The creation of the digital models for the protection of cultural heritage: the Baptistery of Cremona

For the architectural representation, the Texture Mapping application is becoming more and more important because it combines the image with all the semantic feature of the object surveyed to the extremely accurate solid modelling. Therefore, at the photogrammetric images we matched some scans taken with the terrestrial Laser Scanner, in order to provide on one side an image with an elevated metrical precision, and on the other side a thick and accurate digital elevation model (DEM). The key point of this method is the geo-referencing of the two surveys, to be carried out possibly with the same targets. In the Spring of 2002, it was realised a survey of the baptistery of Cremona (Italy) combining photogrammetric to laser scanner techniques; it is a building in Romanesque style, with octagonal plan and a side close to the cathedral. The height, almost 25 meters, has involved a special work for survey of the roof. All the survey is composed of 20 photogrammetric images taken with the digital camera Nikon D100 18/2.8 and 12 scans collected by Optech's ILRIS-3D Laser Scanner. The geo-referencing was carried out with some natural and artificial targets in the structure of the baptistery. The artificial targets used for this survey, were purposely projected by means of several tests, in order get the best radiometric response and univocal geometrical definition. Considering that targets must be in the best location to fix the external orientation of the images, it is often more helpful using natural targets surveyed with a topographic work. It is not easy to determinate these natural targets in the scans and it cannot be done in an unambiguous way. This work was made to investigate which should be the best configuration of reference points both for the laser scan and for the photogrammetric image and to evaluate whether the final result of the Texture Mapping can be improved by adding some artificial targets.