Journal of Geodesy (2005) 78: 558–568 DOI 10.1007/s00190-004-0423-0 The International Association of Geodesy 1862 to 1922: from a regional project to an international organization W. Torge Institut fu¨ r Erdmessung, Universita¨ t Hannover, Schneiderberg 50, 30167 Hannover, Germany; e-mail: [email protected]; Tel: +49-511-762-2794; Fax: +49-511-762-4006 Received: 1 December 2003 / Accepted: 6 October 2004 / Published Online: 25 March 2005 Abstract. Geodesy, by definition, requires international mitment of two scientists from neutral countries, the collaboration on a global scale. An organized coopera- International Latitude Service continued to observe tion started in 1862, and has become today’s Interna- polar motion and to deliver the data to the Berlin tional Association of Geodesy (IAG). The roots of Central Bureau for evaluation. After the First World modern geodesy in the 18th century, with arc measure- War, geodesy became one of the founding members of ments in several parts of the world, and national geodetic the International Union for Geodesy and Geophysics surveys in France and Great Britain, are explained. The (IUGG), and formed one of its Sections (respectively manifold local enterprises in central Europe, which Associations). It has been officially named the Interna- happened in the first half of the 19th century, are tional Association of Geodesy (IAG) since 1932. described in some detail as they prepare the foundation for the following regional project. Simultaneously, Gauss, Bessel and others developed a more sophisticated Key words: Arc measurements – Baeyer – Helmert – definition of the Earth’s figure, which includes the effect Figure of the Earth – History of geodesy – of the gravity field. In 1861, the retired Prussian general International Association of Geodesy J.J. Baeyer took up earlier ideas from Schumacher, Gauss, Struve and others, to propose a Central European Arc Measurement in order to study the figure of the Earth in that region. This led to a scientific organization, which soon extended from Central Europe to the whole continent and later to the globe, and changed its name in 1 The origin of modern geodesy and first international 1886 to ‘Internationale Erdmessung’ (International Geo- cooperation: 18th century detic Association). The scientific programme also wid- ened remarkably from more local studies based on The beginning of modern geodesy may be reckoned geometric data to regional and global investigations, from the 17th century, when physics and astronomy with gravity measurements as an important source of postulated an oblate figure of the Earth (Perrier 1939; information. The Central Bureau of the Internationale Bialas 1982; Torge 2001). Based on the theory of Erdmessung was hosted at the Prussian Geodetic Insti- hydrostatic equilibrium, the flattening of a rotating tute in Potsdam, and with Baeyer as Director, developed Earth was calculated, with values ranging between as an efficient tool of the Association. The scientific 1/230 (I. Newton 1687: ‘Philosophiae Naturalis Prin- research extended and deepened after 1886, when F.R. cipia Mathematica’) and 1/576 (C. Huygens 1690: Helmert became Director of the Central Bureau. A ‘Discours de la Cause de la Pesanteur’). This was a stronger international participation then took place, great challenge for geodesists: to prove the polar while the influence of the German states reduced. Of flattening by geometric methods and to determine the great practical importance were questions of standard- actual value of the flattening, which then would ization and reference systems, but first attempts to represent a boundary value for physical Earth models. interpret gravity field variations and to monitor geody- It started the period of arc measurements at different namic phenomena by geodetic methods indicated future latitudes, whereby latitude arc measurements prevailed tendencies. With the First World War and the expiry of in practice. Here, the length of a part of a meridian is the last international convention in 1916, the interna- determined by triangulation, a method which had been tional cooperation within the frame of the Association successfully introduced by the Dutchman Snellius at the practically came to an end, which ended the first epoch of beginning of the 17th century, and which was employed the Association. Nevertheless, due to the strong com- for nearly 300 years. By measuring the corresponding 559 difference of the latitudes using well-known astronom- set an example for international collaboration. It also ical procedures, the Earth’s curvature at that latitude demonstrated a fruitful cooperation between scientists could be calculated. and military state organizations, which were soon found Naturally, the solution to this problem required