How can we describe the shape of the Earth? One can use the surface of Earth’s oceans to describe its physical shape. This makes sense because water covers about 70% of the Earth’s surface. Since ancient times, the definitions of “horizontal” and “vertical” were directly related to the gravity field. Builders found the vertical by using a heavy object on a string — a“plumb line.” Today, it is possible to create a physical model of Earth’s shape using gravity observations. The Earth’s surface using gravity as a physical property is defined as the “Earth’s geopotential field,” and is characterized as a smoothly undulating shape that follows a certain constant value of Earth's gravity field.
The physical shape of the Earth is used to define a vertical reference system in the US and its territories. Applications using the geopotential surface include: navigation, coastal modeling, agriculture, risk assessment for flooding, and many other applications. NOAA’s National Geodetic Survey conducts research for applications such as model and surface definitions, guidance for gravity data collection, data-processing workflows, and publication of new reference systems.
The geopotential surface is defined as Earth’s water surface at rest without coastal and ocean processes (e.g., tides and currents). Because the water surface is very close to the geopotential surface, it was easy to define the surface using traditional surveying techniques that have been around for centuries, namely spirit leveling. With modern-day technologies that include space observations and aerial observations, it is possible to define a new more accurate geopotential surface without accumulating measurement errors associated with the traditional leveling survey campaigns.
Read moreNOAA’s National Geodetic Survey incorporates a large suite of gravity observations that are acquired on land, on the water surface, in air, and out in space. The devices that measure gravity with respect to a uniform density gravity surface (i.e., the anomaly in the magnitude of gravity) are called gravity meters or gravimeters. NOAA’s National Geodetic Survey research evaluate procedures for cleaning and processing gravity survey data collected using traditional approaches, and investigating what new methods can we use in the future.
Read moreOnce all the available gravity datasets (space, airborne, land and marine) collected over North America and the Pacific Ocean have been cleaned for outliers and errors related to the platform or the instrument, NOAA works closely with counterpart offices in Canada, Mexico, and Denmark on geoid model development and sharing data. The outcome of the international collaboration is to produce a high quality geoid that has a seamless geopotential surface over all the U.S. and its territories.
Read moreNGS uses multiple approaches to evaluate a geoid model or a geopotential surface. Once a geoid model has been developed, it is important to evaluate the model and check it against independent data sets, including survey marks where heights have been measured with both Global Navigation Satellite System (GNSS) measurements and leveling. The National Geodetic Survey has also conducted evaluation of spatial trends of the geoid using astro-geodetic observation and water-surface slopes calculated from water models.
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