Such question tormented members of the French academy of Sciences at the end of the 18th century. The new metric system of measures was introduced on March 19, 1791. The meter theoretically equaled one length of a terrestrial meridian, ten-million from a quarter. And the length of a meridian in practice was not measured yet. Decided to measure by a triangulation method.
Triangulation method
By method of a triangulation planned to measure distance between Dunkirk and Barcelona. This distance makes nine and a half degrees of an arch of a meridian. The degree is one hundred eightieth from meridian length. Work was charged to Caesar Francois Cassini, Andriyen Mari Lezhandra and to Pierre Mechen.
The triangulation consisted in marking of a route on network of highly visible reference points: tower, peaks, spires of churches, etc. Points represented a number of the connected triangles. Knowing all corners formed by two adjacent triangles and lengths at least in one of triangles, it is possible to determine by trigonometry lengths of all parties in both triangles.
The method was already successfully used in 1718 by Jean Cassini, Caesar François's father, for measurement of distance between Dunkirk and Kolliur. During work the surveyors had to endure many adventures and to overcome a set of difficulties. Because of a difficult political situation in the country in days of the Great French revolution, they were repeatedly arrested, damaged and destroyed the geodetic equipment. As a result measurements were complete only in 1799, for three years after the planned term.
Space triangulation
With an accuracy of millimeter length of a meridian was established in the second half by means of a space triangulation. The essence of this method is simple. Along with the satellite several objects on the earth's surface are observed. Their coordinates are given to a uniform system. The triangulable points located on different continents communicate. Thus with a fine precision established distances between continents. Before they were known only approximately. On a water smooth surface it is not possible to apply methods of a classical triangulation. Besides, by method of a space triangulation specified a form which our planet has. It appeared, it is a little rejected from spherical and slightly pear-shaped. "Pear" is a little extended to the North and is sensitive the priplusnut from the South. And the surface of the World Ocean to a degree copies outlines of an oceanic bottom. On the surfaces of the seas and oceans ledges and deepenings are found.