The precursors to the IAG were
arc measurement campaigns. The IAG was founded in 1862 as the
Mitteleuropäische Gradmessung (
Central European Arc Measurement), later became the
Europäische Gradmessung (European Arc Measurement) in 1867, the
Internationale Erdmessung (
Association Geodésique Internationale in French and "International Geodetic Association" in English) in 1886, and took its present name in 1946. As early as 1861,
Johann Jacob Baeyer sent a memorandum to the King of
Prussia recommending international collaboration in
Central Europe with the aim of determining the shape and dimensions of the Earth. At the time of its creation, the association had sixteen member countries:
Austrian Empire,
Kingdom of Belgium,
Denmark, seven German states (
Grand Duchy of Baden,
Kingdom of Bavaria,
Kingdom of Hanover,
Mecklenburg,
Kingdom of Prussia,
Kingdom of Saxony,
Saxe-Coburg and Gotha),
Kingdom of Italy,
Netherlands,
Russian Empire (for
Poland),
United Kingdoms of Sweden and Norway, as well as
Switzerland. The Central European Arc Measurement created a Central Office, located at the Prussian Geodetic Institute, whose management was entrusted to Johann Jacob Baeyer. Baeyer's goal was a new determination of anomalies in the shape of the Earth using precise triangulations, combined with gravity measurements. This involved determining the
geoid by means of gravimetric and leveling measurements, in order to deduce the exact knowledge of the terrestrial spheroid while taking into account local variations. To resolve this problem, it was necessary to carefully study considerable areas of land in all directions. Baeyer developed a plan to coordinate geodetic surveys in the space between the parallels of
Palermo and Christiana (
Oslo) and the meridians of
Bonn and Trunz (German name for
Milejewo in
Poland). This territory was covered by a triangle network and included more than thirty observatories or stations whose position was determined astronomically. Bayer proposed to remeasure ten arcs of meridians and a larger number of arcs of parallels, to compare the curvature of the meridian arcs on the two slopes of the
Alps, in order to determine the influence of this mountain range on
vertical deflection. Baeyer also planned to determine the curvature of the seas, the
Mediterranean Sea and
Adriatic Sea in the south, the
North Sea and the
Baltic Sea in the north. In his mind, the cooperation of all the States of
Central Europe could open the field to scientific research of the highest interest, research that each State, taken in isolation, was not able to undertake. Since the
arc measurement of Delambre and Méchain,
Friedrich Wilhelm Bessel using the
method of least squares had calculated from several
arc measurements a new value for the flattening of the Earth, which he determined as . Seventeen years after Bessel calculated his
ellipsoid of reference, some of the meridian arcs the German astronomer had used for his calculation had been enlarged. This was a very important circumstance because the influence of random errors due to
vertical deflections was minimized in proportion to the length of the meridian arcs: the longer the meridian arcs, the more precise the image of the
Earth ellipsoid would be.|left After the
Struve Geodetic Arc measurement, it was resolved in the 1860s, at the initiative of
Carlos Ibáñez e Ibáñez de Ibero, future president of both the International Geodetic Association and the
International Committee for Weights and Measure, to remeasure the arc of meridian from
Dunkirk to
Formentera and to extend it from
Shetland to the
Sahara. This did not pave the way to a new definition of the metre because it was known that the theoretical definition of the metre had been inaccessible and misleading at the time of Delambre and Mechain arc measurement, as the
geoid is a ball, which on the whole can be assimilated to an oblate
spheroid, but which in detail differs from it so as to prohibit any generalization and any extrapolation from the measurement of a single meridian arc. In 1859,
Friedrich von Schubert demonstrated that several meridians had not the same length, confirming an hypothesis of
Jean Le Rond d'Alembert. He also proposed an ellipsoid with three unequal axes. In 1860, Elie Ritter, a mathematician from
Geneva, using Schubert's data computed that the Earth ellipsoid could rather be a spheroid of revolution accordingly to
Adrien-Marie Legendre's model. However, the following year, resuming his calculation on the basis of all the data available at the time, Ritter came to the conclusion that the problem was only resolved in an approximate manner, the data appearing too scant, and for some affected by
vertical deflections, in particular the latitude of
Montjuïc in the French meridian arc which determination had also been affected in a lesser proportion by systematic errors of the
repeating circle. When
Carlos Ibáñez e Ibáñez de Ibero took part to the remeasurement and extension of the arc measurement of Delambre and Méchain, mathematicians like
Legendre and
Gauss had developed new methods for processing data, including the "
least squares method" which allowed to compare experimental data tainted with
observational errors to a mathematical model. The Earth measurements thus underscored the importance of the scientific method at a time when
statistics were implemented in geodesy. with variant of
Repsold-Bessel pendulum Significant improvements in gravity measuring instruments must also be attributed to Bessel. He devised a gravimeter constructed by
Adolf Repsold which was first used in
Switzerland by
Emile Plantamour, This would allow
Friedrich Robert Helmert to determine a remarkably accurate value of for the flattening of the Earth when he proposed his
ellipsoid of reference. This was also the result of the
Metre Convention of 1875, when the metre was adopted as an international scientific unit of length for the convenience of continental European geodesists following forerunners such as
Ferdinand Rudolph Hassler later Carlos Ibáñez e Ibáñez de Ibero. In the 19th century, astronomers and geodesists were concerned with questions of longitude and time, because they were responsible for determining them scientifically and used them continually in their studies. The International Geodetic Association, which had covered Europe with a network of fundamental longitudes, took an interest in the question of an internationally-accepted prime meridian at its seventh general conference in Rome in 1883. Indeed, the Association was already providing administrations with the bases for topographical surveys, and engineers with the fundamental benchmarks for their levelling. It seemed natural that it should contribute to the achievement of significant progress in navigation, cartography and geography, as well as in the service of major communications institutions, railways and telegraphs. From a scientific point of view, to be a candidate for the status of international prime meridian, the proponent needed to satisfy three important criteria. According to the report by Carlos Ibáñez e Ibáñez de Ibero, it must have a first-rate astronomical observatory, be directly linked by astronomical observations to other nearby observatories, and be attached to a network of first-rate triangles in the surrounding country. Four major observatories could satisfy these requirements:
Greenwich,
Paris,
Berlin and Washington. The conference concluded that Greenwich Observatory best corresponded to the geographical, nautical, astronomical and cartographic conditions that guided the choice of an international prime meridian, and recommended the governments should adopt it as the world standard. The Conference further hoped that, if the whole world agreed on the unification of longitudes and times by the Association's choosing the Greenwich meridian, Great Britain might respond in favour of the unification of weights and measures, by adhering to the
Metre Convention. The International Geodetic Association gained global importance with the accession of
Chile,
Mexico and
Japan in 1888;
Argentina and
United-States in 1889; and
British Empire in 1898. The convention of the International Geodetic Association expired at the end of 1916. It was not renewed due to the
First World War. However, the activities of the
International Latitude Service were continued through an thanks to the efforts of
H.G. van de Sande Bakhuyzen and Raoul Gautier (1854–1931), respectively directors of
Leiden Observatory and
Geneva Observatory. == Overview ==