Sigma 20 mm o
C 20.7944 mm +0.0064 mm
Xo 02235 mm +0.0115 mm
Yo -0.0954 mm +0.0092 mm
K, -2.51E-04 mm! | +4.24E-06 mm!
K; 5.49E-07 mm” | +2.56E-08 mm”
rms,y 0.091 mm
rms, 0.159 mm
Prop. error XY 1:27000
Prop. error Z 1:15000
Canon 35 mm o
C 35.3578 mm 0.0143 mm
Xo 0.1694 mm +0.0031 mm
Yo -0.0425 mm +0.0034 mm
K, -6.44E-05 mm | +3.42E-07 mm”
rms,, 0.199 mm
rms, 0.463 mm
| Prop. error XY 1:20000
Prop. error Z 1:9000
Table 1. Results of self calibration of the Sigma 20 mm and
Canon 35 mm lenses.
The root mean square errors (rms) in planimetry (rms,,) and
depth (rms,) are also expressed in Table 1. These errors (in
object space coordinates) have been obtained from comparison
on 35 target points from the two epochs. The largest distance in
object space (defined by the targets) has been compared with
the errors and the proportional accuracies (rms per distance),
both in planimetry and depth, are given. Better accuracy has
been obtained with the 20 mm lens (near 1:30000 in
planimetry). This can be explained as consequence of the
network configuration. In the 20 mm lens network, the object-
camera distance was around 2 m (adequate depth of field was
attained at that distance) and the network had a strong
convergent geometry. While, in the 35 mm lens network the
average object-camera distance was 5.5 m and the convergent
geometry was less strong than in the 20 mm case. Probably a
better network configuration in a wider workspace had
improved these results (see Atkinson, 1996, for a detailed
revision of network design and optimization).
These results indicate a good response of the tested camera and
lenses for metric applications in archaeology and architecture,
even in medium accuracy works. But it is necessary to check
out if the inner parameters are representative in field conditions
and at other object-camera distances, although with lens
focused to infinity. Calibrations were made at laboratory
conditions with retro targets and digital measurement
techniques (at subpixel accuracy). They are not usual work
conditions in cultural heritage projects, unless in case of special
works (wall deformations, high precision measurements, etc.).
Also convergent network have been used. Maybe in case of
using stereopairs, the weak geometry is not enough to
compensate for systematic errors that remain unsolved.
4. CANON D30 IN HERITAGE PROJECTS
4.1 Introduction
The Canon D30 camera can be used for documentation in
general cultural heritage projects just like any digital or
analogue camera. The reduced resolution (3.2 Mp) is the main
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part BS. Istanbul 2004
drawback of this camera. In our own experience (with the tested
lenses in this paper) when camcera-object distance increases
above 15 m, lack of image resolution appears.
An example is shown in Figure 4, which illustrates an
Almohade watchtower (XII" century) exceptionally well
preserved since the construction material is mainly mud (cob
wall) and there are rests of the original battlements (Orcera,
Spain). Figure 4 includes photographs of the tower walls and
the WRML model of the tower. At the time of the publication
of this work, the tower will be being restoring.
Photorealistic WRML
model of an Almohade
watchtower (XII^ century) in Orcera (Spain).
Photos taken with Canon D30 with 35mm lens.
Figure 4.
But this camera can be used with metric purposes as it is shown
by Mata et al (this volume). The Canon D30 camera has been
used in combination with other camera types (metric, semi-
metric and non metric analogue cameras) in a complete
photogrammetric documentation project for the restoration of
the St. Domingo de Silos’ Church (XIV" century) in Spain
(UJA, 2003).In next sections some examples are given in order
to show the possibilities of using this camera in real field
works.
4.2 Study of a rib vault and walls in a restoration project
4.2.1 Vault. First example is the study of a rib vault in the
St. Domingo de Silos Church (Mata et al, this volume). The
vault covers the main chapel of the church (Figure 5). It has an
asymmetrical plan (between rectangular and trapezoidal) of 6 x
7 m and a height between 3.5 m (at the rib springing) and 7 m.
Zenithal shots were made from the ground organised in three
strips with the 20 mm lens (Figure 6). Projection centers were
separated 1.5 m (in the same strip) and the strip axes were
approximately separated 3 m. That configuration allowed high
end and side laps (80% and 25-45%, respectively) in order to
minimize the relief displacement in the images because one of
the objectives was an orthophotograph (Figure 5).
Control, check and pass points were manually measured in the
images with ENVIC. Phototriangulation was carried out with a
routine developed under IDLO.
Interne
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