accurate drawings. Adequate fieldwork
must be undertaken and special techniques
of recording and of taking lines of the
hull of the craft must be developed. Sev-
eral methods of recording the lines of
historical ships were used by historians
in the past (HABS 1988) and fine drawings
based on recording fieldwork were pub-
lished since the second half of the 19th
century (Paris, EF. 1882). However, even
until today it does not seem that a com-
mon and satisfactory method exists for
recording all kinds of historical ship.
shown that a simple
ship is not sufficient to
geometry and construc-
tion, as it would be for a building. For
a Naval Engineer to be in a position to
study a certain ship, specific plans are
necessary. These are called ship-lines
plans. They represent sections of the
ship's hull with planes parallel to those
of a specific three dimensional cartesian
system (Figure 1). This co-ordinate sys-
tem is defined with the help of the
ship's main axis and the vertical. The
three sets of sections (waterlines, body
Common practice has
drawing of the
reconstruct her
plan lines, and bow and buttock lines)
are related to each other in this draw-
ing, in such a way that all inter—
sectional points may be accurately inter-
polated with their help.
B2 B1 | B1 |
RT
25
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T
p
I
As
AT
Figure 1
The accuracy required for this kind of
documentation is not extremely high. A
few centimetres are usually enough, con-
sidering that it is almost impossible to
have access to the original wooden hull,
after so many years and so many repairs.
The form of the hull, as it is preserved
today, is usually all it is asked for.
The usual method of recording a vessel is
with the help of orthogonal co-ordinates.
The various lines are materialised
through plumb bobs and measuring tapes.
Simple readings on the tapes determine
the three dimensional position of the
points observed (Figure 2). The main dis-
advantages of this method are the low ac-
curacy achieved, the need for immediate
access to the hull of the ship and the
time necessary to complete the fieldwork.
As an alternative classical geodetic
methodology has been tested (Papapolitou
& Frangaki 1990). Although accuracy has
been greatly improved, tediousness has
increased, as every point had to be indi-
vidually marked.
Figure 2
3. PHOTOGRAMMETRIC METHODOLOGIES
is obviously a lucrative
alternative. It should, however, be
approached cautiously. Three different
photogrammetric methodologies have been
put- to test in order to be extensively
examined for their potential and for
their usefulness.
Photogrammetry
The three methodologies are analogue,
analytical and a hybrid low cost one. The
instrumentation used and the procedure
for each method is presented in Table 1.
The relative merits and demerits for each
one are quite clear from the above. À few
control points are naturally needed for
all methods. For the analogue method ca-
reful planning of the photography should
be made, for the accommodation of the
model on the specific instrument. The
achievable accuracy lies within the cen-
timetre boundary, considering the diam-
eter of the measuring mark and the gen-
eral capabilities of the instrument.
The analytical method needs no special
reference. It has proved beneficial to
use colour slides on the MPS-2 instead of
B&W negative film, for obvious reasons of
better detail recognition. The accuracies
attained with this method are well within
the specifications, although the calibra-
tion parameters of the camera were not
taken into account and the scale of the
photography was relatively small. As a
by-product a side elevation of a tradi-
tional ship was produced (Figure 3).
The low cost hybrid method deserves per-
haps a special mention, as it combined
simple digitizer measurements on 3x
enlarged P31 prints with a photogrammet-
ric software developed by the Laboratory
of Photogrammetry of NTUA (Georgopoulos &
Spinou 1992) and a commercial DTM produc-
ing software (SURFER v.4.00). Thus the
