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Finnish Geodetic Institute
Geodeetinrinne 2
P.O.Box 15
FI-02431 MASALA
FINLAND
Tel. +358 9 295 550
The Finnish Geodetic Institute maintains standards for geodetic and photogrammetric measurements and acts as a National Standards Laboratory of length and acceleration of free fall (Law no. 581/2000). Standards in geodetic measurements include quartz meters, geodetic baselines, precision tacheometers and other high precision electronic distance measurement (EDM) instruments, laser interferometers and comparators for levelling rods. We take care of the traceability of these, and perform high precision measurements and calibrations in various geodetic applications.
In 2002 the Finnish Geodetic Institute joined the Mutual Recognition Arrangement (MRA) of national measurement standards and of calibration and measurement certificates issued by national metrology institutes. Our new quality system meets the requirements of the standards ISO/IEC 17025 and ISO 9001.
The metre is the length of the path travelled by light in vacuum during a time interval of 1 / 299792458 of a second. The traceability expresses the connection between this definition, international and national standards, and actual length measurements. A traceability chain may consist of various comparison methods and known uncertainties in them.
We use quartz meters to get the scale in the measurements with the Väisälä interference comparator. The lengths of these 1-m-long measurement standards have been determined in absolute calibrations and comparisons.
Absolute measurements have been performed e.g. by the Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, Germany, for the quartz meters no. 30, 49 and 51 in 1995. A Kösters comparator, a modification of the Michelson interferometer for measuring gauge blocks, was used. Recently, in 2000, the Centre for Metrology and Accreditation (CMA) in Finland performed new absolute calibrations for the quartz meters no. VIII, 49 and 51.
The comparisons are based on optics, too, on recording and measuring interference fringes. Comparisons have been performed in the Tuorla Observatory of the University of Turku, Finland, since 1930s. The quartz meter no. 29 is the principal normal in the comparisons; the new quartz meter system BTM00 is based on measurements in both PTB, CMA and Tuorla. The quartz meters used in the baseline measurements (usually 49 and 51, or VIII in Nummela) are compared with the normal before and after every measurement project.
Absolute calibrations and comparisons together create the traceability to the definition of the metre. The uncertainty (k=2) in the absolute measurements is ±70 nm, and ±10 nm in the comparisons.
In the interference observations with the Väisälä comparator the lengths of quartz meters are multiplied into lengths serviceable in geodesy and surveying, especially for calibration of EDM instruments.
Calibration baselines can be measured with high precision EDM instruments, too. The uncertainty (k=1) of the Väisälä baselines is ±0.03 mm - ±0.09 mm, or ±0.1 ppm, and ±0.1 mm - ±0.2 mm, or ±0.1 ppm - ±0.2 ppm, for the most accurate EDM baselines.
We use a laser interferometer to bring the traceability in the calibration of levelling rods. The frequency of the light source has been compared with the iodine stabilized laser of the CMA, last in 1998. The uncertainty (k=2) of the laser interferometer is ±20 nm, and ±0.4 ppm in the rod calibration.
Still in the 21st century the Väisälä interference comparator is the most accurate instrument to measure lengths less than 1 km. The Nummela Standard Baseline, 40 km north-west of Helsinki, is the most famous baseline measured with the method developed by the academician Yrjö Väisälä in the 1920s.
A baseline is a permanently marked distance, the length of which is known and traceable to the definition of the metre with a known (small) uncertainty. Baselines are used as measurement standards e.g. in length transfer to electronic distance measurement (EDM) instruments and measurement wires and tapes. Standard baselines are established as national or international length standards. These and other calibration baselines serve in various tasks of geodetic metrology.
Nummela Standard Baseline (In finnish).
The Finnish Geodetic Institute has measured the Nummela Standard Baseline with the Väisälä interference
comparator 13 times during 1947 - 1996. The latest remeasurement gave the result 864122.75 mm ±0.07 mm for
the baseline in its entirety; the length has varied less than 0.6 mm in a half a century. The results are published
in the Publ. of the FGI, no. 127. New remeasurements will take place in Autumn 2005.
