic
stereo correlation was investigated. The results are
published in a separate paper (Boochs and Heinz, 1996).
Vector plans scaled 1:10 are needed for the more
important sculptures. A front view is essential. In most
cases, a side view is added, but up to four views can occur.
Those are derived from stereo models which are taken
with a 6 cm x 6 cm réseau camera. A 120 mm lens is used
in order to minimize hidden object regions. Stereo models
have to be taken from positions above and to the left and
right of the figure to make sure that all outlines are visible
in the models. This results in six to twelve stereo models
per view. Control points are drawn on adhesive tape which
is fixed to the stone surface. The coordinates of those
points are derived by intersections observed from
a conveniantly placed baseline with the total station.
A common setup is shown in figure 5.
An analytical stereo plotter is used to digitize the stereo
models directly into a CAD system. If the stereo models
are photographed in the proper way, the outlines of the
objects can be drawn with good results. Also, drawings of
object parts showing sharp edges do not cause any
problems. The representation of all other details, such as
faces or the drapery of robes is very dependent on the
interpretation of the operator. As a rule, the results lead
to arguments between the archaeologist ("the expression
of the face in the drawing is different from the original")
andthe photogrammetrist ("why do you wantto map things
which cannot be objectified?"). A rather long revision
process usually leads to a compromise accepted by both
sides.
Figure 6: Stone sculpture, reduced to 1 : 20.
Left: Photogrammetric plan. Right: Hand drawing.
52
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B5. Vienna 1996
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