Irene Fischer

The present extent of the astro-geodetic geoid and the geodetic world datum derived from it

Since the presentation of “The Hough Ellipsoid” at the Toronto meeting in 1957, the astro-geodetic geoid was extended into the Caribbean, India, and Japan. The two hemispheres were tentatively connected across the North Atlantic by a reasonable assumption about the unknown geoid profile there. A series of solutions for a world ellipsoid and world datum was made, with and without enforcing the flattening of 1/298.3, with and without gravimetric orientation. The resulting ellipsoids are very small, with an equatorial radius of around 6378 160 m. The agreement between astro-geodetic and gravimetric geoid profiles is greatly improved by the small ellipsoid.

New pieces in the picture puzzle of an astrogeodetic geoid map of the world

The astrogeodetic geoid map presented to the IUGG at Helsinki in 1960 has been updated to reflect the accumulation of new data. The geoid map of North America has been recomputed, that of South and Central America enlarged. A geoid chart of Australia has been added. Various other improvements were made Terrestrial gravity was used for interpolation, and satellite observations for intercontinental connection. World datum parameters were derived in various solutions. Satellite positioning of the major astrogeodetic datum blocks leads to an equatorial radiusa=6378142 m for a flatteningf=1/298.25. If surface gravity is included, the radius is larger. A reference figurea=6378150 m andf=1/298.3 is recommended for practical applications?

At the dawn of geodesy

The first land surveyors were rope stretchers and rope knotters, remembered in ancient documents and tomb paintings and also in some terminology. The L-shaped carpenter’s square, one of the earliest and most versatile basic tools, represents the observed direction of the plumb line versus the water level and appears as the shadow-casting gnomon and also as the geometrical gnomon in magically-restricted enlargements of altars. The related “Pythagorean” theorem was known in antiquity centuries before Pythagoras, with algebraic proofs in Babylonia and China. The spherical shape of the earth, deduced from the observation of circumpolar stars, was part of a complete equatorial astronomical system in ancient China. But although shadow measurements were generally used to establish north-south distances, only the Greeks derived from them the size of the earth. The striking difference between the abstract, geometric approach of Greece and the concrete, algebraic approach of Babylonia and China represents not a difference in talents but a difference in culture-bound interests.