Section 1 – What Is NADCON Used for?
1.1 The Purpose of NADCON
NADCON applies a standard method to calculate mathematical transformations between horizontal geodetic datums. |
In 1990, the Federal Geodetic Control Committee (FGCC) announced a decision to adopt a standard method for calculating mathematical transformations between two horizontal geodetic datums: the North American Datum of 1927 (NAD 27) and the North American Datum of 1983 (NAD 83(86)). To accomplish this task, the FGCC chose the North American Datum Conversion Utility (NADCON) software as the Federal standard for performing these datum transformations. This recommendation was made with the intent of maintaining consistency in the calculations and minimizing the potential for errors. |
1.2 What Does NADCON Do?
NADCON transforms latitude and longitude coordinate values between any two datums for which a conversion grid is available. |
NADCON transforms latitude and longitude coordinate values between two geodetic coordinate systems known as “datums.” To do so, NADCON uses a pair of conversion grids that essentially provide a transformation “bridge” between the two datums. NADCON can transform between any two datums for which grids are available. See Available Datum Shifts. To perform the conversions, the NADCON application reads input coordinate data and uses an algorithm developed by Warren T. Dewhurst, Ph.D., and described in NOAA Technical Memorandum NOS NGS-50, dated January, 1990, to calculate new coordinate values and shift offsets. Results are displayed in seconds and meters and the new values are written and displayed. Each conversion requires the presence of conversion grids that are pairs of binary files (*.las, *.los) made up of a grid array of datum shift values plus spatial location and parameter information. Datum shifts for each input value are calculated based on an algorithm that interpolates the positional data using a minimum curvature function. This version of NADCON reproduces the conversions exactly the same as prior versions of NADCON. NADCON conversions between datums are approximate values based on models of real data. NADCON should be used only when data for the point(s) to be shifted does not exist in the NGS data base (NGSIDB). The accuracy of the transformations should be viewed with some caution. At the 67 percent confidence level, this method introduces approximately 0.15 meter uncertainty within the conterminous United States, 0.50 meter uncertainty within Alaska, 0.20 meter uncertainty within Hawaii, and 0.05 meter uncertainty within Puerto Rico and the Virgin Islands. In areas of sparse geodetic data coverage, NADCON may yield less accurate results, but seldom in excess of 1.0 meter. Accordingly, NADCON results should not be used as a navigational aid. Transformations between NAD 83(86) and States/Regions with High Accuracy Reference Networks (HARNs) introduce approximately 0.05 meter uncertainty. Transformations between old datums (NAD 27, Old Hawaiian, Puerto Rico, etc.) and HARN combine uncertainties with the transformation uncertainties occurring in the intermediate step of transforming to NAD 83(86). For example, NAD 27 to HARN requires a transformation from NAD27 to NAD 83(86) first and so the uncertainty would be 0.15 meter + 0.05 meter = 0.20 meter. In near offshore regions, results will be less accurate but seldom in excess of 5.0 meters. Farther offshore NAD 27 was undefined. Therefore, the NADCON computed transformations are extrapolations and no accuracy can be stated. NADCON cannot improve the accuracy of data. Stations that are originally third-order will not become first-order stations. NADCON is merely a tool for transforming coordinate values between datums. Remember, this program is based exclusively upon data within the official National Spatial Reference System (NSRS). Data originating from stations not part of this official reference may not be compatible. Be sure that the data to be transformed is actually referenced to the NSRS.
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