GRAV-D Science Fundamentals
Within NOS, NOAA’s National Geodetic Survey (NGS) has a federal mandate to provide accurate positioning, including heights, to all federal non-military mapping activities in the USA. In 2007, NOAA’s National Geodetic Survey (NGS) embarked on the GRAV-D Project, one of the most ambitious projects in the history of the agency.
This undertaking was driven by the fundamental connection between Earth’s gravity field and the very definition of “height” itself. The specific goal of GRAV-D is therefore to model and monitor Earth’s geoid (a surface of the gravity field, very closely related to global mean sea level) to serve as a zero reference surface for all heights in the nation. Accurate heights are critical to many scientific endeavors, but particularly to understanding and protecting low-lying coastal ecosystems. [read more...]
What is the geoid?
There have been many definitions of the "geoid" over 150 years or so. Here is the one currently adopted at NGS:
Geoid: The equipotential surface of the Earth's gravity field which best fits, in a least squares sense, global mean sea level. [more...]
The Canadian Spatial Reference System describes this equipotential gravity surface, the geoid, very well:
"The geoid is theoretical only. You can't see it, touch it or even dig down to find it. Simply put, the geoid is the natural extension of the mean sea level surface under the landmass. We could illustrate this idea by digging an imaginary trench across the country linking the Atlantic and Pacific oceans. If we allowed the trench to fill with seawater, the surface of the water in the trench would represent the geoid. Not a bad way to imagine the geoid, but in reality not something we could easily do." [more...]
For an excellent overview of geodesy, datums, the science behind GRAV-D, and how these impact your life every day, tune in to the National Ocean Service Podcast: "Diving Deeper" on Geodesy by Dr. Dru Smith, Chief Geodesist, May 2009.
National Ocean Service (parent office for NGS):
Geodesy and Global Positioning
Podcast: "Making Waves, Episode 30" entitled "NGS Positioning Activities Worth Billions", July 2009
Geodesy for the Layman
Natural Resources Canada, Canadian Spatial Reference System:
Main Geodesy Education Page (many tutorials, including the two below)
The Geoid (multiple pages)
Gravity (multiple pages)
UNAVCO:
Tutorial on Datums (Geoid/Ellipsoid) and GPS Receiver Measurements
