OFFICIAL COORDINATES OF U.S. OFFSHORE INSTALLATIONS WILL CHANGE

The U.S. offshore oil and gas industry will be changing the official coordinate designation for its seismic surveys, leases, wells, and platforms. This undertaking is expected to require more than 1 0 years to complete. The change is a result of the U.S. Minerals Management Service (MMS), starting in 1991, to adapt the North American Datum of 1983 (NAD-83) to the outer continental shelf. The new datum depicts more accurately the shape of the earth.

June 24, 1991

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The change is a result of the U.S. Minerals Management Service (MMS), starting in 1991, to adapt the North American Datum of 1983 (NAD-83) to the outer continental shelf. The new datum depicts more accurately the shape of the earth.

The datum change was in response to a Dec. 6, 1988, mandate from the Federal Geodetic Control Committee to change from the existing NAD-27 to the new NAD-83.

The magnitude of the coordinate change will range from a few feet in the continental U.S. to several hundred meters in Alaska.

Conversion will be made by the Nacon method (Federal Register, Vol. 55, No. 155, Aug. 10, 1990). Nacon is federally endorsed datum transfer software which can convert existing NAD-27-based coordinates into NAD-83 equivalents (Federal Register, Vol. 56, No. 84, May 1, 1991).

In the first phase, NAD-27 will still be acceptable as long as it is identified. After a lease is terminated, relinquished, or expires, the block may be redefined by NAD-83.

By the end of Phase 1, about 1993, a full constellation of global positioning satellites is expected to be in place, and NAD-83 should become the dominant navigation and positioning datum.

During Phase 2, most of MMS computer systems will be adapted to NAD-83. Data submitted with NAD-27 coordinates will no longer be ac-cepted. The official registers will still list NAD-27 coordinates along with the equivalent NAD-83.

Full implementation of NAD-83 will take place during the third phase. Coordinates will have to adhere to the universal transverse mercator projection using geodetic reference system of 1980 (GRS-80) as the reference ellipsoid. Official protraction diagrams will depict a metric grid with full grid blocks being 4,800 x 4,800 m.

Because of the distance from the mainland, for Hawaii and U.S. dependencies, the world geodetic system of 1984 may be used as the ellipsoid. Also, the grid block size may be different for this area.

The table lists the presently recommended time frame for implementation.