NOAA’s National Geodetic Survey (NGS) released the results of three major improvements to the National Spatial Reference System (NSRS). The NSRS is the consistent coordinate system that defines latitude, longitude, height, scale, gravity and orientation throughout the United States and its territories. NGS maintains and provides access to the NSRS, which is the foundation for the nation’s transportation, mapping, and charting infrastructure and serves a multitude of scientific and engineering applications.
The three improvements to the NSRS are:
new realization of the North American Datum of 1983 (NAD 83);
gravimetric and hybrid geoid models; and
determination of updated elevations in the Gulf Coast Height Modernization
provide users with positions and elevations that are both spatially and
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,
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.
The National Geodetic Survey Improves National Spatial Reference System
July 16, 2012