GRAV-D Science Fundamentals
This site is currently under construction. Please check back later for additional information!
An excellent overview of geodesy, datums, the science behind GRAV-D, and how these impact your life every day, is available for you to listen to from the National Ocean Service Podcast: "Diving Deeper" on Geodesy by Dr. Dru Smith, Chief Geodesist, May 2009.
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...]
National Ocean Service (parent office for NGS):
Geodesy and Global Positioning
Podcast: "Diving Deeper" on Geodesy, May 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
