Fachbeitrag Torge, 150 Years of International Cooperation in Geodesy … 150 Years of International Cooperation in Geodesy: Precursors and the Development of Baeyer’s Project to a Scientific Organisation* Wolfgang Torge Summary ternationaler Kooperation wird ausführlicher eingegangen, Geodesy commemorates this year the foundation of the abschließend wird auf die spätere Erweiterung zur globalen “Mit tel euro päische Gradmessung”, which started its activi- Wissenschaftsorganisation und die immer stärker werdende ties 150 years ago. Originated from an initiative of the Prus- Einbindung in die interdisziplinäre Geoforschung hingewiesen. sian General Baeyer, this regional geodetic project required international cooperation. Outstanding organization soon led Keywords: Arc measurements, Earth figure, General Baeyer, to a scientific body which continuously extended over the International Association of Geodesy, Mitteleuropäische globe and finally built a global network of geodetic science, Gradmessung represented today by the “International Association of Geo- desy” (IAG). The following paper first remembers the start of international 1 The Origin of Modern Geodesy and Early cooperation at the beginning of modern geodesy, as real- International Cooperation: 18th Century ized through the arc measurements of the 18th and the early 19th century. First attempts for arc measurements covering Geodesy, by definition, requires international collabora- central Europe then are found in the first half of the 19th cen- tion on a global scale. This becomes clearly visible with tury – connected with the names of Zach, Schumacher, Gauß, the beginning of modern geodesy, which may be reck- Müffling, Bessel and Struve, among others. Based upon a oned from the 17th century. The heliocentric world system memorandum presented 1861, Baeyer proposed a coordinat- with the annual revolution of the Earth around the Sun ed proceeding and clearly defined the problems to be solved. and the daily rotation of the Earth had been accepted at This idea soon led to a governmental supported project and an that time, and physics and astronomy postulate an Earth organization which rapidly extended beyond the boundaries figure flattened at the poles (Perrier 1939, Bialas 1982). of central Europe. This early phase of international coopera- The corresponding Earth model is based on the theory of tion is discussed in more detail. Finally, the later extension of hydrostatic equilibrium (Newton** 1687, Huygens 1690), this early structure to a global scientific organization and the the observed polar flattening of Jupiter (Jean-Dominique increasing incorporation into interdisciplinary geoscientific Cassini 1666), and the latitude-dependence of gravity research is shortly described. found by pendulum measurements (Richer 1672/73, Hal- ley 1677/78). This is a great challenge for geodesy: to Zusammenfassung prove the polar flattening by geometric methods and to In diesem Jahr gedenkt die Geodäsie der Entstehung ihrer in- determine the parameters of such an Earth model! For ternationalen wissenschaftlichen Vereinigung vor 150 Jahren. a rotational ellipsoid, these parameters would be the Auf Initiative des preußischen Generals Baeyer entstand die semi-major axis and the geometric flattening. The arc »Mitteleuropäische Gradmessung«, ein zunächst räumlich be- measurement method known since antiquity is avail- grenztes Projekt. Die hierzu notwendige internationale Zusam- able for this purpose. Triangulation introduced by Snel- menarbeit wird effektiv organisiert und führt – bei laufender lius (1614/15) offers an efficient procedure to determine Erweiterung der Aufgabenstellung – schließlich zur globalen geodetic (ellipsoidal) differences in length, while the de- Vernetzung der geodätischen Wissenschaft, heute repräsen- termination of latitude, longitude and azimuth can be tiert durch die »International Association of Geodesy« (IAG). based on well-known astronomical methods. A first at- Der folgende Beitrag erinnert zunächst an die mit Beginn der tempt in this direction is carried out by J.-D. Cassini and modernen Geodäsie einsetzende länderübergreifende Zu- his son Jacques carrying out a triangulation (1683–1718) sammenarbeit bei den Gradmessungen des 18. und des frü- along the meridian of Paris. Dividing this meridian arc of hen 19. Jahrhunderts. Erste Gradmessungen in Mitteleuropa 8°20’ extension into a northern and a southern part and oder Ansätze hierzu finden sich dann in der ersten Hälfte des 19. Jahrhunderts – verbunden u. a. mit den Namen Zach, * Modifizierte und ins Englische übersetzte Fassung eines Vor- Schumacher, Gauß, Müffling, Bessel und Struve. Baeyers Idee trages im Rahmen des Festkolloquiums »150 Jahre Mittel euro- eines koordinierten Vorgehens mit klarer Aufgabendefinition päische Gradmessung in Sachsen« am 1. Juni 2012 in Dresden. führt schließlich – auf der Grundlage einer 1861 vorgelegten ** Names written in italics refer to historical persons, with rel- Denkschrift – zu einer zwischenstaatlichen Zusammenarbeit, evant years of important publications or surveys, and not to list die bald über Europa hinausreicht. Auf diese frühe Phase in- of references. 