The National Spatial Reference System (NSRS) contains both the official “horizontal” and “vertical” datums of the United States: the North American Datum of 1983 (NAD83) and the North American Vertical Datum of 1988 (NAVD88). Horizontal datums provide reference latitude and longitude on Earth's surface; vertical datums are used for describing elevation. The responsibility for defining, maintaining, and providing access to these datums falls to the National Geodetic Survey (NGS), an office within the National Oceanic and Atmospheric Administration (NOAA).
These datums were created to correct deficiencies in the previous (1920's era) datums. Still, they relied almost exclusively on terrestrial surveying and the use of passive marks for their realization. Once the Global Positioning System (GPS) and other space geodetic techniques were widely operational, some significant issues became apparent. First, NAD83 has a nongeocentricity (i.e., its origin differs from the known center of Earth's mass) of approximately 2.2 meters. Second, the use of NAVD88's unmonitored passive control benchmarks to determine elevations lacks critical vertical velocity information. Additionally, the zero-elevation surface of NAVD88 is now known to be biased (50 centimeters) and tilted (2 meters from Florida to Alaska) relative to the geoid.