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How did measurements of Earth help define the original standard meter during the French Revolution?

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Diagram showing the deflection of the vertical from the geoid to the ellipsoid or spheroid, with labeled lines for geoid, spheroid, rock surface, ocean, and geoid height in a geology context. — geoid Deflection of the vertical from the geoid to the spheroid.

During the French Revolution, the French Academy of Sciences sought to establish a rational, universal system of measurement to replace inconsistent regional standards. They defined the metre as one ten-millionth of the distance from the North Pole to the Equator along the meridian passing through Paris.12

To determine this length, astronomers Jean Delambre and Pierre Mechain conducted an arduous geodetic survey between 1792 and 1798, measuring the meridian arc from Dunkirk to Barcelona. By employing modernized geodesy—a method involving triangulation and precise stellar observations—they calculated the distance of the meridional quadrant.1342

Although these efforts provided the basis for the standard meter, the resulting value was slightly inaccurate because the Earth is not a perfect sphere but rather an oblate spheroid, and the survey data combined with earlier measurements resulted in a meter that was approximately 0.02 percent shorter than the intended definition.42