This target marker was added to the collection of scientific instruments and teaching aids together with around 320 other surveying instruments from the Institute of Geodesy and Photogrammetry. It was used for surveying work during the construction of the Simplon Tunnel.
The Simplon Tunnel
The Simplon Tunnel, consisting of two single-track tubes, was built in 1898—1905 and 1912—1921. At a length of almost 20 km, the tunnel connects Switzerland (Brig, Valais) with Italy (Iselle di Trasquera, Piedmont). Compared to the three main Alpine tunnels at the time, Mont-Cenis, Vorarlberg and Gotthard, the Simplon Tunnel was not only the longest, but at 705 m. above sea level, it had also the lowest apex.1. It remained the longest tunnel in the world until 1982.
Rosenmund and surveying the tunnel
Max Rosenmund was Professor of Surveying and Geodesy at ETH from 1904 to 1908. He had been working on determining the direction of the Simplon Tunnel and carried out the surveying work starting in 1898. In the process, he calculated the vertical deflections in the network calculation, which caused a stir and attracted much attention as a pioneering improvement.2
On the occasion of the breakthrough of the first tube in 1905, Max Rosenmund published an article describing the tunnel’s construction in detail. The triangulation method he chose to survey the tunnel is described on p. 85 et seq. The “triangulation field work”, i.e. the on-site angle measurements, was carried out in the summer of 1898 and took 70 days.3
However, directional measurements also had to be taken inside the tunnel, using illuminated target or tunnel markers just like this one. Rosenmund vividly describes how arduous this surveying work in the middle of the mountain must have been:
«The surveying engineer sets up a theodolite or a transit telescope at the last safe point surveyed. The previous safe point outbound from the tunnel is used as a point of direction. A lamp is set up here and once it has been sighted, he penetrates the telescope and re-sights a point further in. But this does not always happen as quickly as described here. No sooner has our engineer set the sights than we hear an “Attentione” from afar. A horse-drawn train is coming along; it doesn’t stop, it just keeps going, and our engineer has to make sure that he gets to the side quickly with his instrument if he doesn’t want it to be damaged. Then we set up again; the instrument is in place, the lamp can be seen in the correct position on the point behind, now it will work! Then explosive shots crackle from the spot, the lamps have been extinguished by the air pressure, everything is in darkness, and by the time the lights are lit again the smoke from the shots is already there, making it impossible to see at a further distance. No wonder the surveying engineer sometimes packs up impatiently and calls out, ‘See you tomorrow!»Rosenmund (1905), pg. 90.
Surveying theodolites and target markers in use
Two Kern Aarau surveying theodolites and target markers or lamps were used in the Simplon Tunnel for the work described here. Acetylene lamps, which were used in mining and tunnelling because of their strong luminosity, are mentioned in Rosenmund’s article. An illustration in the article shows a heavy tripod on which a kind of target marker is set up, but without the plate with the five circles as shown in our model. The gas tank for the acetylene is attached below and connected to the lamp.
The target marker in our collection contains a candle as a light source. This illuminates the metal plate with the five circles, which can be sighted using the telescope or theodolite. The brass box protects the candle from wind or the aforementioned detonations. This video, which we produced for the ETH Library’s Treasure Troves campaign, shows how the target marker works:
More important than the tunnel marks or lamps were, of course, the surveying theodolites. The collection of scientific instruments and teaching aids comprises a surveying theodolite from Kern. However, this was not used in the construction of the Simplon, rather in that of the Lötschberg tunnel (blog post to follow on 27/09/2023). One of the two Simplon instruments, which looks almost identical, is now in Stadtmuseum Aarau. As can be seen from the Stadtmuseum Aarau inventory sheet, the Kern theodolite was initially brought to the museum on loan from the Institute of Geodesy and Photogrammetry at ETH. Therefore, it was also originally in the possession of the Geodetic Institute. Max Rosenmund would have given it to the Geodetic Institute together with the target marker.
Selecting this object for the collection was easy for us, as we knew that it was used in the construction of the Simplon Tunnel. Such a precise attribution to a historical event and its use is rather rare in our collection and therefore of special value.
Using the 3D model, we were also able to gain further valuable information about the object: The stickers on the underside remained hidden from us during the initial inventory (see annotations 6-8), as we never looked at the object from below. It was only when attaching the annotations, i.e. when moving the digital object, that we came across these stickers. Especially with heavy or bulky exhibits, where the underside or other hidden details are difficult to see, 3D models prove to be an invaluable source of added value for inventory and documentation.
Photogrammetry and modelling
The black painted metal was easy to capture using photogrammetry. The level with the glass cover had to be added manually as usual. The candle was also difficult to capture using photogrammetry and was therefore added manually. The animated structures were also created manually.