THE NORTH AMERICAN DATUM OF 1983 By JOHN W. McCREE Illinois Department of Transportation

The North American Datum of 1983 is the result of a readjustment of the existing horizontal control network that has been established and maintained by the National Geodetic Survey (formerly United States Coast & Geodetic Survey). The horizontal control network consists of over 250,000 monumented control stations that are spread across the continental United States and interconnected by surveying observations. These monumented stations are used by surveyors from federal, state, county, city or other entities for referencing boundaries, providing control for mapping, and charting and many other purposes.

The previous adjustment, entitled North American Datum of 1927, has generally satisfied lower order survey requirements for engineering and mapping purposes. With the advent of the satellite program in 1957 and the technological advances in surveying instrumentation, geodesists and surveyors became aware of deficiencies in azimuth and base line measurements that existed in the 1927 Datum. Since 1927, approximately 200,000 stations across North America have been added to the horizontal network. Many of the 200,000 stations were established using greatly enhanced surveying techniques as compared to those used prior to 1927. This more accurate observation data has, in the past, been forced to fit into the existing datum. The result was a horizontal control system with varying accuracies from area to area. A readjustment and recomputations of the entire United States network became imperative.

The 1983 datum uses a new reference ellipsoid which is a mathematical figure that most nearly matches the actual shape of the earth. The ellipsoid is formed by rotating an ellipse about the minor axis. An ellipse is used because of the flattening of the earth at the poles. The axis of the earth through the poles is used as the minor axis of the ellipse. New data collected by using satellites, Global Positioning Systems, and electronic distance measuring equipment has been used in developing a more precise ellipsoid. This new ellipsoid is called the Geodetic Reference System 1980 (GRS-80).

The new adjustment was started in the mid 70's and originally was estimated to be completed in 1983. However, the final adjusted values did not become available in final published format until 1987. All 250,000 horizontal control stations across the United States received a new geodetic position as a result of this new reference datum and adjustment.

Since new geodetic coordinates (latitude and longitude) have been developed with the new adjustment, new Illinois State Plane Coordinates (ISPC) for each monument are required to be calculated also. The National Geodetic Survey (NGS) will provide the formulas and/or software to compute ISPC's for any point in Illinois.

The National Geodetic Survey has computed the new State Plane Coordinates for all of their monumented control stations in Illinois. These are now available from the NGS.

To reduce the confusion for the user of state plane coordinates, the NGS assigned different values to the origin point of each Illinois zone. The east coordinate has been drastically changed so that a particular set of coordinate values will not appear in a zone on each datum.

The NGS is publishing state plane coordinates in meters only. The user will have to convert these values to feet using the U.S. survey foot definition: 3937/1200 meters, or the International Survey Foot definition: 1 inch equals 2.54 centimeters exactly.

November 1988 / Illinois Municipal Review / Page 17

The Illinois Department of Transportation commenced using Illinois State Plane Coordinates referenced to the North American Datum of 1983 in August of this year. All new control surveys being done by or for the Department of Transportation which are based on the Illinois State Plane Coordinate System are to use the North American Datum (NAD) of 1983 as the reference datum and the U.S. survey foot definition for converting from meters to feet.

The best procedure to use to convert NAD 27 coordinates to NAD 83 is by recomputation using the original field data. If this is not possible, software is available from the NGS for a nominal fee that will convert coordinates from NAD 27 to NAD 83 and vice versa.

Additional information can be obtained by contacting the Bureau of Location and Environment, Aerial Surveys Section in Springfield, Illinois, or by contacting the National Geodetic Survey in Rockville, Maryland. •

Local agency representatives have inquired regarding who they should contact in the Illinois Department of Transportation when they have highway or street related questions. These questions should be addressed to the appropriate District Local Roads Engineer shown below. DISTRICT LOCAL ROADS ENGINEERS DISTRICT 1 R. M. Harris PHONE 312/705-4201 DISTRICT 2 L E.Reed PHONE 815/284-5380 DISTRICT 3 W. Kasza PHONE 815/434-8441 DISTRICT 4 R. E. McCumber PHONE 309/691-2110 DISTRICT 5 W. A. Newlin, Jr. PHONE 217/485-4181 DISTRICT 6 Z. Wickham PHONE 217/782-4690 DISTRICT 7 J. Cunningham PHONE 217/342-3951 DISTRICT 8 D. Weisenstein PHONE 618/346-3330 DISTRICT 9 C. L. Hager PHONE 618/549-2171

Page 18 / Illinois Municipal Review / November 1988