What ellipsoid model is used in the N.A.D. 83 reference system?

The correct answer is that the ellipsoid model used in the North American Datum of 1983 (NAD 83) reference system is the Geodetic Reference System 1980 (GRS 80). This choice is appropriate because NAD 83 was designed to provide a continuous and consistent reference system for mapping and surveying across North America, and it is based on the GRS 80 ellipsoid.

The GRS 80 model was developed to provide a better fit to the Earth's surface and was based on satellite Doppler measurements, making it suitable for modern surveying applications. This model supports the geodetic needs of national mapping agencies, enabling precise positioning and cartographic work.

In contrast, WGS 84 is primarily utilized for global positioning systems and is intended for worldwide applications; it is not the foundational ellipsoid for NAD 83. The Hayford 1910 and Clarke 1866 models, while historically significant in surveying, do not serve as the basis for NAD 83, as they were used in earlier datums that have since been superseded by the more accurate and widely applicable GRS 80.