In: Paparoditis N., Pierrot-Deseilligny M.. Mallet C.. Tournaire O. (Eds). 1APRS. Vol. XXXVIII. Part ЗА - Saint-Mandé, France. September 1-3, 2010 
170 
2. METHODOLOGY 
It is important to stress the difficulties that arise from the 
particular features of the walls of Avila, mainly its size. The 
wall towers have an average height of 15 m (up to 20 m in some 
of the doors) and this feature influences the position of the laser 
stations in order to guarantee an adequate covering of the object 
having in mind the device vertical field of view. In addition, the 
length of the perimeter is 2516 m and so, a high number of 
stations become necessary. This, in time, demands a careful 
design of the framework to guarantee the adequate coherence 
and soundness in the results. 
Figures 1 and 2 show the procedures and methodology followed 
in both the laser scanner and the camera work flows. 
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Figure 1. Scheme of the laser data flow 
Figure 2. Scheme of the camera data flow 
2.1. Field Work 
Data acquisition with laser scanner: The instrument used is a 
time of flight laser scanner from Trimble. This instrument 
collects data with a horizontal field of view of 360° and a 
vertical field of view of 60°. This poses an important limitation 
when rendering a high object from a close distance as in the 
present case. So, a special head must be used to cover the higher 
parts of the wall. A previous analysis was carried focusing on 
the main obstacles, occlusions and restrictions of the 
environment. The goal of this analysis is to reach a balance 
between the maximum coverage of the object and the minimum 
number of stations (figure 3). It is of the major importance to 
note that a large number of stations can easily spoil all the work 
because of the cumulative fashion of error propagation when 
orientation is transmitted from one laser station to another. On 
the other side, an adequate spatial resolution is set to obtain a 
good level of detail of the object. Last but not least, a local 
coordinate system is fixed that permits a precise reference of all 
the data in a unique and straightforward way. 
v" 
Figure 3. Example of laser station with adequate overlap 
High resolution images: The taking of high resolution images 
(both terrestrial and aerial) completes the data acquisition phase. 
This data overcomes the main laser scanner drawback, that is. 
the lack of radiometric information. The reflex camera is a 
Nikon D80, previously calibrated at the laboratory. The 
planning must ensure a complete acquisition of the radiometric 
information of the walls. Some guidelines must be followed to 
achieve the best results: to avoid every cast shadow produced by 
any object in the surroundings; to choose cloudy days and to use 
a tripod to obtain the best illumination conditions. It is 
recommended to use a low ISO number and a diaphragm 
aperture that meets a compromise between image quality, noise 
and depth of field. Another relevant issue is to acquire the 
images in a raw format because it happens to work as the 
conventional negative film, collecting information with the 
lowest level of noise. In this way the image may contain as 
much as 12-14 bits per channel. In addition, while processing 
the digital developing, the radiometry may be optimized by 
modifying parameters such as the vignetting, the exposition, the 
white balance,... The images must provide a pixel size projected 
on the object (GSD-Ground Sample Distance) between 5 and 
10mm to achieve the optimum resolution on the final results, as 
stated by the equation: 
GSD = -^ (1) 
f 
where D is the distance between the camera and the object, s x is 
the pixel size and / , the camera focal length. Multiple 
overlapped images are taken trying to ensure the same object 
registration of that of the laser scanner. To render the side part 
of the towers, some additional images are taken pursuing to 
avoid major perspective effects. (Figure 4) 
Figure 4. Adequate positions of the reflex camera 
Images from the blimp: The aerial images are taken from a 
captive blimp. A self stabilizing platform attached to the blimp 
lodges de camera. This camera provides images that complete 
those acquired from the ground. The aerial view permits to 
access a privileged “bird point of view” that completes the 
means of object exploration. As in the case of the terrestrial 
images, a previous planning must be accomplished in order to 
guarantee a thorough documentation of the object. A critical 
factor in this case is the wind: if its velocity is above 5km/h 
problems arise (Figure 5). 
Figure 5. Blimp (left) and aerial image of the northern wall 
(right).