×

You are using an outdated browser that does not fully support the intranda viewer.
As a result, some pages may not be displayed correctly.

We recommend you use one of the following browsers:

Full text

Title
New perspectives to save cultural heritage
Author
Altan, M. Orhan

CIPA 2003 XIX th International Symposium, 30 September - 04 October, 2003, Antalya, Turkey
specifications: Agfa Rodinal and Kodak Technidol, the second
one being specific for Technical Pan film.
Figure 4. Design of the converging network.
3.1.3 Negative digitisation. The digitisation was done with
the use of a Vexcel Ultrascan 5000 scanner with a pixel size of
5 pm. The pixel dimension was calculated in relation to the
resolution of the APX 100 (150 lines/mm) film and the formula
proposed by Kraus (1993). The size of the pixel represents
approximately 1.8mm of the central area of the façade.
3.2 Photogrammetric process
The steps followed are the typical ones in the conventional
photogrammetric process: internal and external orientation and
DSM Construction. The difference in this case is that the
Orthobase Pro allows a simultaneous adjustment of these 11
images.
The DSM construction is accomplished by automatic methods
of stereo matching. However, Orthobase Pro cannot take
advantage of the network as a whole and the models are
constructed from only a pair of images. Thus, each combination
of two images allows for a different DSM. The results are
points that permit the construction of a Triangulated Irregular
Network (TIN) structure (Peucker et al., 1978).
3.3 Error control
The error control was carried out by comparing the z
coordinates (depth) out of the 75 checkpoints of the DSM
(Figure 5). The results are expressed as Root Mean Square
Error (RMSE). The DSM data for a concrete localization (x,y)
is estimated through interpolation based on the TIN. Thus, it is
interesting to have a dense TIN, formed by an elevated number
of points. The contrary is equivalent to a process of
generalization that would have foreseeable visible negative
influences on the error statistics.
The control points will not always be available in their totality,
since, according to the photographic pair, some may be in
hidden areas. In this case, the points are eliminated before the
error control is performed.
Figure 5. Distribution of the check points.
4. RESULTS
4.1 Adjustment error and network design
The internal orientations were adjusted with errors that were
always inferior to the pixel (5 pm) size. The external
orientations were adjusted with standard deviation values of
0.49 and 0.66 pixels. The calculation of the centres of
projection had identical results with both films, with variations
that never surpassed 9 mm with the exception of point 7 (14
mm). Network adjustments have been carried out from a
minimum of three images up to a maximum of 11. The
exactitude of the bundle adjustment was independent of the
number of exposures included in the network. The standard
deviation of the adjustment in the minor block (3 images) was
of 0.48 pixels with 980 tie points. In the 11 image block, the
standard deviation was of 0.49 pixels with 3400 tie points.
4.2 Stereo-matching Strategy
Although Orthobase Pro uses a “simplified structural matching”
algorithm (Wang, 1998), it is not clear in which stages of the
process this algorithm or other alternatives take part (Karras et
al., 1998). The configuration process of stereo matching by the
user can only be modified in minor factors: the selection of the
search window sizes and their correlation. It should be noted
that in epipolar images, the first of these factors might play a
less important role. However, our tests reveal a surprising result
that is detailed below.
The determination of the tie points is done automatically once
the support points and the size of the search windows and
correlation are defined. Apparently, the number of tie points
that are recognized is determined by a threshold value of the
correlation coefficient. The data associated to the tie points
include information on two interesting characteristics: the
number of images in which the point has been acknowledged
and what those images are. Analyzing the reports, two facts
may be determined: 1) most tie points are recognized in only
two images and 2) these pairs are formed by images with the
minimum convergence angle allowed by the block. When the
network is formed by 11 images, the majority of the tie points
are recognized in pairs with a convergence angle of 5°. When a
network is formed, for example, by images 1-5-9 the tie points