Earth geometry

DM-44 - Earth's Shape, Sea Level, and the Geoid

C. F. Gauss set the modern definition of the shape of the Earth, being described as the shape the oceans would adopt if they were entirely unperturbed and, thus, placid—a surface now called the geoid.  This surface cannot be observed directly because the oceans have waves, tides, currents, and other perturbations. Nonetheless, the geoid is the ideal datum for heights, and the science of determining the location of the geoid for practical purposes is the topic of physical geodesy. The geoid is the central concept that ties together what the various kinds of height mean, how they are measured, and how they are inter-related.

DM-44 - Earth's Shape, Sea Level, and the Geoid

C. F. Gauss set the modern definition of the shape of the Earth, being described as the shape the oceans would adopt if they were entirely unperturbed and, thus, placid—a surface now called the geoid.  This surface cannot be observed directly because the oceans have waves, tides, currents, and other perturbations. Nonetheless, the geoid is the ideal datum for heights, and the science of determining the location of the geoid for practical purposes is the topic of physical geodesy. The geoid is the central concept that ties together what the various kinds of height mean, how they are measured, and how they are inter-related.

DM-44 - Approximating the Earth's shape with geoids
  • Explain why gravity varies over the Earth’s surface
  • Explain how geoids are modeled
  • Explain the role that the U.S. National Geodetic Survey plays in maintaining and developing geoid models
  • Explain the concept of an equipotential gravity surface (i.e., a geoid)
DM-44 - Approximating the Earth's shape with geoids
  • Explain why gravity varies over the Earth’s surface
  • Explain how geoids are modeled
  • Explain the role that the U.S. National Geodetic Survey plays in maintaining and developing geoid models
  • Explain the concept of an equipotential gravity surface (i.e., a geoid)
DM-44 - Approximating the Earth's shape with geoids
  • Explain why gravity varies over the Earth’s surface
  • Explain how geoids are modeled
  • Explain the role that the U.S. National Geodetic Survey plays in maintaining and developing geoid models
  • Explain the concept of an equipotential gravity surface (i.e., a geoid)
DM-44 - Approximating the Earth's shape with geoids
  • Explain why gravity varies over the Earth’s surface
  • Explain how geoids are modeled
  • Explain the role that the U.S. National Geodetic Survey plays in maintaining and developing geoid models
  • Explain the concept of an equipotential gravity surface (i.e., a geoid)
DM-44 - Approximating the Earth's shape with geoids
  • Explain why gravity varies over the Earth’s surface
  • Explain how geoids are modeled
  • Explain the role that the U.S. National Geodetic Survey plays in maintaining and developing geoid models
  • Explain the concept of an equipotential gravity surface (i.e., a geoid)
DM-44 - Approximating the Earth's shape with geoids
  • Explain why gravity varies over the Earth’s surface
  • Explain how geoids are modeled
  • Explain the role that the U.S. National Geodetic Survey plays in maintaining and developing geoid models
  • Explain the concept of an equipotential gravity surface (i.e., a geoid)
DM-44 - Approximating the Earth's shape with geoids
  • Explain why gravity varies over the Earth’s surface
  • Explain how geoids are modeled
  • Explain the role that the U.S. National Geodetic Survey plays in maintaining and developing geoid models
  • Explain the concept of an equipotential gravity surface (i.e., a geoid)
DM-44 - Approximating the Earth's shape with geoids
  • Explain why gravity varies over the Earth’s surface
  • Explain how geoids are modeled
  • Explain the role that the U.S. National Geodetic Survey plays in maintaining and developing geoid models
  • Explain the concept of an equipotential gravity surface (i.e., a geoid)

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