Nummela in Finland is home to the world’s most accurate line

People have travelled to Nummela in Finland even from the Far East to use the faultless distance measurement of nearly a kilometre required for the accurate construction of nuclear power plants and bridges. The Nummela Standard Baseline is the world’s most accurate distance measurable between two points in the terrain, which attracts industrial customers, researchers and representatives of land survey organisations from across the world. The Finnish Geospatial Research Institute (FGI) of the National Land Survey of Finland (NLS) verifies the line once or twice a decade with measurements, for which fieldwork was recently completed.

8.1.2025
Research Manager Jorma Jokela using the Väisälä interference comparator
Measuring the normal baseline is a demanding project in terms of expertise and weather conditions. The measurement was carried out using the Väisälä interference comparator, which measures length with the help of light beams reflected between mirrors.

Nummela is home to a line 864 meters long, whose end-to-end distance is the most accurate in the world. It is known to less than one tenth of a millimetre. Originally, the geodetic standard baseline in Nummela was used to provide the basis of a scale for Finland’s maps. Today, the line is used to check the accuracy of the world’s most accurate electronic distance measurement instruments (EDM). It brings customers from all over the world to Nummela to check their devices. Such level of accuracy is needed, for example, in the construction of dams, bridges and nuclear power plants, as well as in the development of new more accurate distance measuring instruments.
Checking the distance of the standard baseline is called ‘interference measurement’ in the language of researchers. Fieldwork for the measurement was carried out this autumn whenever the weather permitted it.

‘This was a demanding project in terms of expertise and weather conditions. The measurement was carried out using the Väisälä interference comparator, which measures length with the help of light beams reflected between mirrors. For this reason, the measurement required the correct weather conditions. In autumn, the thick cloud cover at night provided the best conditions, when we conducted measurements for as long as possible,’ says Research Manager Jorma Jokela from FGI.

A Finnish export product  

The measurement method used in Nummela was developed by academician Yrjö Väisälä, who is known as a renowned geodetic engineer, astronomer and physicist. The esker in Nummela was selected as the standard baseline location because of its stable soil. Since 1947, the length of the line has varied by only 0.7 mm. The preliminary measurement result of this autumn’s measurements was the same as the last one in 2013 with an accuracy of one tenth of a millimetre: 864,122.9 millimetres.
The method developed in Finland is known internationally, but the scale transfer has also been an attractive export product for expertise. The Nummela scale has already been exported to about 30 countries, including Estonia, Lithuania, Austria, Germany, Spain, China and South Korea.

Further information

Jorma Jokela, Research Manager, +358 29 531 4743 firstname.lastname@nls.fi

 

Research
Finnish Geospatial Research Institute (FGI)
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