The North American Datum of 1983

T he North American Datum comprises a network of more than 250,000 control points (survey monuments) covering the US, Canada, Mexico, Central America, Greenland, and the Caribbean islands. This network provides a means of accomplishing very accurate horizontal positioning, both on land and offshore. The last major adjustment of this system of control points resulted in the geodetic reference system known as the North American Datum of 1927 (NAD 27). “Adjustment” in this sense is the determination and application of corrections to survey observations, for the purpose of reducing errors or removing internal inconsistencies in derived results. Survey information, position coordinates, and maps used in the oil industry in North America are generally based on NAD 27. In July 1974, the governments of the US and Canada began a new geodetic adjustment of the horizontal NAD. Government authorities from Mexico, Central America, Denmark (for Greenland), and many countries within the Caribbean agreed to cooperate in this endeavor. The readjustment was scheduled for completion in 1983; thus, NAD 83. Project delays postponed completion until July 1986, but now there is finally a new, more accurate geodetic reference system spanning Panama through the Arctic islands and Greenland. Its useful life span is estimated to be 50 years. (A new adjustment of the North American Vertical Datum, the level surface to which elevations are referenced, is currently in progress and should not be confused with the horizontal datum discussed here.) This article will provide the reader with an understanding of the effects that this reference system will have on the North American petroleum industry; being unaware of it or ignoring its technical implications, can potentially cause endless problems with the positioning of seismic lines, drilling locations, and other geophysical and geologic data the end result of which could lead to dry holes or incomplete recovery of reserves. In order to fully appreciate and understand the readjustment of the North American Datum, the reader must first understand a few basic principles of geodesy. These include reference surfaces, the terrestrial coordinate system, the concept of a horizontal geodetic datum, and mathematical datum transformations.