Datum

A datum is the starting frame for coordinates. NOAA's National Geodetic Survey (NGS) defines it: "A geodetic datum or reference frame is an abstract coordinate system with a reference surface (such as sea level) that serves to provide known locations to begin surveys and create maps." NGS "defines the official geodetic datums for all federal mapping activities in the U.S. and its territories."

Horizontal datums

• NAD 27: superseded. NGS: "The NAD 27 was based on the Clarke Ellipsoid of 1866," computed with station MEADES RANCH in Kansas as the datum point. NGS lists it for 1927 to 1986.
• NAD 83: "the horizontal and geometric control datum for the United States, Canada, Mexico, and Central America," released in 1986 and based on the Geodetic Reference System of 1980. Later realizations include NSRS2007 and NAD 83(2011). NGS: "Current surveys should use NAD 83 (2011) Epoch 2010.00 for the conterminous U.S., Alaska and Puerto Rico."
• WGS 84: NGS lists it as "Defined by the U.S. Department of Defense. Commonly used within civilian GPS software."

USGS Professional Paper 1395 (1987), written as NAD 83 was coming in, said coordinates of almost every point in North America would change, and "The difference can reach 300m."

Vertical datums

Heights use a separate datum. NGS: "The current vertical datum for the contiguous United States and Alaska is the North American Vertical Datum of 1988." It replaced NGVD 29, where NGS reports distortions "amounting to as much as 9 meters."

Coming change

NGS plans to replace NAD 83 with four plate-fixed frames, including the North American Terrestrial Reference Frame of 2022 (NATRF2022), and NAVD 88 with a geopotential datum, NAPGD2022. NGS says "NAD 83 is misaligned to the earth's center by about 2.2 meters," and that coordinates will change "by as much as four meters." In the meantime, "the current NSRS will remain the official NSRS of the United States." When this entry was last checked, in September 2026, NGS's datums page still listed NAD 83 and NAVD 88 as current; check NGS for the status of the new datums before relying on either.

The PLSS datum

BLM's Manual of Surveying Instructions (2009) uses the word in a survey sense: "A datum is the reference system by which the measurements of a survey system are reported" (section 2-9). For the PLSS, "Most boundaries are lines of constant bearing measured at ground elevation" (section 2-10).

Why it matters in mining claim research

BLM's MLRS mining claim layer is on NAD 83. Older plats, mineral surveys, and location notices may give positions on NAD 27 or name no datum at all. Each source's datum has to be known, and transformed where needed, before a mining claim is placed or an overlap is called. See Coordinate System.