Length in Geodesy – On Metrological Traceability of a Geospatial Measurand

Jorma Jokela

    Research output: ThesisDoctoral ThesisMonograph

    Abstract

    The metre is one of the base units in the International System of Units (SI). The traceability chain connects length measurements to the definition of the metre. Metrological institutes implement this with sequential measurements ranging from the realization of the metre using internationally recommended procedures to practical length or distance measurements with high-precision electro-optical or mechanical instruments. Estimating the uncertainty of measurement at every stage in the traceability chain is an essential part of the measurement result and its usability. This publication examines the traceability of a geospatial measurand, a length used in geodesy, beginning with lengths of 1-m-long quartz gauge blocks and ending with terrestrial distance measurements of up to 1 km or more. The Finnish Geodetic Institute (FGI) started measuring geodetic standard baselines with the Väisälä interference comparator in 1947. Based on the author’s 25 years of experience, this publication includes the most detailed description of the interference measurement method to date. New results and inspiring experiences are presented from five Väisälä baseline measurements. Especially interesting is the FGI’s 864-m Nummela Standard Baseline, recognized as a world-class measurement standard due to its extreme accuracy and stability. In addition, a few alternative standard baseline designs are presented. From standard baselines, the FGI transfers the traceable scale to other geodetic baselines or test fields using calibrated, high-precision electro-optical distance measurement instruments as transfer standards. Using the calibrated objects, the traceable scale is then transferred forward for the calibration of surveyors’ instruments or for scientific purposes. This publication shows the capability of the method, which is not utilized elsewhere, and discusses 11 scale transfer examples of it to seven baselines or test fields. The influence of atmospheric conditions is a major source of uncertainty of measurement, and it is discussed in detail in connection with a few cases. The traceable scale transfer service of the FGI has become internationally in demand, and it makes a remarkable contribution to the ongoing European research and development projects in length metrology. Most baselines that we have measured are alive and well, and interest in them is growing. The latest measurements with the Väisälä interference comparator at standard baselines produced total expanded uncertainties ranging from 0.04 mm to 0.14 mm for baseline section lengths between 5 m and 864 m. After applying the scale transfer measurements to calibration baselines and test fields, the comparable uncertainty values were from 0.1 mm to 1.2 mm for baseline section lengths between 2 m and 1 488 m. A total expanded uncertainty of 0.5 mm/km is achievable under favourable conditions, and when the scale transfer is performed as a continuation of interference measurements at the same baseline, it is possible to reach 0.2 mm/km.
    Translated title of the contributionPituus geodesiassa – Geospatiaalisen mittaussuureen metrologisesta jäljitettävyydestä
    Original languageEnglish
    QualificationDoctor's degree
    Awarding Institution
    • Aalto University
    Supervisors/Advisors
    • Vermeer, Martin, Supervising Professor
    • Poutanen, Markku, Thesis Advisor, External person
    Print ISBNs978-951-711-309-0
    Electronic ISBNs978-951-711-310-6
    Publication statusPublished - 2014
    MoE publication typeG4 Doctoral dissertation (monograph)

    Keywords

    • geodesy
    • metrology
    • geodetic baseline
    • Väisälä interference comparator
    • Nummela Standard Baseline
    • length measurement
    • scale transfer
    • calibration
    • EDM
    • traceability

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