3.1 Surveying activity
Firstly a surveying plan phase was performed in order to
determinate the most appropriate image scale for the wanted
accuracy; particularly the aim was a restitution scale of about
1:100, which associated accuracy is, in according with
graphicism-error, ±2cm.
Due to the object height, the acquisition of images at the
necessary scale (1:400) should required two strips, but for logistic reasons, it was preferred to adopt a little inclination of
the optique axis: consequently the photos present a lightly lower
scale in the top part. The medium image scale for each object
side, with the associated restitution scale and accuracy, are
reported in the table 1. The plan foresaw photos both in pseudo-normal and convergent
trim, suited respectively for a stereo-scopic and a mono-scopic
restitution.
For the image acquisition a 24 mm semi-metrical Leica R5 film
based camera and a non-metrical Nikon CoolPix 5400 digital
camera were adopted, whose inner orientation parameters were
successively obtained by means of a self-calibration procedure.
The images were then digitized by KODAK Photo CD system.
To define the 3D coordinates of 10 natural control points, a
topographic survey was performed using a modern prism less
Total Station (Topcon GPT 6001). It was preferred to use
natural points in spite of a loss of accuracy in the collimation
phase, because of the aim of simulate common conditions of a
cultural heritage application and so in order to simplify on the
whole the survey procedure .
The adopted surveying scheme was the three-dimensional
spatial intersection from three vertex; the control points are
localized on two tomb facades.
Figure 2 A and 3 B show respectively a San Domenico square
plan with in evidence the position of the tomb and the three
topographical survey vertex (0A, 0B, 0C), and the scheme of
chosen control points.
Besides, direct object distance measurements were collected by
means of a metric ribbon, in order to evaluate the possibility of
carrying out a speditive model orientation using only distance
constraints.