Geodesy is the science of measuring and monitoring the size and shape of the Earth including its gravity field and determining the location of points on the Earth’s surface. Learn more.
NOS's National Geodetic Survey (NGS) recently rolled out several important updates to the National Spatial Reference System (NSRS), the nationwide network of interconnected survey points used for transportation, mapping, and charting. With these new improvements, users of the spatial reference system now have access to better positioning and elevation data.
The first improvement to the NSRS, NGS’ National Adjustment of 2011 Project, marries up new position information (latitude, longitude, and height) for more than 80,000 survey marks with that of NGS's more advanced GPS receiver-based control points, known collectively as the Continuously Operating Reference Stations network. As a result, the coordinates of the nation's reference system are now consistent across space (all GPS-observed points in the reference system are updated) and time (all points are updated at the same point in time, which is important considering the Earth is always moving and changing).
Consistency in the National Spatial Reference System is critical, because people rely on positioning data for countless engineering and scientific applications—to plan roads to improve traffic flow, to check the integrity of buildings, to ensure airplanes land safely on the runway, and even to measure small changes in sea level over time. Surveyors and others use the NSRS throughout the country to ensure their positional coordinates are compatible with those determined by others, so that when experts create maps; mark off property boundaries; and plan, design, and build roads, bridges, and other structures, everything matches up.
The next update from NGS is the release of two new models of the geoid, which can be described as the 'shape' of mean sea level around the globe. Surveyors, engineers, and the scientific community use the geoid—a model derived through complex math and gravity measurements—to measure surface elevations with a high degree of accuracy.
While the science behind the construction of geoid models is very complex, one reason why we need such models is surprisingly simple:
a small difference in height measurement on land determines which way water flows, and this makes the development of new and better geoid models an ongoing priority.
By providing a more accurate means of determining elevations, the new models will assist those involved in floodplain management, coastal and emergency response, port operations, and river flow monitoring. More accurate elevations also help to lessen risks to coastal communities caused by hurricanes, storm surges, tsunamis, and other flood-related events. Engineering and other activities requiring accurate elevations also benefit, including precision agriculture and the construction and maintenance of dams, levees, roads, and other infrastructure.
Rounding out NGS’ new releases is the Gulf Coast Height Modernization Project. This project is in a region of known subsidence (where the land is sinking) that spans eastern Texas, southern Louisiana, Mississippi, Alabama, and westernmost Florida.
Due to subsidence, the elevation values on many survey control marks in the region are obsolete. To fix this problem, NGS experts calculated new elevations by combining new and old leveling observations using a technique called Vertical Time-Dependent Positioning. This method compares leveling performed at different times to estimate rates of elevation change, and these rates are used in leveling adjustments to compute updated elevations.