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Photograms – an Ever-Growing Treasure

For over 100 years, swisstopo has been surveying Switzerland with the help of photographs. During the surveying work on the ground and from the air, new images of the landscape have continuously been produced in addition to measurement data. Originally, the images were mainly used to produce maps. However, over the decades, a historical treasure has been unintentionally created. Today, it opens up new doors for research.

28.10.2022 | DKW

Analogue photograms from the swisstopo image collection.

Survey images have formed an important basis for Swiss national surveying since the beginning of the last century. Since about 100 years, swisstopo has been taking photographs of the Swiss landscape every year. They were used to produce and update maps. As a side effect, regular surveying of the same places created a valuable treasure: the largest collection of landscape photographs in Switzerland.

This is how it all began

In the 1910s, swisstopo began to use survey images for the production of maps. These were photographs taken from the ground, so-called terrestrial images. In the period up and extending to beyond the Second World War, more than 50,000 images were produced. This type of photogram is only possible in mountainous regions as the angle of view of the terrain to be photographed area must not be too flat. The topographers climbed hundreds of exposed station points to collect measurement data for map production. 

The future was in the air

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About ten years after the introduction of terrestrial photogrammetry at swisstopo, aerial images were also used for the first time. Aerial images are photographs taken from an aircraft. In early tests even from a balloon! The office founded its own aerial service as early as 1926. swisstopo began to take aerial images of areas of the country every year. By the late 1940s, the systematic taking of aerial images had completely replaced terrestrial photographs. Thanks to the rapid development of aircrafts and the camera systems on board, this rapidly simplified the recording of photograms.

For the first time, aerial images made it possible to conduct photogrammetric surveys of whole Switzerland. 

The Twin Otter - one of the aircraft currently in service at swisstopo.

For almost 100 years of close cooperation with the Swiss Air Force, the swisstopo air service uses federal surveying aircrafts to survey a third of Switzerland every year. This means that there is always an up-to-date photograph of the country. In addition, the environmental observation component is becoming increasingly important due to the rapidly changing landscape: swisstopo produces additional aerial images for this purpose.

Half a million images

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In total, the surveyors produced over 500,000 analogue images, most of which show Switzerland from a bird's eye view. Since 2010, swisstopo has been exclusively producing digital survey images. From the beginning, however, half of the images were not used directly in map production and tracking but rather for environmental monitoring on behalf of external institutions. Today, the historical survey images serve as valuable contemporary witnesses and offer unique past-present comparisons of the Swiss landscape.

The largest landscape memory

The continuity of swisstopo's image collection is unique. Photographs of whole Switzerland exist for the last 70 years and beyond. This is also unique in that the same place has been photographed repeatedly over decades. swisstopo has set itself the goal of digitising all existing analogue images and making them freely accessible to the public. The digitised photograms open up new possibilities for environmental monitoring in many areas. Calculating the volume of landslides, glacier melt, rock displacements after landslides, to name just a few applications.

Terrestrial Photogrammetry

The photograms used for terrestrial image measurement are photographs taken from an exposed point. They record the visible terrain. Photograms were once used in map production and are now increasingly used in environmental observation. Photogrammetry requires at least two photos (a pair of images) taken from different angles relative to the same reference object. Terrestrial photogrammetry is only possible in the mountains as the angle relative to the photographed landscape must be sufficiently sharp. Consequently, this is not possible on the Central Plateau.

If the distance between the two photograph locations (the so-called baseline) is known, the distance from the photographed object can be calculated together with the camera parameters (such as the focal length). Photogrammetry is used, among other things, in glacier monitoring. If you now compare the photogrammetric calculations from pictures taken in different years, you can visualise changes in the environment. 

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