The two springs known as 'Goul de la Tannerie' and 'Goul du Pont' are placed in the 'vallon de Tourne', Ardeche, near a Roman archaeological site.
'Goul du Pont' is a famous underwater cave for technical scuba diving, whose exploration began in the middle 50's by French underwater speleologists.
The maximum depth of -185 m, at a distance of 575 m from the entrance, was reached in 2006.
The underwater photogrammetric survey was very challenging due to the morphology of the cave and covered the sections described below. The entrance is located at -12 m, along a slope of pebbles. An almost horizontal tunnel with an oval section leads to the first well at a distance of 75 m from the entrance and a depth of -18 m. The path becomes almost vertical up to – 33 m, followed by a succession of vertical steps up to - 60 m (125 m from the entrance). Here the section reduces and, with a smoother slope, the tunnel reaches a depth of -79 m at 157 m from the entrance. At this point, the section dimensions further decrease up to about 1.2 x 2 m.
The survey was realised by Olivier Bianchimani and his team with a professional-grade DSLR camera (Nikon D700) equipped with a 14mm Nikkor lens, secured in a Nauticam waterproof housing coupled with a dome port. To maximize the image overlap in such a narrow space, the image acquisition followed a helical path.
4452 images were acquired and processed following a semi-automatic pipeline. In the most critical and narrow sections, the automatic feature extraction and matching algorithms failed; consequently, tie points were manually identified to connect and orient together the different parts.
Scale bars with circular coded targets provided the scale to the photogrammetric model. and connect underwater survey with aerial one.