Legacy datum to or from WGS 84

Model: EPSG geocentric translation or 7-parameter Helmert (geog2D domain): geographic → ECEF on the datum's ellipsoid → Helmert → WGS 84

Accuracy: As stated by EPSG for each transformation: 2 m (OSGB36) to 35 m (Arc 1960); matches PROJ's implementation of the same EPSG operation to 1e-9°

When to use this: Use this to place an old map, chart, or survey record against a modern position. Coordinates printed before satellite positioning are on a datum fitted to one region — ED50 across western Europe, NAD27 in North America, Tokyo in Japan and Korea, Arc 1960 in east Africa — and reading them as WGS 84 puts a point tens to hundreds of meters from where it belongs, far enough to move a boundary onto the wrong side of a road. The answer reports how far the point moves and in which direction, so the size of the correction is visible, along with what the transformation is actually worth.

Limitations: This is meters of accuracy, not centimeters, and the stated figure is the point: from 2 m for OSGB36 to 35 m for Arc 1960. A single set of parameters is being asked to stand for a whole continental datum that was never that consistent, so the error varies across the region and no single number describes any particular point. Each transformation has a published area of use and the answer is flagged outside it, where it is extrapolation rather than transformation. Where a national grid exists — the NADCON5 grids for North America, OSTN15 for Great Britain — that grid is more accurate than this and should be preferred; the neighbouring NADCON5 tool covers the American cases. Only latitude and longitude are converted; heights are untouched, and nothing here concerns a vertical datum.

Worked example: Paris on ED50 to WGS 84. Source: PROJ 9.3.0 asked for EPSG:1133 (ED50 to WGS 84 (1)) by code alone, through pyproj, returns 48.85568546266484 and 2.350914333016232 — identical to the tool in every digit a double carries. 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: 2026-09-19, when a maintainer last confirmed this tool's sources at the issuer. See the sources ledger.

Status: version 1.0.0, core 0.1.0. See this tool in the verification report.

Checked against: 20 golden test vectors (download the test vectors, each with its source and tolerance). See how results are checked and every source.

Sources

• EPSG Geodetic Parameter Dataset, IOGP Geomatics Committee, v10.094, as shipped in PROJ 9.3.0. Transformations 1108, 1122, 1133, 1173, 1267, 1305, 1314, 1864 to WGS 84.
• Coordinate Conversions and Transformations including Formulas, IOGP Publication 373-7-2, International Association of Oil & Gas Producers (IOGP), Revised September 2019. §4.2.3 Helmert 7-parameter and §4.2.5 time-dependent transformations (EPSG methods 1032, 1033, 1053, 1056).

Terms

AGD66: Australian Geodetic Datum 1966

A legacy Australian datum on the Australian National Spheroid. Source: EPSG Geodetic Parameter Dataset

ED50: European Datum 1950

A legacy European datum on the International 1924 ellipsoid. Source: EPSG Geodetic Parameter Dataset

NAD27: North American Datum of 1927

The older North American datum on the Clarke 1866 ellipsoid. Its positions can differ from NAD 83 by tens of meters. Source: NADCON5 datum transformations

OSGB36: Ordnance Survey Great Britain 1936

The legacy datum of the British National Grid, on the Airy 1830 ellipsoid. Source: EPSG Geodetic Parameter Dataset

SAD69: South American Datum 1969

A legacy South American datum on the GRS 1967 ellipsoid. Source: EPSG Geodetic Parameter Dataset

WGS: World Geodetic System

The US Department of Defense global reference frame and ellipsoid that GPS uses; the current version is WGS 84. Source: Department of Defense World Geodetic System 1984