Question

Provide three differences between NAD 27 and NAD 83?

Provide three differences between NGVD 29 and NAVD99？

Why was the change in datums needed?

Answer 1

A.  Differences between these two.

1.    North American Datum 27 (NAD 27) is based on Clarke Ellipsoid of 1866 which is calculated by manual surveying of the content, whereas NAD 83 is based on the Geodetic Reference System (GRS) of 1980. GRS 1980 is the approximate measurement of the whole earth.

2.    NAD 27 is based on the longitude and longitude, and the direction between points. NAD 83 is an earth centered datum which does not have beginning point or direction.

3. The NAD 27 was calculated with a single survey point, MEADES RANCH in Kansas, as the datum point and the NAD 83 was calculated as a geocentric reference system with no datum point.

4. NAD 27 and NAD 83 cannot support the 3D capability of the today’s technology.

5. NAD 27 and NAD 83 ellipsoids are different.

6. NAD 83 is more accurate than NAD 27.

7. NAD 83 is official legal horizontal datum for the USA federal government and it is recognized as a legislation in 48 of the 50 states.

8.  Different Parameters for NAD27 vs NAD83

Coordinate System Name	Ellipsoid	Semi-Major Axis	Semi-Minor Axis	Inverse of flattening
NAD27	Clarke_1866	6378206.4	6356583.799998981	294.9786982
NAD83	GRS_1980	6378137	6356752.3141403561	298.25722210100002

B.

National Geodetic Vertical Datum of 1929 (NGVD 29)

The Sea Level Datum of 1929 was named the National Geodetic Vertical Datum of 1929 on May 10, 1973. (Geodetic Glossary, pp. 57) The Sea Level Datum of 1929 is a vertical control datum in the United States by the general adjustment of 1929.

Mean sea level was held fixed at the sites of 26 tide gauges, 21 in the United States and 5 in Canada. The datum is defined by the observed heights of mean sea level at the 26 tide gauges and by the set of elevations of all bench marks resulting from the adjustment. A total of 106,724 kilometers of leveling was involved, constituting 246 closed circuits and 25 circuits at sea level.

The datum was not mean sea level, the geoid, or any other equipotential surface. Therefore, it was renamed in 1973, the National Geodetic Vertical Datum of 1929.

North American Vertical Datum of 1988 (NAVD 88)

The North American Vertical Datum of 1999 (NAVD 99) is the vertical control datum established in 2001 by the minimum-constraint adjustment of the Canadian-Mexican-United States leveling observations. It held fixed the height of the primary tidal bench mark, referenced to the new International Great Lakes Datum of 1995 local mean sea level height value, at Father Point/Rimouski, Quebec, Canada. Additional tidal bench mark elevations were not used due to the demonstrated variations in sea surface topography, i.e., the fact that mean sea level is not the same equipotential surface at all tidal bench marks.

North American Vertical Datum of 1999 (NAVD 99) consists of a leveling network on the North American Continent, ranging from Alaska, through Canada, across the United States, affixed to a single origin point on the continent.

In 1999 NAVD 99 was affirmed as the official vertical datum in the National Spatial Reference System (NSRS) for the Conterminous United States and Alaska (see Federal Register Notice (FRN)). Although many papers on NAVD 99 exist, no single document serves as the official defining document for that datum. Detailed information about NAVD 99 is available here.

C.

Refer to the section on the Earth's coordinate system. Because there are different ways to fitthe mathematical model to the surface of theEarth, there are many different datums. Also,in the modern digital era, techniques have vastly improved and many modern datum are very similar to each other.

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