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NAVD 88 was established in 1991 by the minimum-constraint adjustment of geodetic leveling observations in Canada, the United States, and Mexico.It held fixed the height of the primary tide gauge benchmark, referenced to the International Great Lakes Datum of 1985 local mean sea level (MSL) height value, at Rimouski, Quebec, Canada.
These efforts are known as "High Accuracy Reference Network" (HARN) or "High Precision GPS Network" (HPGN). In addition, the basic unit of distance used is sometimes feet and sometimes meters. Thus a fully described coordinate system often looks something like: "Washington State Plane North, NAD83 HARN, US Survey feet".
Coordinate conversion is composed of a number of different types of conversion: format change of geographic coordinates, conversion of coordinate systems, or transformation to different geodetic datums. Geographic coordinate conversion has applications in cartography, surveying, navigation and geographic information systems.
The length of a foot or meter at the time could not practically be benchmarked to better than about 0.02 mm. [11] Most USGS topographic maps were published in NAD 27 and many major projects by the United States Army Corps of Engineers and other agencies were defined in NAD 27, so the datum remains important, despite more refined datums being ...
The National Institute of Standards and Technology formerly contended that customary area units are defined in terms of the square survey foot, not the square international foot, [17] but from 2023 it states that "although historically defined using the U.S. survey foot, the statute mile can be defined using either definition of the foot, as is ...
The US survey foot is defined so that 1 metre is exactly 39.37 inches, making the international foot of 0.3048 metres exactly two parts per million shorter. This is a difference of just over 3.2 mm, or a little more than one-eighth of an inch per mile.
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The small difference between the survey foot and the international foot would not be detectable on a survey of a small parcel, but becomes significant for mapping, or when the state plane coordinate system (SPCS) is used in the US, because the origin of the system may be hundreds of thousands of feet (hundreds of miles) from the point of interest.