Old US datums to NAD 83 (NADCON5)

Model: NADCON5 first-step grid for the region, biquadratic interpolation (NGS qterp); the reverse by iteration

Accuracy: Matches PROJ's NADCON5 transformations to 1e-9°; NGS states each first step itself at the decimeter level (about 0.15 m, 1σ, for NAD 27 in CONUS)

When to use this: Use this for old American coordinates when the answer has to be better than a meter. NAD 27 and the other regional datums — Old Hawaiian, Puerto Rico 1940, St. Paul 1952, American Samoa 1962, Guam 1963 — differ from NAD 83 by an amount that changes from place to place across the country, and the NGS grids model that variation directly instead of averaging it into one set of parameters. This is the transformation NGS itself publishes for the job, so it is what a survey, a parcel record, or a republished map should be brought forward with.

Limitations: It only covers what the grids cover: the conterminous United States, Alaska, Hawaii, Puerto Rico and the Virgin Islands, St. Paul Island, American Samoa, and Guam and the Northern Marianas. A point outside all of them is refused rather than extrapolated, and the regional helmert transformation is the fallback for anywhere else. This is the first step of NADCON5 — the one that carries the old datum to NAD 83 (1986) — and NGS states it at the decimeter level, about 0.15 m for NAD 27 in the conterminous states, so the result is decimeters, not centimeters. Later NAD 83 realizations are a separate step, which the NAD 83 tool handles. Only latitude and longitude are converted; the vertical grids are not part of this.

Worked example: Central Kansas (Meades Ranch, the NAD 27 origin). Source: PROJ 9.3 applying the NADCON5 grid us_noaa_nadcon5_nad27_nad83_1986_conus.tif through pyproj, which the 34 grid vectors come from across all seven regions; the grid moves this point 29.815 m where the single-parameter EPSG helmert moves it 31.793 m, the 2.5 m between them being the regional variation a grid carries and a helmert cannot. It is golden test vector v001, and every build checks the tool still gives its answer within its tolerance.

Review: Not yet independently reviewed by a geodesist.

Last verified: 2026-09-23, when a maintainer last confirmed this tool's sources at the issuer. See the sources ledger.

Status: version 1.0.1, core 0.1.0. See this tool in the verification report.

Changes

• 2026-09-23, fixed: NADCON5 (version 1.0.1) cited NOAA Technical Memorandum NOS NGS 84 as the NADCON5 report. That memo is Dru Smith's Biquadratic Interpolation (2022). The tool now cites the NADCON 5.0 report, NOAA Technical Report NOS NGS 63 (Smith and Bilich, 2017, revised 2020), and cites TM 84 separately for the interpolation between grid nodes, which the tool does use. Results are unchanged. Changelog

Checked against: 36 golden test vectors (download the test vectors, each with its source and tolerance). See how results are checked and every source.

Sources

• NADCON 5.0: Geometric Transformation Tool for Points in the National Spatial Reference System, NOAA Technical Report NOS NGS 63, Smith, D. A., and Bilich, A. L., National Geodetic Survey, NOAA TR NOS NGS 63 (revised August 31, 2020); grids release 20160901. The transformation grids by region and how they are applied. Read free.
• Biquadratic Interpolation, NOAA Technical Memorandum NOS NGS 84, Smith, D. A., National Geodetic Survey, NOAA TM NOS NGS 84 (January 25, 2022). Section 2: quadratic interpolation over the nearest 3 × 3 grid nodes.

Terms

NAD: North American Datum

The family of North American horizontal datums: NAD 27 on the Clarke 1866 ellipsoid and NAD 83 on GRS 80. Source: NADCON5 datum transformations

NADCON5: NGS datum conversion grids, version 5

NGS grids that convert positions between US datums, such as NAD 27 to NAD 83, by interpolating measured shifts. Source: NADCON5 datum transformations