From beneath your feet to outer space

The national survey conducted in 1903 was replaced in the 1990s. Since 1995, swisstopo has been operating a new network of surveying points throughout Switzerland. The first measurements relied on GPS, an American satellite navigation system. Nowadays, the surveying antennae receive signals from the four major satellite navigation systems – GPS, Europe’s Galileo system, Russia’s GLONASS and China’s Beidou system. These four systems also come together under the name of «Global Navigation Satellite Systems», or GNSS for short. Only a handful of changes have been made to the network of points since the introduction of the new surveying process, as there is a great interest in being able to measure the same points over a long period. The decision to have 220 fixed points is a compromise between the resulting benefits and the associated costs. Significantly fewer points would result in lower quality, whereas many more points in the network would drastically intensify the effort required to complete the measuring campaign. In addition to the number of points, the location of the surveying points also plays a role. Ideally, the fixed points should be drilled into bedrock in order to prevent them from moving. The points also need to be easily accessible and clearly visible from the sky to ensure a consistently reliable contact with the GNSS satellites.

15 weeks on tour

Every six years, two teams of surveyors from swisstopo spend one summer travelling around Switzerland. They re-survey and verify the 220 points (see map). This work makes it possible to identify any movements in the earth’s crust. Thanks to the frequency of the monitoring, discrepancies are always very small. The data remains up to date and expert knowledge can be retained and passed on. The work proceeds section by section. During one week (= one section), each person surveys six to eight points. This involves mounting an antenna onto a tripod and positioning it within a millimetre above the point fixed into the ground. The antenna is then connected to a special GNSS receiver that records the measurements. Each point is visited three times. During the first visit, the antenna and receiver are set up. On the second visit, they are monitored, and on the third visit, they are dismantled. This process guarantees that each point is surveyed for a period of at least 24 hours.

Reducing the workload with digitalisation

Permanent stations are fixed antenna and receiver systems that function without human involvement. The 41 stations operated by swisstopo constantly receive signals from the GNSS satellites. Permanent stations deliver highly accurate and daily up-to-date results. However, they are expensive to build and operate, which means that it isn’t possible to replace the 220 fixed points with permanent stations. As a compromise, the permanent stations serve as a reference for the 220 fixed points. The data collected by this configuration are subsequently processed. This processing is largely automatic thanks to the advance of digitalisation. The combination of permanent stations and fixed points ensures that measurements are accurate to within a few millimetres.

Planimetric fixed points of the first categorie LFP1 provide the reference framework for all survey work in Switzerland.

Challenges

Once the surveying stations are set up, they operate independently. An external battery supplies the stations with electricity for at least three days. Neither the energy supply nor the surveying process pose the major challenge in ensuring high quality data. Wind, weather and animals can interfere with the surveying process and jeopardise the results. For this reason, points located in exposed areas need to be secured with tension belts, guy ropes or even short railway tracks. With a bit of creativity and resourcefulness, a receiver may even be able to withstand strong winds or curious deer.

The current GNSS surveying campaign will continue until late October 2022. Experts at swisstopo will proceed with their work over the coming months. The data they are collecting will guarantee the accuracy of the Swiss coordinate system. As a result, it will remain possible to determine every location in Switzerland accurately for years to come. It will also be possible to continue to accurately plan routes with navigation systems and display jogging routes with swisstopo data.