In: Paparoditis N., PieiTOt-Deseilligny M.. Mallet C.. Tournaire O. (Eds). 1APRS. Vol. XXXVIII. Part 3A - Saint-Mandé, France. September 1-3. 2010 
m 
When the model is unified and optimized we proceed to register 
the images. The next step is the creation of a surface model 
based on the generation of a triangular mesh. This surface leads 
to an improvement of the photo-realism of the results. 
The last phase of the project is the generation of by-products 
such as (Figures 9-12): 
Total and partial 3D Model: this is the final result of work flux 
described above. 
Figure 9. Complete 3D laser model of the Wall of Avila. 
XY map: easily derived from the 3D model. Since the model is 
geo-referenced in a geodetic national system it can be easily 
exported to other cartographic data bases. It leads to metrical 
analysis and comparison. 
Figure 10. XY map of the wall from laser data. 
Videos and virtual fly-through: with the 3D model impossible 
points of view may be accessed. A variety of fly-through can be 
rendered aiming at the dissemination and promotion of the 
cultural heritage. 
Figure 11. 3D laser photo-realistic model (Western wall) 
Height maps and contours: is an easy derived product from the 
laser geo-referenced model. 
Figure 12. Contours of the cathedral apse, integrated in the wall 
(equidistance: 20cm). 
True Orthophotos: This product allows realizing dimensional 
analysis such as measuring the distances and computing the area 
of cracks, humiditys and towers. These measures sustain the 
development of control and monitoring tasks in zones that are 
under risk. 
Cross sections and profiles: We can obtain a series of 
longitudinal and lateral profiles from the original model. These 
documents permit thorough analysis of the geometric 
dimensions of the wall. 
Accuracy control: one of the most important aspects of this 
project considering the large-size and closed shape of the object 
is the achieved accuracy of the global model, especially the 
accumulative fashion of error propagation which involves a lot 
of laser scanner stations. To this end. a network of control 
points distributed along the battlements of the medieval wall has 
been designed and measured by RTK GPS. These control points 
have allowed us to guarantee the global adjustment 
convergence. Finally, in order to control the accuracy of the 
global model several distances have been checked in favourable 
cases (distances measured over the same wall) and unfavourable 
cases (distances measured between different walls). The 
following expression has been used to this control: 
< #' = JÊÈ[( J>i )is -( d¡ >)ars} 2 /■». ' < J 
(5) 
being (djj) LS the distance between laser control points, and 
(dji)is the distance between GPS control points. 
As a result, discrepancies of 2 cm have been achieved in the 
favourable cases, since discrepancies of 5 cm have been 
obtained for the unfavourable situations. 
4. CONCLUSIONS AND FUTURE PERSPECTIVES 
In this paper a real work is reported in documentation of a large- 
size and closed shape historical site. Several practical methods, 
such as terrestrial laser scanner and low-cost aerial 
photogrammetry have been integrated and applied in this 
project. 
The results attained are due to the sustained effort of a large 
number of students, researchers and teachers from the 
University of Salamanca. The size of the work is clearly 
expressed on the figures related to the data volume and 
dimension of the object. Nevertheless, this numbers must not 
eclipse the huge work and effort of the processing task. Even 
though laser technology is already completely developed and 
extended, the size of the object conveys a special meaning to 
173