2011 - Geoinformatics

A r t i c l e
Fig. 5 – Wireframe model with local photo-textures
images, bypassing the heavy step dedicated to each photo projection
and the need for several homologous tie points.
Altogether, fifteen scans (each of 800 Mb) were produced, seven
over the exteriors and eight for the interiors, with the aid of thirteen
plane targets (mounted over a mobile support to optimize the survey
inter-visibility) located inside a selected area and suitable for the
cloud connection.
The external survey was undertaken in the morning under a clouded
sky, this way avoiding the unwanted effects of image shadowing.
The interiors were measured under artificial lighting due to the low
level of natural illumination provided by the windows.
The church model and photo-texturing
With the aid of targets, the sanctuary point model was generated by
connecting each cloud to the central one, selected as reference. From
the same scanning positions, proper overlapping images were acquired
via manual shooting using the Nikon camera.
The following photo-texturing process was automatically carried out
thanks to the PTGui stitching package, in order to perform spherical
multi-image panoramas, which were then imported inside 3D
Reconstructor software, switching on spherical projectors for model rendering.
A series of photographic images is shown describing the building façade
Fig. 6 – Perspective view of the textured model
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and the corresponding spherical panorama produced
with PTGui (Fig. 4).
Finally, the following figures (Figs. 5, 6) show some
striking perspective views of the reconstructed 3D
church model (points and meshes), with the phototexture
superimposition.
Entity edge detection, for the building vector drawing
generation, was done by automatic extraction
of the angular discontinuity lines from the point
model; this 2D product is performed by applying
geometric tests regarding the local attitude of the
surface-perpendicular in every point of the model.
The process provides sketchy elevations of a building.
Figure 7 shows an example of the church
main elevation, achieved both through the angular
surface discontinuity analysis (sketchy drawing
being completed in CAD) and by tracing a preproduced
colour orthophoto.
Final remarks
Laser scanning techniques provide an interesting
3D point or surface model, which can be
integrated with colour images and is fully measurable.
The extraction of 2D plots from this model is still a complex step:
starting from the scanned data and using cutting planes, it is possible
to produce horizontal and vertical cross-profiles to be used
together with orthophoto backdrops to complete building cross sections.
However, 2D elevations have to be realized with the heavy
support of manual editing and the photo-texturing step causes the
greatest difficulties and errors.
Nevertheless, the 3D model and its derived 2D plots represent an
effective tool for the metric analysis of a building (geometry, shape,
symmetries, alignments, parallelisms), for thematic inspections
(colours, materials, preservation and/or decay condition) and globally
for the generation of a knowledge database of an historical
monument.
Luigi Colombo, luigi.colombo@unibg.it, is professor of Geomatics and
Barbara Marana is assistant professor at the University of
Bergamo - Faculty of Engineering - DPT - Dalmine (Italy)
Acknowledgements:
Thanks are due to some graduated students of Geo-Technology Lab
at the University of Bergamo and to 3DTarget for Z+F technology support.
Fig. 7 – Sketchy elevation for the main façade (to the left); the same drawing performed by orthophoto
tracing (to the right)
October/November 2011