Is there a relationship between horizontal datums and ellipsoids?

A horizontal datum is defined in part by the parameters of a reference ellipsoid.

What are the ellipsoids associated with the horizontal datums listed in the domain of the "Horizontal Datum Name" element?

Horizontal Datum............................ Ellipsoid
North American Datum of 1927..... Clarke 1866
North American Datum of 1983..... Geodetic Reference System 80

What are the values for the semi-major axis and denominator of flattening ratio for some commonly used ellipsoids?"

The table below provides the values for some commonly used ellipsoids. Note, however, that it is important that you record the values that were used in your computer software with your data, and not merely those that are available in a reference book.

                 (equitorial)  Denominator of
Name       Date  axis, meters  Flattening Ratio     Source         
System 80  1980  6,378,137*    298.257              Snyder 1987**

                                                    Department of 
                               298.25722210088***   Commerce 1989

System 72  1972  6,378,135*    298.26               Snyder 1987****

Clarke     1866  6,378,206.4*  294.98               Snyder 1987
     * Taken as exact values.
     ** "Ellipsoid derived from adopted model of Earth.  
            World Geodetic System 84 has same dimensions 
            within accuracy shown." (Snyder 1987).
     *** To 14 significant digits by computation.
     **** Ellipsoid derived from adopted model of Earth.

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