## 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.

Semi-major
(equitorial) Denominator of
Name Date axis, meters Flattening Ratio Source
___________________________________________________________________
Geodetic
Reference
System 80 1980 6,378,137* 298.257 Snyder 1987**
Department of
298.25722210088*** Commerce 1989
World
Geodetic
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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