SUOMEN GEODEETTISEN LAITOKSEN JULKAISUJA VERÖFFENTLICHUNGEN DES FINNISCHEN GEODÄTISCHEN INSTITUTES PUBLICATIONS OF THE FINNISH GEODETIC INSTITUTE ======N:o 144 ======

INTERFERENCE MEASUREMENTS OF THE NUMMELA STANDARD BASELINE IN 2005 AND 2007

by

Jorma Jokela and Pasi Häkli

KIRKKONUMMI 2010

ISBN-13: 978-951-711-282-6 (printed) ISBN-13: 978-951-711-283-3 (PDF) ISSN: 0085-6932

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Contents

Abstract ...... 4 1 Introduction ...... 5 2 Landmarks in the history of the Nummela Standard Baseline ...... 8 2.1 First international recognitions ...... 8 2.2 Change from invar wires to EDM instruments as transfer standards...... 8 2.3 Recent construction works ...... 10 2.4 Importance in the 2010s ...... 13 3 Traceability chain of geodetic length measurements ...... 14 4 Quartz gauges in the determination of the scale ...... 16 4.1 Review of quartz gauge systems at Tuorla Observatory ...... 16 4.2 Comparisons at Tuorla Observatory ...... 17 4.3 Determination of the length of quartz gauge no. VIII in BTM00 ...... 21 5 Preparing the baseline for interference measurements...... 24 5.1 Principle of the Väisälä interference comparator ...... 24 5.2 Preparing the observation pillars for interference measurements ...... 25 5.3 Precise levellings – start of the measurements...... 25 5.4 Aligning the mirrors ...... 27 5.5 Setting the mirrors at correct positions in the baseline direction ...... 28 5.6 Installing the transferring bars onto the observation pillars ...... 31 5.7 Installations on the telescope pillar ...... 32 5.8 Installations on pillars 0 and 1 ...... 35 6 Interference observations ...... 36 6.1 Observation procedure ...... 36 6.2 About weather conditions ...... 41 6.3 Personnel ...... 43 7 Determination of corrections ...... 44 7.1 Compensator corrections ...... 44 7.2 Refraction correction ...... 45 7.3 Corrections due to mirrors ...... 49 7.4 Geometric corrections ...... 49 7.5 Projection corrections ...... 51 8 Computation of baseline lengths ...... 58 8.1 Computation of the actual length of the quartz gauge ...... 58 8.2 Results from interference observations in 2005 ...... 61 8.3 Results from interference observations in 2007 ...... 68 8.4 Final lengths ...... 77 9 Estimation of uncertainty of measurement ...... 78 9.1 Combined uncertainty of the lengths between the underground markers . 78 9.2 Some supplementary analysis of uncertainty of measurement ...... 80 10 Summary and conclusions ...... 82 References ...... 84

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Abstract The Nummela Standard Baseline of the Finnish Geodetic Institute is a unique national and international measurement standard for length measurements in geodesy. The design of the 864-m baseline was originally, in 1933, fitted for the calibration of 24-m invar wires to determine a uniform scale for triangulation. Since 1947, the baseline has been regularly measured with the Väisälä interference comparator. As a continuation to the impressive time series, the performance and results of the latest interference measurements in 2005 and 2007 are presented in detail in this publication. Two consecutive measurements within a short time span were necessary, since only half of the baseline could be measured in 2005, due to unfavourable weather conditions. The new results again confirm the excellent stability and unique accuracy, 9×10 -8, of the baseline. The 6-pillar baseline now serves in the calibration, testing and validation of electronic distance measurement (EDM) instruments for precise surveying and mapping and in scale transfer measurements to other geodetic baselines, test fields and local geodynamical networks. The measurements are metrologically traceable to the definition of the metre through a quartz gauge system. This publication provides a summary of the rare measurement method and is a detailed supplement to the previously published or internal instruction manuals. First, we present the quartz gauge systems, which determine the scale in the Väisälä interference comparator. After a description of the present c