Measuring zero at sea: on the delocalization and abstraction of the geodetic framework

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Highlights

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    Investigates debates about the limitations of measuring sea levels along coasts.

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    Discusses the reliability of alternative approaches proposed to tackle the issue.

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    Notes the growing importance of gravity at sea as a proxy for the level of water.

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    Argues that the geodetic framework became increasingly delocalized and abstracted.

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    Contributes to scholarship on the history of geodesy and ocean topography.

Abstract

Starting from the issue of how mean sea level has come to represent, since the early nineteenth century, the most commonly adopted vertical datum, this paper reviews the inherent biases and limitations of attempting to measure the sea from individual locations tucked away in bays and inlets along the coast. It then delves into a history of the attempts made to move measurements away from continental shores, including efforts to collect water height data from the bottom of the sea and from remote islands. Finally, it provides a history of the shift from attempting to measure the level of water at coastal locations to estimating, instead, where water ought to be. To achieve this, the article focuses on early developments in gravimetry at sea as a tool to map more precisely the geoid and, thus, the oceans’ topography. The idea behind these developments was to obtain more accurate results and a more trustworthy geodetic framework by getting rid of the variability caused by relative movement of land and sea and by contextual topographical challenges. It was also an attempt to delocalize the data collection endeavour in order to focus on inferring and estimating average values. It was thought that increasing the degree of abstraction would enable an improved representation of the planet and its oceans.

Section snippets

Assessing sea levels beyond the shore

No land-based technology in the nineteenth century ever overcame the intrinsic limits of measuring the level of a body of water at one point along the coast, and therefore potentially from within inlets and bays that affect the circulation of water, from places which are subjected to the possibility of extreme tidal occurrences, local currents, or even peculiar patterns of gravitational attraction. This meant that the geodesists’ mean sea level was still just a rough estimate of the sea’s

Islands in a corrugated sea

By the end of the nineteenth century the interpolation of data, made possible by the introduction of new mathematical methods and tools, had allowed scientists to build upon measurements taken close to shore or during the odd high sea expedition to make estimates about the height of water over wider stretches of the seven seas. It became clear, as exemplified by the work done on the topography of the Norwegian Sea by oceanographer and meteorologist Henrik Mohn in the 1870s, that measurements

High sea gravimetry

The German oceanographer Otto Krümmel asserted, in his 1907 revised edition of the Handbuch first edited by Georg von Boguslawski in 1884, that the only hope of obtaining more precise values for the level of the sea lay in taking a radically different approach to how to determine and calculate it. He suggested that attention needed to be paid, primarily, to the planet’’s gravitational field, rather than attempting to estimate the position of the sea’s waters at any particular moment in any

Conclusion

As this paper has shown, every attempt to obtain reliable measurements of sea level was marked by the need to recur to proxies and estimates. The limits of first human, then mechanized measurements along the coasts, disturbed as they were by local oceanographic and geomorphological conditions, as well as the limits of both surveyors and instruments and the impossibility of reliably measuring the sea level from the bottom up, led a number of nineteenth-century geodesists to try to devise ways to

Acknowledgements

I would like to express my gratitude to all the participants of the ‘Estimated Truths: Water, Science, and the Politics of Approximation’ workshop held at the Max Planck Institute for the History of Science in August 2017 for an extremely interesting and productive discussion, which framed deeply the way this paper has developed. The feedback provided on various drafts of this manuscript by my co-conveners, Etienne Benson and Giacomo Parrinello, as well as the anonymous reviewers and the

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