Transforming NAD 27 and NAD 83 positions : making legacy mapping and surveys GPS compatible.
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2015-06-01
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Abstract:The Connecticut Department of Transportation (CTDOT) and the University of Connecticut are creating a real-time network (RTN) to make real-time surveying widely available in Connecticut. This RTN uses global navigation satellite system (GNSS) technology (such as the U.S. Global Positioning System [GPS]) and is inherently most compatible with the North American Datum of 1983 (NAD83). An RTN will be most effective if it can be used throughout the State. There are still towns in Connecticut that have maps and surveys based on the North American Datum of 1927 (NAD 27), and global navigation satellite system (GNSS) surveying instruments cannot provide real-time positioning in NAD 27 with the accuracy levels demanded by land surveying for lack of a high-accuracy reference-frame transformation between NAD 27 and the reference frame of the GNSS (one of the modern International Terrestrial Reference Frames). Re-mapping and re-surveying the legacy maps is cost prohibitive. Therefore, the least-squares method was used to try to provide a transformation. We used weighted and non-weighted, ordinary and total least squares on both geocentric Cartesian (Earth-centered, Earth-fixed) positions and on positions in the Connecticut State Plane coordinate system. No transformation was best so, by parsimony, we recommend the model producing using ordinary least squares in State Plane Coordinates (SPC). The transformed coordinates’ residuals differed from control values in eastings by 0.040 m (0.159 m 1-sigma) and by 0.001 m (0.128 m 1-sigma) in northings. Although inadequate for many surveying and engineering projects, this result should be adequate for topographic mapping and resource-location projects throughout Connecticut.
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