The National Geodetic Survey Improves National Spatial Reference System
The three improvements to the NSRS are:
1. A new realization of the North American Datum of 1983 (NAD 83);
2. New gravimetric and hybrid geoid models; and
3. The determination of updated elevations in the Gulf Coast Height Modernization Project. These improvements provide users with positions and elevations that are both spatially and temporally consistent.
The first improvement, the NGS update of NAD 83, is referred to as the NAD 83(2011) epoch 2010.00 realization. This new realization aligns the Continuously Operating Reference Station (CORS) network with the network of passive geodetic control monuments positioned using GNSS technology. The CORS and passive GNSS networks are now optimally aligned at a common epoch date of 2010.00 (January 1, 2010), with the positions referring to a single moment in time. Aligning the two systems best meets user needs by providing consistent and accurate access to the latest realization of the official national geometric coordinate system.
NAD 83(2011) epoch 2010.00 coordinates are now available for all of the approximately 2,300 CORS and nearly 80,000 passive control stations distributed throughout the conterminous United States, Alaska, the Caribbean and the Pacific. With the completion of the new realization, all NGS datasheets listing coordinates in NAD 83(2011) epoch 2010.00 will be consistent with results obtained by CORS and the NGS Online Positioning User Service (OPUS). The (2011) datum tag refers specifically to control stations located in the conterminous United States, Alaska and the Caribbean, all of which are in a common North American reference frame. Control stations located in the Pacific have either a (PA11) or (MA11) datum tag. Stations in Hawaii, American Samoa and other locations on the Pacific tectonic plate have a datum tag of (PA11). Control stations located in Guam, the Commonwealth of the Northern Mariana Islands (CNMI) and other locations on or near the Mariana tectonic plate have a datum tag of (MA11).
The second major NGS product release is a set of two new and improved geoid models, GEOID12 and the U.S. Gravimetric Geoid of 2012 (USGG2012). A geoid model is a complex mathematical representation of global mean sea level with many practical applications for surveyors, engineers and the scientific community.
USGG2012 is a geoid model based on gravity data and is used mainly for scientific purposes. In contrast, GEOID12 is a hybrid geoid model derived from USGG2012 by incorporating orthometric heights (“elevations”) from leveling along with NAD 83(2011) epoch 2010.00 ellipsoid heights. The purpose of a hybrid geoid model is to give users an improved method for measuring elevations. GEOID12 provides an efficient mechanism for transforming GNSS heights to elevations relative to the North American Vertical Datum of 1988 (NAVD 88) and other vertical datums of Caribbean and Pacific island communities that are part of the NSRS.
By providing a more accurate means of determining elevations, GEOID12 will assist those involved in floodplain management, coastal and emergency response, port operations and river/stream flow monitoring. When used with GNSS, GEOID12 makes it possible to obtain more accurate elevations anywhere in the nation, helping to mitigate risks to coastal communities caused by hurricanes, storm surges, tsunamis and other flood-related events. Engineering and other activities requiring accurate elevations will also benefit, including precision agriculture and the construction and maintenance of dams, levees, roads and other infrastructure.
Rounding out NGS’ new product releases is the Gulf Coast Height Modernization Project. This project is in a region of known subsidence spanning southeastern Texas, southern Louisiana, Mississippi, Alabama and westernmost Florida. Due to subsidence, many control station elevations in the region are obsolete. New elevations were determined by combining new and old leveling observations using a technique called Vertical Time-Dependent Positioning (VTDP). VTDP compares leveling performed at different times to estimate rates of elevation change, and these rates are used in leveling adjustments to compute updated elevations. The VTDP results were also used in development of GEOID12 in this region to ensure consistency between leveling and NAD 83(2011) epoch 2010.00 ellipsoid heights. In the near future, NGS will combine VTDP results with GEOID12 and (2011) ellipsoid heights from recent GNSS surveys to determine accurate elevations at other control stations in the region.
NGS invites users to visit its website (http://geodesy.noaa.gov) and explore these major new releases.