Nummela Calibration Baseline.
We use the Nummela Standard Baseline in the calibration of high precision length measurement instruments and in
scientific research. For the more common calibration needs we have the Nummela Calibration Baseline in the immediate
vicinity. Usually people just want to determine the scale error and the additive constant of their field work EDM
instrument/prism combination. This is possible at the calibration baseline, which consists of five observation
pillars within a 840-m distance. Observing in all combinations ten different lengths can be measured; the uncertainties
in the known lengths are ±0.1 mm - ±0.3 mm. There is a rail for determining short-periodic errors
at the baseline, too. The calibration baseline is free and open to all the public. The latest comparison between
the standard and calibration baselines was made in 2004.
Slope distances between centre holes of observation pillars, with total uncertainties (95%):
| 1 - 2 | 120 198.5 mm ±0.3 mm |
| 1 - 3 | 240 226.1 mm ±0.3 mm |
| 1 - 4 | 480 238.0 mm ±0.3 mm |
| 1 - 5 | 840 231.9 mm ±0.3 mm |
| 2 - 3 | 120 027.6 mm ±0.2 mm |
| 2 - 4 | 360 039.5 mm ±0.2 mm |
| 2 - 5 | 720 033.4 mm ±0.2 mm |
| 3 - 4 | 240 011.9 mm ±0.1 mm |
| 3 - 5 | 600 005.8 mm ±0.1 mm |
| 4 - 5 | 359 993.9 mm ±0.1 mm |
Since 1947 the FGI has measured standard and calibration baselines around the world. Many
of them now belong to the history of geodesy; some of them still are of great importance in maintaining and developing
national measurement standards.
The Gödöllö Standard Baseline in Hungary, 30 km north-east of Budapest, was founded in 1938. In
1987 the FGI measured it first time using the Väisälä interference comparator (432 m) and a Kern
Mekometer ME3000 (864 m). The lengths were 432018.40 mm ±0.06 mm and 864032.16 mm ±0.10 mm. Since
then the stability has been monitored with yearly measurements with a Kern Mekometer ME5000. The interference measurements
were repeated in 1999. The new results were 432 018.35 mm ±0.06 mm and 864 032.12 mm ±0.10 mm, i.e.
the same as the old results. Besides Nummela, Gödöllö seems to be extremely stable, and suitable
for the calibration of the most precise EDM instruments. Our partner in co-operation in Hungary is the Institute
of Geodesy, Cartography and Remote Sensing (FÖMI).
The Gödöllö Standard Baseline
The Chengdu Standard Baseline in Sichuan, China
The FGI and the Sichuan Bureau of Surveying and Mapping measured the Chengdu Standard Baseline in Sichuan, China,
in 1998. This old calibration baseline was measured with the Kern Mekometer ME5000 of the HUT, and partly (384
m) with the Väisälä interference comparator. The result, the baseline in its entirety 1488017.4
mm ±0.2 mm, was as expected; the uncertainties of the 12 distances between pillars varied between ±0.03
mm and ±0.08 mm. Monitoring the stability of the baseline will be continued.
The Chang Yang Standard Baseline in China, 35 km south-west of Beijing was constructed in 1984. In 1985 the FGI
and the National Bureau of Surveying and Mapping measured it first time with the Väisälä interference
comparator. Remeasurements were made in 1990 and 1994. The results have not been the best possible, but yet one
order of magnitude more accurate than with any other method.
The Taoyuan Standard Baseline in Hsinchu, Taiwan, China, was measured with the Väisälä interference
comparator in 1993. Since then seismic activity has caused deformations. The high precision EDM instruments used
in the monitoring of stability were calibrated at the Nummela Standard Baseline in 1997 by the Center for Measurement
Standards of the Industrial Technology Research Institute.
High precision EDM is a much simpler method to measure baselines than the interference measurements. However, only
the most precise instruments should be used, and calibrations of them are essential, too; accuracy parameters in
the manual of the instrument are not sufficient. The Nummela Standard Baseline is an excellent place for such calibrations.
