TOPEX Outreach
Geoid Modeling Errors

To obtain ocean dynamics information from sea-surface height measurements requires a detailed knowledge of the geoid shape. This necessity arises from having to reference the acquired surface height and slope data to the reference ellipsoid/geoid reference frame in order to yield a reliable geoid undulation and corresponding sea-surface topography. The deviations of the geoid from the reference ellipsoid range from -100 m (South of India) to +64 m (near New Guinea).

The spatial variability can be large. For example, the geoid can vary by several meters over a few kilometers in areas where there are ocean trenches or ridges. This is in stark contrast to the variability of sea surface topography which is typically +/- 1.5 m. In order to determine the ocean-surface topography accurately it is necessary to know the geoid shape to around 10 cm accuracy over length scales the size of the ocean length scales being monitored.

There are essentially three ways in which to measure the earth's geoid:

  • Satellite orbit tracking
  • Direct measurements of gravity on land or ships
  • Satellite altimetry
Gravity fields deduced from satellite orbit tracking data include spherical harmonic expansion terms of rather low degree and order (typically less than 50). Discrepancies in the spherical harmonic coefficients and truncation of the spherical harmonic expansion produce errors in these models, the majority of which is due to inadequate global observation information at the short spatial scales referred to previously.

Effects of Temporal Variations


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Last Modified: Sat Jul 17, 1998
CSR/TSGC Team Web