in other countries. During this time, there were signifi- international collaboration. A driving force in this cant improvements in the measurement and computa- direction was the French Academy of Sciences, founded tion techniques, characterized by the introduction of the in 1666, which emphasized the importance of the prob- Borda repetition circle and baseline apparatus in France, lem of the Earth’s figure. First attempts to calculate the the Ramsden second theodolite in Great Britain, and flattening from measurements carried out in France mathematical developments for the calculation of tri- along the meridian through Paris by J.D. Cassini and angulation networks on the ellipsoid by Legendre, others (around the turn of the 18th century) failed, and Lagrange, Laplace and others. the French Academy consequently initiated the well- known arc measurements in Lapland (1736–1737) and in the Spanish Vice-Kingdom of Peru (today Ecuador) 2 Collaboration in central Europe in the 19th century: (1735–1744). These enterprises required political agree- the forerunners ment between the countries involved, but, through the participation of non-French scientists and engineers, The political situation in central Europe was less favour- these French expeditions obtained an international able in the 19th centuary, due to the strong division into character. The Lapland expedition, led by the French local territorial units, which was especially pronounced in Academy member Maupertuis,1 included the Swedish Germany. On the other hand, this diversity led to a variety astronomer Celsius. In addition to the French acade- of different solutions for geodetic surveys, and several micians Godin, Bouguer and La Condamine, the interesting attempts to contribute to the determination of Spanish officers Jorge Juan and Antonio de Ulloa par- the size and shape of the Earth. As the organized ticipated in the measurements and the evalution of the international cooperation in geodesy started in central Peru expedition. Europe during the second half of the 19th century, we next The early combination of the Lapland arc measure- consider the developments that led to this step (Torge ment with the revised Paris meridian arc then confirmed 1997). the oblate ellipsoidal Earth model, with a flattening of There were a number of state surveys in the last 1/304, which is close to the value known today decades of the 18th century, which were based on the (1/298.25). Further arc measurements in the 18th cen- method of triangulation, such as the survey of Denmark tury delivered, by different combinations and calculation (1764–1792) under the direction of Thomas Bugge, and procedures, flattening values of between 1/144 and the survey of the principality, and later kingdom, of 1/352. Gravity measurements, although carried out in Saxonia in Germany (1780–1811), under Friedrich connection with most of the arc measurements, were not Ludwig Aster. On the other hand, in the survey of the then exploited for the determination of the Earth’s principality of Hannover (1764–1786) under Josua de flattening according to Clairaut’s theorem (published in Plat and Johann Ludwig Hogrewe, plane table mea- 1743 in ‘The´ orie de la Figure de la Terre, tire´ edes surements were only locally connected by short base- Principes de l’Hydrostatique’). However, the common lines, and orientated by a few astronomically derived adjustment of only 19 measurements performed until positions. The situation was even worse in the kingdom 1760 would have led to a reasonable flattening value of of Prussia, which covered a large part of northern 1/328 (Ekman and Ma¨ kinen 1998). Germany. Topographical information was kept secret With the strengthening of the European national for military reasons, and mapping of larger areas uti- states, and the related changes in economy, adminis- lized existing local information, supplemented and tration and military organization, the 18th century also connected by occasional plane table surveys. hosted the first national geodetic surveys, organized or A radical change happened at the turn of the 19th sponsored by the respective national governments. century, with the extension of French influence over These surveys provided a network of geodetic control central Europe through the Napoleonic wars. The geo- points, which then served as a basis for topographical detic tradition built up by the French arc measurements and partly also cadastral maps, as well as for large-scale and the triangulation of France, but also the establish- engineering projects. France was the leading nation, ment of a land recording system (cadastre) on the ini- with a country-wide triangulation for the ‘Carte tiative of Napoleon, strongly influenced the German ge´ ome´ trique de la France’ (Cassini de Thury,