This paper presents the use of photogrammetry connected to mine surveying and stability analysis at the Mining Department, the Norwegian Institute of Technology. Photogrammetry in general and the equipment in USB, a stereo camera and an ordinary photogrammetric plotting instrument are also shortly described.
Cet article montre l'utilisation de la photogrammetrie en connexion avec la topographie minière et l'analyse de la stabilite du terrain, faite à l'Institut des Mines, Ecole Polytechnique de Norvège, NTH. On donne aussi une courts description de la photogrammetrie en general et de l'equipment utilise, une camera stereo et un autographe ordinaire.
Es handelt sich um die Anwendung von Photogrammetrie in Verbindung mit markshceiderischen Arbeiten, bei dem Institut fuer Bergbau der Norwegischen Technischen Hochschule. Allgemeine Photogrammetrie und die dafuer verwendeten Gerate, eine Stereokamera und ein gewöhnlicher Autograf, sind auch kurz beschreiben.
As early as in the middle of the last century, photogrammetry, or the derivation of quantative information from photographs, was used for mapping the earth's surface. Still the main application of photogrammetry is the construction of topographic maps, but the method has demonstrated its potential in several other fields. At first this paper outlines the principles, and describes a possible instrument combination for close-range photogrammetry. Then the application of the technique to some mine surveying tasks and to a stability analysis is shown with case histories.
When the photographs are taken with a stereocamera, the situation is as shown in fig. 1. Three dimensional information can be obtained from the overlap area, the area shaded in fig. 1. This is done by placing the pictures in a plotting instrument, thus forming a stereoscopic model, which can be viewed and measured. The difference between the model and the object is in principle only that the rays go in different directions and that the scale is reduced. The expected accuracy depends on the square of the distance from the camera to the object (D2), the focal length (f), the base (B), and the parallax error (dp). The so called parallax error depends on the resolution of the photographic plates and on how accurately the photogrammetric operator works. The parallax error is generally 0.01 mm. A stereocamera with a base 1.2 m and a focal length 64 mm, gives an expected photogrammetric accuracy, dependent on the distance from the camera to the object, as shown in fig. 3. The graph in fig. 3 shows the accuracy connected with absolute measurements. Often only relative measurements are required, which give a better accuracy.
Photogrammetry demands an expensive instrumentation, and there is a need for specialized personell. Often the time needed for evaluation is long. On the other hand, the time on the site can be made short. Photogrammetry makes it possible to increase the number of basic data from an area without the same increase in time, contrasted with direct measurements. This is for example useful when the volume of an irregular body is searched. The method makes it possible to measure areas with limited access. The photographic plates provide a permanent record, and can be analysed when convenient. For example if the point is to find the changes after inbursts or other unexpected events. Because of the synchronisation of the shutters in the stereocamera, moving objects can be measured.
The camera used in these studies is a Wild C 120 stereocamera. The stereocamera consists of a stable base tube with a camera fixed at each end, symmetrical about the centre of the tube. The lens of the camera is focused at a distance of 10 m, in order to produce sharp images over a range of stereoscopic evaluation of 5 m to 30 m. Each camera has a Compur between lens shutter, giving exposure time between 1 second and 1/500 second, as well as time exposures in position "B". The electro-magnetic shutter release guarantees an accuracy of synchronisation of approximately 20 ms. The shutters can also be released by hand. There is a tilting head between the camera and the tripod. The tilting head permits photographs with axes at 10°, 30°, 60°, 80° and 90° above and below the horizontal, as well as horizontal axes. The tilting head also permits photographs to be taken with the base tube vertical. A picture of the stereocamera is shown in fig. 4.
The optic model of the object is formed in a Wild A 40 plotting instrument. The optical system used to view the model also provides the possibility of stereoscopic setting of a so called floating mark. The floating mark, a black dot seen on the model, is in a way the measuring tool. It is moved by two handwheels and a foot disc.
