American Society of Civil Engineers

Coordinate Conversion from NAD 27 to NAD 83

by D. R. Graff, M.ASCE, (Consultant, Surveying and Mapping, P.O. Box 311, Beaver Dam, WI 53916)

Journal of Surveying Engineering, Vol. 114, No. 3, August 1988, pp. 125-130, (doi:

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Document type: Journal Paper
Abstract: A method for converting North American Datum (NAD) 27 coordinates to NAD 83 coordinates is presented. The method is tested on over 1,000 control points, known in both NAD 27 and NAD 83, obtained from the National Geodetic Survey. Input data are maintained as geodetic latitude and longitude to allow for nationwide application. In order to use the transformation equations of plane geometry, latitudes and longitudes are projected to transverse Mercator X,Y with a unique projection for each point considered to be unknown in NAD 83. The methodology uses three or four (depending on the type of transformation) nearby points, known both in HAD 27 and HAD 83, to determine transformation coefficients which are then applied to a point considered to be known only in NAD 27. Results from the use of first-degree conformal, second-degree conformal, and affine coordinate transformation equations are presented. The final results indicate that greatest accuracy is obtained with the use of first- and second-order control points only. The use of the method in practical applications is discussed.

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