Baselines measured with calibrated high precision EDM instruments can then be used in calibration and quality control
of lower order EDM instruments.
Sometimes a "stable" baseline proves to be unstable just after the interference measurements, because
of the extreme measurement accuracy. High precision EDM is a good solution before or instead of laborious interference
measurements, if the stability of a baseline is questionable.
The Kyviškes Calibration Baseline
The 1320-m Kyviskes Calibration Baseline, 25 km east of Vilnius, Lithuania, was established for calibration of
EDM instruments in 1996. In 1997 the FGI, together with the Institute of Geodesy of the Vilnius Gediminas Technical
University, measured it using the Kern Mekometer ME5000 of the HUT. The instrument was calibrated at the Nummela
Standard Baseline before and after the measurements in Lithuania. The baseline was extended to a triangle-shaped
test field in 2000, and remeasured in 2001. The results confirm the good stability of the baseline, which now can
be used in calibration of EDM instruments and testing of tacheometer and GPS measurement equipments. A similar
scale transfer project was performed in 2000 to the old Vääna baseline (1728 m) in Estonia, in co-operation
with the Maa-amet, Estonian Land Survey.
In the calibration of a measurement instrument we compare the results given by the instrument with the more accurate values represented by the measurement standard. In geodesy the measurement standards include e.g. baselines (for EDM instruments) and laser interferometers (for levelling rods). Angle measurement instruments are not calibrated in the FGI. Facts related in calibrations in gravimetry can be found at the web-site of our National Standards Laboratory of acceleration of free-fall (In finnish).
In the calibration of EDM instruments the observed values are compared with the known values. Refraction and geometric corrections are needed before further computation. Adjustments of observations produce the instrument errors, necessary corrections and accuracy estimates.
The Nummela Standard Baseline is the right place for calibration of high precision EDM instruments and for scientific
research. Our recent projects include consulting and measurements for geophysical deformation studies, for industry
and commerce, and international scale transfers.
As actual measurements are usually performed in field conditions, instruments should be calibrated in real field
conditions, too. The medium always have an impact on the propagation of measurement signal. Correct air temperature,
pressure and humidity values are of great importance, and must be determined with calibrated weather observation
instruments.
Self-service calibration facilities are available at the Nummela Calibration Baseline and at the Jämijärvi
Calibration Baseline.
The FGI has an air-conditioned laboratory for calibration of levelling rods in its main building in Masala.
The calibrations are usually performed in vertical position, but rods or other scales can be measured in horizontal
position, too. A laser interferometer is used as a measurement standard, and the comparator works in accordance
with the Abbe's principle.
To improve quality and efficiency the vertical rod comparator was fully automatized in 1997. The rod is moved in
the comparator using a linear conveyor, lift system and stepping motor. The position of the rod is measured using
the laser interferometer, and scale lines are registered using a CCD camera. An automatic weather station registers
the ambient temperature, pressure and humidity; reductions are needed for the rod length and the propagation of
laser light. The operation of the system is controlled by computer programs.
The vertical rod comparator can be used in calibration of rods with scales of both equal and varying (bar code)
distribution. The temperature of the laboratory can be adjusted between 5º and 50ºC, and the determination
of temperature dependency of rod scales is possible.
Our National Standards Laboratory of acceleration of free-fall serves in the calibration of gravimeters (In finnish).
| Markku Poutanen | Department head, Prof. | NSL Quality Manager |
| Pasi Häkli | Senior research scientist, M. Sc. (Tech), Tel. +358-9-29555222 |
EDM |
| Jorma Jokela | Specialist research scientist, Lic.Sc.( Tech.) Tel. +358-9-29555219 |
Laboratory Head; baselines, EDM and calibration |
| Paavo Rouhiainen | Senior research scientist, M. Sc., Tel. +358-9-47890490 |
Deputy Laboratory Head; rod calibration, precise levelling techniques |