166 zfv 3/2012 137. Jg. Torge, 150 Years of International Cooperation in Geodesy … Fachbeitrag separate evaluation led to an Earth model elongated at 2 Central Europe at the Turn of the 19th Century: the poles, with a negative flattening of –1/95. This result Military Triangulations and the Beginning of was in agreement with a few older arc measurements, Arc Measurements but contradicted the hypothesis of an Earth flattened at the poles. The political situation in central Europe is less favour- This is the beginning of the well-known dispute be- able for large-scale triangulations of high standard. This tween the followers of Newton (flattening at the poles) is due to the strong separation into local territorial units, and the Cassinis (elongation at the poles) which is solved which is especially pronounced in Germany; for the his- by the famous arc measurements in Lapland (Maupertuis, tory of geodesy in Germany we refer to Torge (2009). Clairaut et al. 1736/37) and in the Spanish Vice-Kingdom Among the remarkable exceptions are the triangula- of Peru – today Ecuador (La Condamine, Bouguer, Godin tions carried out in the dukedom of Oldenburg (Georg et al. 1735–1744), initiated and organized by the French Christian von Oeder, 1781–1785) and in the principality, Academy of Sciences. The early combination of the Lap- and later kingdom, of Saxony (Friedrich Ludwig Aster, land arc with the Paris meridian arc revised by Cassini 1780–1811), both following the example of the geodetic de Thury and La Caille (1739/40) already yields a polar survey of Denmark (1762–1779) under Thomas Bugge. flattening of 1/304. This is confirmed by the combina- An important intermediate phase occurs at the turn of the tion of further arc measurements, although the flattening 19th century, with the extension of French influence over values vary between 1/144 and 1/352. These great French central Europe through the Napoleonic wars. The military arc measurements not only require political agreements surveys carried out by the well-trained French engineer- between the countries involved, but also are the begin- geographers are based on triangulation, and shall extend ning of international participation at large-scale geodetic the French map of scale 1: 86400 on the occupied or al- projects. The Lapland expedition, for instance, included lied countries. Triangulation of the occupied German re- the Swedish astronomer Celsius, and the Spanish navy gions left of the Rhine (1802–1809) is connected with the officers Jorge Juan and Antonio de Ulloa actively con- name of Tranchot. The upper Rhine area is surveyed by tributed to the measurements and the evaluation of the Henry (1804), while Epailly performs a rapid triangula- Peruvian arc measurement. tion of Hannover (1805–1806), and Bonne establishes a The large differences found for the flattening (and nat- first trigonometric network in the allied Bavaria (1801– urally also for the semi-major axis) of an Earth ellipsoid 1807). The Batavian Republic is triangulated by Krayen- led to first discussions on the figure of the Earth, starting hoff (1802–1811). All these triangulations are connected already in the 18th century. In the beginning it is tried to to the new arc measurement along the Paris meridian keep the ellipsoidal Earth model, and to explain local and (1792–1798) carried out by Delambre and Méchain in or- later also large-scale deviations through the effects of der to define the metre as a natural unit of length, and topography, geology, and continent-ocean distribution. thus refer to the geodetic datum provided by the Paris Different strategies are proposed in order to solve this observatory. This is also valid for the Prussian military problem, including the reduction of local anomalies and triangulation of Westphalia (1795–1805) carried out un- the introduction of a latitude- or longitude-dependent der the direction of Colonel von Lecoq, in a period when flattening parameter. Around 1800, the time is ripe for Prussia had withdrawn from the coalition against France. a new definition of the Earth figure, which is introduced At the end of the Napoleonic wars, Prussia reorganized soon after by Gauß, Bessel, and others (see below). This its mapping organizations and concentrated them on the coincides with national geodetic surveys organized or General Staff (Torge 2002). Under the direction